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Editor:

Serge Planton (France)

Annex III: Glossary

This annex should be cited as:

IPCC, 2013: Annex III: Glossary [Planton, S. (ed.)]. In: Climate Change 2013: The Physical Science Basis. Contribution

of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F.,

D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge

University Press, Cambridge, United Kingdom and New York, NY, USA.

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Annex III Glossary

AIII

This glossary deﬁnes some speciﬁc terms as the Lead Authors intend

them to be interpreted in the context of this report. Red, italicized

words indicate that the term is deﬁned in the Glossary.

Abrupt climate change A large-scale change in the climate system

that takes place over a few decades or less, persists (or is anticipated to

persist) for at least a few decades and causes substantial disruptions in

human and natural systems.

Active layer The layer of ground that is subject to annual thawing and

freezing in areas underlain by permafrost.

Adjustment time See Lifetime. See also Response time.

Advection Transport of water or air along with its properties (e.g., tem-

perature, chemical tracers) by winds or currents. Regarding the general

distinction between advection and convection, the former describes trans-

port by large-scale motions of the atmosphere or ocean, while convection

describes the predominantly vertical, locally induced motions.

Aerosol A suspension of airborne solid or liquid particles, with a typical

size between a few nanometres and 10 μm that reside in the atmosphere

for at least several hours. For convenience the term aerosol, which includes

both the particles and the suspending gas, is often used in this report in

its plural form to mean aerosol particles. Aerosols may be of either natural

or anthropogenic origin. Aerosols may inﬂuence climate in several ways:

directly through scattering and absorbing radiation (see Aerosol–radiation

interaction) and indirectly by acting as cloud condensation nuclei or ice

nuclei, modifying the optical properties and lifetime of clouds (see Aero-

sol–cloud interaction).

Aerosol–cloud interaction A process by which a perturbation to

aerosol affects the microphysical properties and evolution of clouds

through the aerosol role as cloud condensation nuclei or ice nuclei, par-

ticularly in ways that affect radiation or precipitation; such processes can

also include the effect of clouds and precipitation on aerosol. The aerosol

perturbation can be anthropogenic or come from some natural source. The

radiative forcing from such interactions has traditionally been attributed

to numerous indirect aerosol effects, but in this report, only two levels of

radiative forcing (or effect) are distinguished:

Radiative forcing (or effect) due to aerosol–cloud interactions

(RFaci) The radiative forcing (or radiative effect, if the perturbation is

internally generated) due to the change in number or size distribution

of cloud droplets or ice crystals that is the proximate result of an aero-

sol perturbation, with other variables (in particular total cloud water

content) remaining equal. In liquid clouds, an increase in cloud droplet

concentration and surface area would increase the cloud albedo. This

effect is also known as the cloud albedo effect, ﬁrst indirect effect, or

Twomey effect. It is a largely theoretical concept that cannot readily be

isolated in observations or comprehensive process models due to the

rapidity and ubiquity of rapid adjustments.

Effective radiative forcing (or effect) due to aerosol–cloud inter-

actions (ERFaci) The ﬁnal radiative forcing (or effect) from the aero-

sol perturbation including the rapid adjustments to the initial change

in droplet or crystal formation rate. These adjustments include changes

in the strength of convection, precipitation efﬁciency, cloud fraction,

lifetime or water content of clouds, and the formation or suppression

of clouds in remote areas due to altered circulations.

The total effective radiative forcing due to both aerosol–cloud and

aerosol–radiation interactions is denoted aerosol effective radiative

forcing (ERFari+aci). See also Aerosol–radiation interaction.

Aerosol–radiation interaction An interaction of aerosol directly

with radiation produce radiative effects. In this report two levels of radia-

tive forcing (or effect) are distinguished:

Radiative forcing (or effect) due to aerosol–radiation interac-

tions (RFari) The radiative forcing (or radiative effect, if the pertur-

bation is internally generated) of an aerosol perturbation due directly

to aerosol–radiation interactions, with all environmental variables

remaining unaffected. Traditionally known in the literature as the direct

aerosol forcing (or effect).

Effective radiative forcing (or effect) due to aerosol-radiation

interactions (ERFari) The ﬁnal radiative forcing (or effect) from

the aerosol perturbation including the rapid adjustments to the ini-

tial change in radiation. These adjustments include changes in cloud

caused by the impact of the radiative heating on convective or larger-

scale atmospheric circulations, traditionally known as semi-direct aero-

sol forcing (or effect).

The total effective radiative forcing due to both aerosol–cloud and

aerosol–radiation interactions is denoted aerosol effective radiative

forcing (ERFari+aci). See also Aerosol–cloud interaction.

Afforestation Planting of new forests on lands that historically have

not contained forests. For a discussion of the term forest and related terms

such as afforestation, reforestation and deforestation, see the IPCC Special

Report on Land Use, Land-Use Change and Forestry (IPCC, 2000). See also

the report on Deﬁnitions and Methodological Options to Inventory Emis-

sions from Direct Human-induced Degradation of Forests and Devegeta-

tion of Other Vegetation Types (IPCC, 2003).

Airborne fraction The fraction of total CO

emissions (from fossil fuel

and land use change) remaining in the atmosphere.

Air mass A widespread body of air, the approximately homogeneous

properties of which (1) have been established while that air was situated

over a particular region of the Earth’s surface, and (2) undergo speciﬁc

modiﬁcations while in transit away from the source region (AMS, 2000).

Albedo The fraction of solar radiation reﬂected by a surface or object,

often expressed as a percentage. Snow-covered surfaces have a high

albedo, the albedo of soils ranges from high to low, and vegetation-cov-

ered surfaces and oceans have a low albedo. The Earth’s planetary albedo

varies mainly through varying cloudiness, snow, ice, leaf area and and

cover changes.

Alkalinity A measure of the capacity of an aqueous solution to neutral-

ize acids.

Altimetry A technique for measuring the height of the Earth’s surface

with respect to the geocentre of the Earth within a deﬁned terrestrial refer-

ence frame (geocentric sea level).

Annular modes See Northern Annular Mode (NAM) and Southern

Annular Mode (SAM).

Anthropogenic Resulting from or produced by human activities.

Atlantic Multi-decadal Oscillation/Variability (AMO/AMV) A

multi-decadal (65- to 75-year) ﬂuctuation in the North Atlantic, in which

sea surface temperatures showed warm phases during roughly 1860 to

1880 and 1930 to 1960 and cool phases during 1905 to 1925 and 1970 to

1990 with a range of approximately 0.4°C. See AMO Index, Box 2.5.

Atmosphere The gaseous envelope surrounding the Earth. The dry

atmosphere consists almost entirely of nitrogen (78.1% volume mixing

ratio) and oxygen (20.9% volume mixing ratio), together with a number

of trace gases, such as argon (0.93% volume mixing ratio), helium and

radiatively active greenhouse gases such as carbon dioxide (0.035%

1449

Glossary Annex III

AIII

volume mixing ratio) and ozone. In addition, the atmosphere contains the

greenhouse gas water vapour, whose amounts are highly variable but typi-

cally around 1% volume mixing ratio. The atmosphere also contains clouds

and aerosols.

Atmosphere–Ocean General Circulation Model (AOGCM) See

Climate model.

Atmospheric boundary layer The atmospheric layer adjacent to the

Earth’s surface that is affected by friction against that boundary surface,

and possibly by transport of heat and other variables across that surface

(AMS, 2000). The lowest 100 m of the boundary layer (about 10% of the

boundary layer thickness), where mechanical generation of turbulence is

dominant, is called the surface boundary layer or surface layer.

Atmospheric lifetime See Lifetime.

Attribution See Detection and attribution.

Autotrophic respiration Respiration by photosynthetic (see photo-

synthesis) organisms (e.g., plants and algaes).

Basal lubrication Reduction of friction at the base of an ice sheet

or glacier due to lubrication by meltwater. This can allow the glacier or

ice sheet to slide over its base. Meltwater may be produced by pressure-

induced melting, friction or geothermal heat, or surface melt may drain to

the base through holes in the ice.

Baseline/reference The baseline (or reference) is the state against

which change is measured. A baseline period is the period relative to which

anomalies are computed. The baseline concentration of a trace gas is that

measured at a location not inﬂuenced by local anthropogenic emissions.

Bayesian method/approach A Bayesian method is a method by

which a statistical analysis of an unknown or uncertain quantity(ies) is car-

ried out in two steps. First, a prior probability distribution for the uncertain

quantity(ies) is formulated on the basis of existing knowledge (either by

eliciting expert opinion or by using existing data and studies). At this ﬁrst

stage, an element of subjectivity may inﬂuence the choice, but in many

cases, the prior probability distribution can be chosen as neutrally as pos-

sible, in order not to inﬂuence the ﬁnal outcome of the analysis. In the

second step, newly acquired data are used to update the prior distribution

into a posterior distribution. The update is carried out either through an

analytic computation or though numeric approximation, using a theorem

formulated by and named after the British mathematician Thomas Bayes

(1702–1761).

Biological pump The process of transporting carbon from the ocean’s

surface layers to the deep ocean by the primary production of marine phy-

toplankton, which converts dissolved inorganic carbon (DIC) and nutrients

into organic matter through photosynthesis. This natural cycle is limited

primarily by the availability of light and nutrients such as phosphate, nitrate

and silicic acid, and micronutrients, such as iron. See also Solubility pump.

Biomass The total mass of living organisms in a given area or volume;

dead plant material can be included as dead biomass. Biomass burning is

the burning of living and dead vegetation.

Biome A biome is a major and distinct regional element of the bio-

sphere, typically consisting of several ecosystems (e.g., forests, rivers,

ponds, swamps within a region). Biomes are characterized by typical com-

munities of plants and animals.

Biosphere (terrestrial and marine) The part of the Earth system

comprising all ecosystems and living organisms, in the atmosphere, on

land (terrestrial biosphere) or in the oceans (marine biosphere), including

derived dead organic matter, such as litter, soil organic matter and oceanic

detritus.

Black carbon (BC) Operationally deﬁned aerosol species based on

measurement of light absorption and chemical reactivity and/or thermal

stability. It is sometimes referred to as soot.

Blocking Associated with persistent, slow-moving high-pressure sys-

tems that obstruct the prevailing westerly winds in the middle and high

latitudes and the normal eastward progress of extratropical transient

storm systems. It is an important component of the intraseasonal climate

variability in the extratropics and can cause long-lived weather conditions

such as cold spells in winter and summer heat waves.

Brewer–Dobson circulation The meridional overturning circulation

of the stratosphere transporting air upward in the tropics, poleward to the

winter hemisphere, and downward at polar and subpolar latitudes. The

Brewer–Dobson circulation is driven by the interaction between upward

propagating planetary waves and the mean ﬂow.

Burden The total mass of a gaseous substance of concern in the atmo-

sphere.

C Stable isotope of carbon having an atomic weight of approximately

13. Measurements of the ratio of

C in carbon dioxide molecules are

used to infer the importance of different carbon cycle and climate pro-

cesses and the size of the terrestrial carbon reservoir.

C Unstable isotope of carbon having an atomic weight of approxi-

mately 14, and a half-life of about 5700 years. It is often used for dating

purposes going back some 40 kyr. Its variation in time is affected by the

magnetic ﬁelds of the Sun and Earth, which inﬂuence its production from

cosmic rays (see Cosmogenic radioisotopes).

Calving The breaking off of discrete pieces of ice from a glacier, ice

sheet or an ice shelf into lake or seawater, producing icebergs. This is a

form of mass loss from an ice body. See also Mass balance/budget (of

glaciers or ice sheets).

Carbonaceous aerosol Aerosol consisting predominantly of organic

substances and black carbon.

Carbon cycle The term used to describe the ﬂow of carbon (in various

forms, e.g., as carbon dioxide) through the atmosphere, ocean, terrestrial

and marine biosphere and lithosphere. In this report, the reference unit for

the global carbon cycle is GtC or equivalently PgC (10

g).

Carbon dioxide (CO

) A naturally occurring gas, also a by-product of

burning fossil fuels from fossil carbon deposits, such as oil, gas and coal,

of burning biomass, of land use changes and of industrial processes (e.g.,

cement production). It is the principal anthropogenic greenhouse gas that

affects the Earth’s radiative balance. It is the reference gas against which

other greenhouse gases are measured and therefore has a Global Warming

Potential of 1.

Carbon dioxide (CO

) fertilization The enhancement of the growth

of plants as a result of increased atmospheric carbon dioxide (CO

) con-

centration.

Carbon Dioxide Removal (CDR) Carbon Dioxide Removal meth-

ods refer to a set of techniques that aim to remove CO

directly from the

atmosphere by either (1) increasing natural sinks for carbon or (2) using

chemical engineering to remove the CO

, with the intent of reducing the

atmospheric CO

concentration. CDR methods involve the ocean, land and

technical systems, including such methods as iron fertilization, large-scale

afforestation and direct capture of CO

from the atmosphere using engi-

neered chemical means. Some CDR methods fall under the category of

geoengineering, though this may not be the case for others, with the dis-

tinction being based on the magnitude, scale, and impact of the particular

CDR activities. The boundary between CDR and mitigation is not clear and

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Annex III Glossary

AIII

there could be some overlap between the two given current deﬁnitions

(IPCC, 2012, p. 2). See also Solar Radiation Management (SRM).

CFC See Halocarbons.

Chaotic A dynamical system such as the climate system, governed by

nonlinear deterministic equations (see Nonlinearity), may exhibit erratic or

chaotic behaviour in the sense that very small changes in the initial state

of the system in time lead to large and apparently unpredictable changes

in its temporal evolution. Such chaotic behaviour limits the predictabil-

ity of the state of a nonlinear dynamical system at speciﬁc future times,

although changes in its statistics may still be predictable given changes in

the system parameters or boundary conditions.

Charcoal Material resulting from charring of biomass, usually retain-

ing some of the microscopic texture typical of plant tissues; chemically it

consists mainly of carbon with a disturbed graphitic structure, with lesser

amounts of oxygen and hydrogen.

Chronology Arrangement of events according to dates or times of

occurrence.

Clathrate (methane) A partly frozen slushy mix of methane gas and

ice, usually found in sediments.

Clausius–Clapeyron equation/relationship The thermodynamic

relationship between small changes in temperature and vapour pressure

in an equilibrium system with condensed phases present. For trace gases

such as water vapour, this relation gives the increase in equilibrium (or

saturation) water vapour pressure per unit change in air temperature.

Climate Climate in a narrow sense is usually deﬁned as the average

weather, or more rigorously, as the statistical description in terms of the

mean and variability of relevant quantities over a period of time rang-

ing from months to thousands or millions of years. The classical period for

averaging these variables is 30 years, as deﬁned by the World Meteorologi-

cal Organization. The relevant quantities are most often surface variables

such as temperature, precipitation and wind. Climate in a wider sense is

the state, including a statistical description, of the climate system.

Climate–carbon cycle feedback A climate feedback involving

changes in the properties of land and ocean carbon cycle in response to cli-

mate change. In the ocean, changes in oceanic temperature and circulation

could affect the atmosphere–ocean CO

ﬂux; on the continents, climate

change could affect plant photosynthesis and soil microbial respiration

and hence the ﬂux of CO

between the atmosphere and the land biosphere.

Climate change Climate change refers to a change in the state of the

climate that can be identiﬁed (e.g., by using statistical tests) by changes

in the mean and/or the variability of its properties, and that persists for an

extended period, typically decades or longer. Climate change may be due

to natural internal processes or external forcings such as modulations of

the solar cycles, volcanic eruptions and persistent anthropogenic changes

in the composition of the atmosphere or in land use. Note that the Frame-

work Convention on Climate Change (UNFCCC), in its Article 1, deﬁnes

climate change as: ‘a change of climate which is attributed directly or indi-

rectly to human activity that alters the composition of the global atmo-

sphere and which is in addition to natural climate variability observed over

comparable time periods’. The UNFCCC thus makes a distinction between

climate change attributable to human activities altering the atmospheric

composition, and climate variability attributable to natural causes. See also

Climate change commitment, Detection and Attribution.

Climate change commitment Due to the thermal inertia of the

ocean and slow processes in the cryosphere and land surfaces, the climate

would continue to change even if the atmospheric composition were held

ﬁxed at today’s values. Past change in atmospheric composition leads to

a committed climate change, which continues for as long as a radiative

imbalance persists and until all components of the climate system have

adjusted to a new state. The further change in temperature after the com-

position of the atmosphere is held constant is referred to as the constant

composition temperature commitment or simply committed warming or

warming commitment. Climate change commitment includes other future

changes, for example, in the hydrological cycle, in extreme weather events,

in extreme climate events, and in sea level change. The constant emission

commitment is the committed climate change that would result from keep-

ing anthropogenic emissions constant and the zero emission commitment

is the climate change commitment when emissions are set to zero. See also

Climate change.

Climate feedback An interaction in which a perturbation in one

climate quantity causes a change in a second, and the change in the

second quantity ultimately leads to an additional change in the ﬁrst. A

negative feedback is one in which the initial perturbation is weakened

by the changes it causes; a positive feedback is one in which the initial

perturbation is enhanced. In this Assessment Report, a somewhat narrower

deﬁnition is often used in which the climate quantity that is perturbed is

the global mean surface temperature, which in turn causes changes in the

global radiation budget. In either case, the initial perturbation can either

be externally forced or arise as part of internal variability. See also Climate

Feedback Parameter.

Climate Feedback Parameter A way to quantify the radiative

response of the climate system to a global mean surface temperature

change induced by a radiative forcing. It varies as the inverse of the effec-

tive climate sensitivity. Formally, the Climate Feedback Parameter (a; units:

W m

–2

°C

–1

) is deﬁned as: a = (ΔQ – ΔF)/ΔT, where Q is the global mean

radiative forcing, T is the global mean air surface temperature, F is the

heat ﬂux into the ocean and Δ represents a change with respect to an

unperturbed climate.

Climate forecast See Climate prediction.

Climate index A time series constructed from climate variables that

provides an aggregate summary of the state of the climate system. For

example, the difference between sea level pressure in Iceland and the

Azores provides a simple yet useful historical NAO index. Because of their

optimal properties, climate indices are often deﬁned using principal com-

ponents—linear combinations of climate variables at different locations

that have maximum variance subject to certain normalisation constraints

(e.g., the NAM and SAM indices which are principal components of North-

ern Hemisphere and Southern Hemisphere gridded pressure anomalies,

respectively). See Box 2.5 for a summary of deﬁnitions for established

observational indices. See also Climate pattern.

Climate model (spectrum or hierarchy) A numerical representa-

tion of the climate system based on the physical, chemical and biological

properties of its components, their interactions and feedback processes,

and accounting for some of its known properties. The climate system can

be represented by models of varying complexity, that is, for any one com-

ponent or combination of components a spectrum or hierarchy of models

can be identiﬁed, differing in such aspects as the number of spatial dimen-

sions, the extent to which physical, chemical or biological processes are

explicitly represented or the level at which empirical parametrizations

are involved. Coupled Atmosphere–Ocean General Circulation Models

(AOGCMs) provide a representation of the climate system that is near or

at the most comprehensive end of the spectrum currently available. There

is an evolution towards more complex models with interactive chemistry

and biology. Climate models are applied as a research tool to study and

simulate the climate, and for operational purposes, including monthly, sea-

sonal and interannual climate predictions. See also Earth System Model,

Earth-System Model of Intermediate Complexity, Energy Balance Model,

Process-based Model, Regional Climate Model and Semi-empirical model.

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AIII

Climate pattern A set of spatially varying coefﬁcients obtained by

“projection” (regression) of climate variables onto a climate index time

series. When the climate index is a principal component, the climate pat-

tern is an eigenvector of the covariance matrix, referred to as an Empirical

Orthogonal Function (EOF) in climate science.

Climate prediction A climate prediction or climate forecast is the

result of an attempt to produce (starting from a particular state of the

climate system) an estimate of the actual evolution of the climate in

the future, for example, at seasonal, interannual or decadal time scales.

Because the future evolution of the climate system may be highly sensitive

to initial conditions, such predictions are usually probabilistic in nature. See

also Climate projection, Climate scenario, Model initialization and Predict-

ability.

Climate projection A climate projection is the simulated response of

the climate system to a scenario of future emission or concentration of

greenhouse gases and aerosols, generally derived using climate models.

Climate projections are distinguished from climate predictions by their

dependence on the emission/concentration/radiative forcing scenario

used, which is in turn based on assumptions concerning, for example,

future socioeconomic and technological developments that may or may

not be realized. See also Climate scenario.

Climate regime A state of the climate system that occurs more fre-

quently than nearby states due to either more persistence or more frequent

recurrence. In other words, a cluster in climate state space associated with

a local maximum in the probability density function.

Climate response See Climate sensitivity.

Climate scenario A plausible and often simpliﬁed representation of

the future climate, based on an internally consistent set of climatological

relationships that has been constructed for explicit use in investigating

the potential consequences of anthropogenic climate change, often serv-

ing as input to impact models. Climate projections often serve as the raw

material for constructing climate scenarios, but climate scenarios usually

require additional information such as the observed current climate. A cli-

mate change scenario is the difference between a climate scenario and the

current climate. See also Emission scenario, scenario.

Climate sensitivity In IPCC reports, equilibrium climate sensitivity

(units: °C) refers to the equilibrium (steady state) change in the annual

global mean surface temperature following a doubling of the atmospheric

equivalent carbon dioxide concentration. Owing to computational con-

straints, the equilibrium climate sensitivity in a climate model is sometimes

estimated by running an atmospheric general circulation model coupled

to a mixed-layer ocean model, because equilibrium climate sensitivity is

largely determined by atmospheric processes. Efﬁcient models can be run

to equilibrium with a dynamic ocean. The climate sensitivity parameter

(units: °C (W m

–2

)

–1

) refers to the equilibrium change in the annual global

mean surface temperature following a unit change in radiative forcing.

The effective climate sensitivity (units: °C) is an estimate of the global

mean surface temperature response to doubled carbon dioxide concen-

tration that is evaluated from model output or observations for evolv-

ing non-equilibrium conditions. It is a measure of the strengths of the

climate feedbacks at a particular time and may vary with forcing history

and climate state, and therefore may differ from equilibrium climate

sensitivity.

The transient climate response (units: °C) is the change in the global

mean surface temperature, averaged over a 20-year period, centred at

the time of atmospheric carbon dioxide doubling, in a climate model

simulation in which CO

increases at 1% yr

–1

. It is a measure of the

strength and rapidity of the surface temperature response to green-

house gas forcing.

Climate sensitivity parameter See climate sensitivity.

Climate system The climate system is the highly complex system

consisting of ﬁve major components: the atmosphere, the hydrosphere,

the cryosphere, the lithosphere and the biosphere, and the interactions

between them. The climate system evolves in time under the inﬂuence of

its own internal dynamics and because of external forcings such as vol-

canic eruptions, solar variations and anthropogenic forcings such as the

changing composition of the atmosphere and land use change.

Climate variability Climate variability refers to variations in the mean

state and other statistics (such as standard deviations, the occurrence of

extremes, etc.) of the climate on all spatial and temporal scales beyond

that of individual weather events. Variability may be due to natural internal

processes within the climate system (internal variability), or to variations

in natural or anthropogenic external forcing (external variability). See also

Climate change.

Cloud condensation nuclei (CCN) The subset of aerosol particles

that serve as an initial site for the condensation of liquid water, which can

lead to the formation of cloud droplets, under typical cloud formation con-

ditions. The main factor that determines which aerosol particles are CCN at

a given supersaturation is their size.

Cloud feedback A climate feedback involving changes in any of the

properties of clouds as a response to a change in the local or global mean

surface temperature. Understanding cloud feedbacks and determining

their magnitude and sign require an understanding of how a change in cli-

mate may affect the spectrum of cloud types, the cloud fraction and height,

the radiative properties of clouds, and ﬁnally the Earth’s radiation budget.

At present, cloud feedbacks remain the largest source of uncertainty in

climate sensitivity estimates. See also Cloud radiative effect.

Cloud radiative effect The radiative effect of clouds relative to the

identical situation without clouds. In previous IPCC reports this was called

cloud radiative forcing, but that terminology is inconsistent with other uses

of the forcing term and is not maintained in this report. See also Cloud

feedback.

-equivalent See Equivalent carbon dioxide.

Cold days/cold nights Days where maximum temperature, or nights

where minimum temperature, falls below the 10th percentile, where the

respective temperature distributions are generally deﬁned with respect to

the 1961–1990 reference period. For the corresponding indices, see Box

2.4.

Compatible emissions Earth System Models that simulate the land

and ocean carbon cycle can calculate CO

emissions that are compatible

with a given atmospheric CO

concentration trajectory. The compatible

emissions over a given period of time are equal to the increase of carbon

over that same period of time in the sum of the three active reservoirs: the

atmosphere, the land and the ocean.

Conﬁdence The validity of a ﬁnding based on the type, amount, quality,

and consistency of evidence (e.g., mechanistic understanding, theory, data,

models, expert judgment) and on the degree of agreement. Conﬁdence is

expressed qualitatively (Mastrandrea et al., 2010). See Figure 1.11 for the

levels of conﬁdence and Table 1.1 for the list of likelihood qualiﬁers. See

also Uncertainty.

Convection Vertical motion driven by buoyancy forces arising from

static instability, usually caused by near-surface cooling or increases in

salinity in the case of the ocean and near-surface warming or cloud-top

radiative cooling in the case of the atmosphere. In the atmosphere con-

vection gives rise to cumulus clouds and precipitation and is effective at

both scavenging and vertically transporting chemical species. In the ocean

convection can carry surface waters to deep within the ocean.

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Annex III Glossary

AIII

Cosmogenic radioisotopes Rare radioactive isotopes that are cre-

ated by the interaction of a high-energy cosmic ray particles with atoms

nuclei. They are often used as indicator of solar activity which modulates

the cosmic rays intensity or as tracers of atmospheric transport processes,

and are also called cosmogenic radionuclides.

Cryosphere All regions on and beneath the surface of the Earth and

ocean where water is in solid form, including sea ice, lake ice, river ice,

snow cover, glaciers and ice sheets, and frozen ground (which includes

permafrost).

Dansgaard–Oeschger events Abrupt events characterized in Green-

land ice cores and in palaeoclimate records from the nearby North Atlantic

by a cold glacial state, followed by a rapid transition to a warmer phase,

and a slow cooling back to glacial conditions. Counterparts of Dansgaard–

Oeschger events are observed in other regions as well.

Deforestation Conversion of forest to non-forest. For a discussion of

the term forest and related terms such as afforestation, reforestation, and

deforestation see the IPCC Special Report on Land Use, Land-Use Change

and Forestry (IPCC, 2000). See also the report on Deﬁnitions and Meth-

odological Options to Inventory Emissions from Direct Human-induced

Degradation of Forests and Devegetation of Other Vegetation Types (IPCC,

2003).

Deglaciation/glacial termination Transitions from full glacial con-

ditions (ice age) to warm interglacials characterized by global warming

and sea level rise due to change in continental ice volume.

Detection and attribution Detection of change is deﬁned as the

process of demonstrating that climate or a system affected by climate has

changed in some deﬁned statistical sense, without providing a reason for

that change. An identiﬁed change is detected in observations if its likeli-

hood of occurrence by chance due to internal variability alone is deter-

mined to be small, for example, <10%. Attribution is deﬁned as the pro-

cess of evaluating the relative contributions of multiple causal factors to

a change or event with an assignment of statistical conﬁdence (Hegerl et

al., 2010).

Diatoms Silt-sized algae that live in surface waters of lakes, rivers and

oceans and form shells of opal. Their species distribution in ocean cores is

often related to past sea surface temperatures.

Direct (aerosol) effect See Aerosol–radiation interaction.

Direct Air Capture Chemical process by which a pure CO

stream is

produced by capturing CO

from the ambient air.

Diurnal temperature range The difference between the maximum

and minimum temperature during a 24-hour period.

Dobson Unit (DU) A unit to measure the total amount of ozone in a

vertical column above the Earth’s surface (total column ozone). The number

of Dobson Units is the thickness in units of 10

–5

m that the ozone column

would occupy if compressed into a layer of uniform density at a pressure

of 1013 hPa and a temperature of 0°C. One DU corresponds to a column of

ozone containing 2.69 × 10

molecules per square metre. A typical value

for the amount of ozone in a column of the Earth’s atmosphere, although

very variable, is 300 DU.

Downscaling Downscaling is a method that derives local- to regional-

scale (10 to 100 km) information from larger-scale models or data analyses.

Two main methods exist: dynamical downscaling and empirical/statistical

downscaling. The dynamical method uses the output of regional climate

models, global models with variable spatial resolution or high-resolution

global models. The empirical/statistical methods develop statistical rela-

tionships that link the large-scale atmospheric variables with local/ regional

climate variables. In all cases, the quality of the driving model remains an

important limitation on the quality of the downscaled information.

Drought A period of abnormally dry weather long enough to cause a

serious hydrological imbalance. Drought is a relative term; therefore any

discussion in terms of precipitation deﬁcit must refer to the particular

precipitation-related activity that is under discussion. For example, short-

age of precipitation during the growing season impinges on crop produc-

tion or ecosystem function in general (due to soil moisture drought, also

termed agricultural drought), and during the runoff and percolation season

primarily affects water supplies (hydrological drought). Storage changes

in soil moisture and groundwater are also affected by increases in actual

evapotranspiration in addition to reductions in precipitation. A period with

an abnormal precipitation deﬁcit is deﬁned as a meteorological drought. A

megadrought is a very lengthy and pervasive drought, lasting much longer

than normal, usually a decade or more. For the corresponding indices, see

Box 2.4.

Dynamical system A process or set of processes whose evolution in

time is governed by a set of deterministic physical laws. The climate system

is a dynamical system. See also Abrupt climate change, Chaotic, Nonlinear-

ity and Predictability.

Earth System Model (ESM) A coupled atmosphere–ocean general

circulation model in which a representation of the carbon cycle is includ-

ed, allowing for interactive calculation of atmospheric CO

or compatible

emissions. Additional components (e.g., atmospheric chemistry, ice sheets,

dynamic vegetation, nitrogen cycle, but also urban or crop models) may be

included. See also Climate model.

Earth System Model of Intermediate Complexity (EMIC) A cli-

mate model attempting to include all the most important earth system

processes as in ESMs but at a lower resolution or in a simpler, more ideal-

ized fashion.

Earth System sensitivity The equilibrium temperature response of

the coupled atmosphere–ocean–cryosphere–vegetation–carbon cycle

system to a doubling of the atmospheric CO

concentration is referred to

as Earth System sensitivity. Because it allows slow components (e.g., ice

sheets, vegetation) of the climate system to adjust to the external pertur-

bation, it may differ substantially from the climate sensitivity derived from

coupled atmosphere–ocean models.

Ecosystem An ecosystem is a functional unit consisting of living

organisms, their non-living environment, and the interactions within and

between them. The components included in a given ecosystem and its spa-

tial boundaries depend on the purpose for which the ecosystem is deﬁned:

in some cases they are relatively sharp, while in others they are diffuse.

Ecosystem boundaries can change over time. Ecosystems are nested within

other ecosystems, and their scale can range from very small to the entire

biosphere. In the current era, most ecosystems either contain people as

key organisms, or are inﬂuenced by the effects of human activities in their

environment.

Effective climate sensitivity See Climate sensitivity.

Effective radiative forcing See Radiative forcing.

Efﬁcacy A measure of how effective a radiative forcing from a given

anthropogenic or natural mechanism is at changing the equilibrium global

mean surface temperature compared to an equivalent radiative forc-

ing from carbon dioxide. A carbon dioxide increase by deﬁnition has an

efﬁcacy of 1.0. Variations in climate efﬁcacy may result from rapid adjust-

ments to the applied forcing, which differ with different forcings.

Ekman pumping Frictional stress at the surface between two ﬂuids

(atmosphere and ocean) or between a ﬂuid and the adjacent solid sur-

face (the Earth’s surface) forces a circulation. When the resulting mass

1453

Glossary Annex III

AIII

transport is converging, mass conservation requires a vertical ﬂow away

from the surface. This is called Ekman pumping. The opposite effect, in case

of divergence, is called Ekman suction. The effect is important in both the

atmosphere and the ocean.

Ekman transport The total transport resulting from a balance between

the Coriolis force and the frictional stress due to the action of the wind on

the ocean surface. See also Ekman pumping.

Electromagnetic spectrum Wavelength or energy range of all elec-

tromagnetic radiation. In terms of solar radiation, the spectral irradiance is

the power arriving at the Earth per unit area, per unit wavelength.

El Niño-Southern Oscillation (ENSO) The term El Niño was initially

used to describe a warm-water current that periodically ﬂows along the

coast of Ecuador and Peru, disrupting the local ﬁshery. It has since become

identiﬁed with a basin-wide warming of the tropical Paciﬁc Ocean east of

the dateline. This oceanic event is associated with a ﬂuctuation of a global-

scale tropical and subtropical surface pressure pattern called the Southern

Oscillation. This coupled atmosphere–ocean phenomenon, with preferred

time scales of two to about seven years, is known as the El Niño-Southern

Oscillation (ENSO). It is often measured by the surface pressure anomaly

difference between Tahiti and Darwin or the sea surface temperatures in

the central and eastern equatorial Paciﬁc. During an ENSO event, the pre-

vailing trade winds weaken, reducing upwelling and altering ocean cur-

rents such that the sea surface temperatures warm, further weakening the

trade winds. This event has a great impact on the wind, sea surface tem-

perature and precipitation patterns in the tropical Paciﬁc. It has climatic

effects throughout the Paciﬁc region and in many other parts of the world,

through global teleconnections. The cold phase of ENSO is called La Niña.

For the corresponding indices, see Box 2.5.

Emission scenario A plausible representation of the future develop-

ment of emissions of substances that are potentially radiatively active

(e.g., greenhouse gases, aerosols) based on a coherent and internally con-

sistent set of assumptions about driving forces (such as demographic and

socioeconomic development, technological change) and their key relation-

ships. Concentration scenarios, derived from emission scenarios, are used

as input to a climate model to compute climate projections. In IPCC (1992)

a set of emission scenarios was presented which were used as a basis

for the climate projections in IPCC (1996). These emission scenarios are

referred to as the IS92 scenarios. In the IPCC Special Report on Emission

Scenarios (Nakićenović and Swart, 2000) emission scenarios, the so-called

SRES scenarios, were published, some of which were used, among others,

as a basis for the climate projections presented in Chapters 9 to 11 of IPCC

(2001) and Chapters 10 and 11 of IPCC (2007). New emission scenarios

for climate change, the four Representative Concentration Pathways, were

developed for, but independently of, the present IPCC assessment. See also

Climate scenario and Scenario.

Energy balance The difference between the total incoming and total

outgoing energy. If this balance is positive, warming occurs; if it is nega-

tive, cooling occurs. Averaged over the globe and over long time periods,

this balance must be zero. Because the climate system derives virtually all

its energy from the Sun, zero balance implies that, globally, the absorbed

solar radiation, that is, incoming solar radiation minus reﬂected solar radi-

ation at the top of the atmosphere and outgoing longwave radiation emit-

ted by the climate system are equal. See also Energy budget.

Energy Balance Model (EBM) An energy balance model is a sim-

pliﬁed model that analyses the energy budget of the Earth to compute

changes in the climate. In its simplest form, there is no explicit spatial

dimension and the model then provides an estimate of the changes in

globally averaged temperature computed from the changes in radiation.

This zero-dimensional energy balance model can be extended to a one-

dimensional or two-dimensional model if changes to the energy budget

with respect to latitude, or both latitude and longitude, are explicitly con-

sidered. See also Climate model.

Energy budget (of the Earth) The Earth is a physical system with

an energy budget that includes all gains of incoming energy and all losses

of outgoing energy. The Earth’s energy budget is determined by measur-

ing how much energy comes into the Earth system from the Sun, how

much energy is lost to space, and accounting for the remainder on Earth

and its atmosphere. Solar radiation is the dominant source of energy into

the Earth system. Incoming solar energy may be scattered and reﬂected

by clouds and aerosols or absorbed in the atmosphere. The transmitted

radiation is then either absorbed or reﬂected at the Earth’s surface. The

average albedo of the Earth is about 0.3, which means that 30% of the

incident solar energy is reﬂected into space, while 70% is absorbed by

the Earth. Radiant solar or shortwave energy is transformed into sensible

heat, latent energy (involving different water states), potential energy, and

kinetic energy before being emitted as infrared radiation. With the average

surface temperature of the Earth of about 15°C (288 K), the main outgoing

energy ﬂux is in the infrared part of the spectrum. See also Energy balance,

Latent heat ﬂux, Sensible heat ﬂux.

Ensemble A collection of model simulations characterizing a climate

prediction or projection. Differences in initial conditions and model formu-

lation result in different evolutions of the modelled system and may give

information on uncertainty associated with model error and error in initial

conditions in the case of climate forecasts and on uncertainty associated

with model error and with internally generated climate variability in the

case of climate projections.

Equilibrium and transient climate experiment An equilibrium

climate experiment is a climate model experiment in which the model is

allowed to fully adjust to a change in radiative forcing. Such experiments

provide information on the difference between the initial and ﬁnal states

of the model, but not on the time-dependent response. If the forcing is

allowed to evolve gradually according to a prescribed emission scenario,

the time-dependent response of a climate model may be analysed. Such

an experiment is called a transient climate experiment. See also Climate

projection.

Equilibrium climate sensitivity See Climate sensitivity.

Equilibrium line The spatially averaged boundary at a given moment,

usually chosen as the seasonal mass budget minimum at the end of

summer, between the region on a glacier where there is a net annual loss

of ice mass (ablation area) and that where there is a net annual gain (accu-

mulation area). The altitude of this boundary is referred to as equilibrium

line altitude (ELA).

Equivalent carbon dioxide (CO

) concentration The concentra-

tion of carbon dioxide that would cause the same radiative forcing as

a given mixture of carbon dioxide and other forcing components. Those

values may consider only greenhouse gases, or a combination of green-

house gases and aerosols. Equivalent carbon dioxide concentration is a

metric for comparing radiative forcing of a mix of different greenhouse

gases at a particular time but does not imply equivalence of the corre-

sponding climate change responses nor future forcing. There is generally

no connection between equivalent carbon dioxide emissions and resulting

equivalent carbon dioxide concentrations.

Equivalent carbon dioxide (CO

) emission The amount of carbon

dioxide emission that would cause the same integrated radiative forcing,

over a given time horizon, as an emitted amount of a greenhouse gas or

a mixture of greenhouse gases. The equivalent carbon dioxide emission is

obtained by multiplying the emission of a greenhouse gas by its Global

Warming Potential for the given time horizon. For a mix of greenhouse

1454

Annex III Glossary

AIII

gases it is obtained by summing the equivalent carbon dioxide emissions

of each gas. Equivalent carbon dioxide emission is a common scale for

comparing emissions of different greenhouse gases but does not imply

equivalence of the corresponding climate change responses. See also

Equivalent carbon dioxide concentration.

Evapotranspiration The combined process of evaporation from the

Earth’s surface and transpiration from vegetation.

Extended Concentration Pathways See Representative Concentra-

tion Pathways.

External forcing External forcing refers to a forcing agent outside the

climate system causing a change in the climate system. Volcanic eruptions,

solar variations and anthropogenic changes in the composition of the

atmosphere and land use change are external forcings. Orbital forcing is

also an external forcing as the insolation changes with orbital parameters

eccentricity, tilt and precession of the equinox.

Extratropical cyclone A large-scale (of order 1000 km) storm in

the middle or high latitudes having low central pressure and fronts with

strong horizontal gradients in temperature and humidity. A major cause

of extreme wind speeds and heavy precipitation especially in wintertime.

Extreme climate event See Extreme weather event.

Extreme sea level See Storm surge.

Extreme weather event An extreme weather event is an event that

is rare at a particular place and time of year. Deﬁnitions of rare vary, but

an extreme weather event would normally be as rare as or rarer than the

10th or 90th percentile of a probability density function estimated from

observations. By deﬁnition, the characteristics of what is called extreme

weather may vary from place to place in an absolute sense. When a pat-

tern of extreme weather persists for some time, such as a season, it may

be classed as an extreme climate event, especially if it yields an average or

total that is itself extreme (e.g., drought or heavy rainfall over a season).

Faculae Bright patches on the Sun. The area covered by faculae is great-

er during periods of high solar activity.

Feedback See Climate feedback.

Fingerprint The climate response pattern in space and/or time to a spe-

ciﬁc forcing is commonly referred to as a ﬁngerprint. The spatial patterns of

sea level response to melting of glaciers or ice sheets (or other changes in

surface loading) are also referred to as ﬁngerprints. Fingerprints are used

to detect the presence of this response in observations and are typically

estimated using forced climate model simulations.

Flux adjustment To avoid the problem of coupled Atmosphere–Ocean

General Circulation Models (AOGCMs) drifting into some unrealistic cli-

mate state, adjustment terms can be applied to the atmosphere-ocean

ﬂuxes of heat and moisture (and sometimes the surface stresses resulting

from the effect of the wind on the ocean surface) before these ﬂuxes are

imposed on the model ocean and atmosphere. Because these adjustments

are pre-computed and therefore independent of the coupled model inte-

gration, they are uncorrelated with the anomalies that develop during the

integration.

Forest A vegetation type dominated by trees. Many deﬁnitions of the

term forest are in use throughout the world, reﬂecting wide differences in

biogeophysical conditions, social structure and economics. For a discussion

of the term forest and related terms such as afforestation, reforestation and

deforestation see the IPCC Report on Land Use, Land-Use Change and For-

estry (IPCC, 2000). See also the Report on Deﬁnitions and Methodological

Options to Inventory Emissions from Direct Human-induced Degradation of

Forests and Devegetation of Other Vegetation Types (IPCC, 2003).

Fossil fuel emissions Emissions of greenhouse gases (in particular

carbon dioxide), other trace gases and aerosols resulting from the combus-

tion of fuels from fossil carbon deposits such as oil, gas and coal.

Framework Convention on Climate Change See United Nations

Framework Convention on Climate Change (UNFCCC).

Free atmosphere The atmospheric layer that is negligibly affected by

friction against the Earth’s surface, and which is above the atmospheric

boundary layer.

Frozen ground Soil or rock in which part or all of the pore water is

frozen. Frozen ground includes permafrost. Ground that freezes and thaws

annually is called seasonally frozen ground.

General circulation The large-scale motions of the atmosphere and

the ocean as a consequence of differential heating on a rotating Earth.

General circulation contributes to the energy balance of the system

through transport of heat and momentum.

General Circulation Model (GCM) See Climate model.

Geoengineering Geoengineering refers to a broad set of methods and

technologies that aim to deliberately alter the climate system in order to

alleviate the impacts of climate change. Most, but not all, methods seek

to either (1) reduce the amount of absorbed solar energy in the climate

system (Solar Radiation Management) or (2) increase net carbon sinks

from the atmosphere at a scale sufﬁciently large to alter climate (Carbon

Dioxide Removal). Scale and intent are of central importance. Two key

characteristics of geoengineering methods of particular concern are that

they use or affect the climate system (e.g., atmosphere, land or ocean)

globally or regionally and/or could have substantive unintended effects

that cross national boundaries. Geoengineering is different from weather

modiﬁcation and ecological engineering, but the boundary can be fuzzy

(IPCC, 2012, p. 2).

Geoid The equipotential surface having the same geopotential at each

latitude and longitude around the world (geodesists denoting this poten-

tial W0) that best approximates the mean sea level. It is the surface of

reference for measurement of altitude. In practice, several variations of

deﬁnitions of the geoid exist depending on the way the permanent tide

(the zero-frequency gravitational tide due to the Sun and Moon) is consid-

ered in geodetic studies.

Geostrophic winds or currents A wind or current that is in balance

with the horizontal pressure gradient and the Coriolis force, and thus is out-

side of the inﬂuence of friction. Thus, the wind or current is directly parallel

to isobars and its speed is proportional to the horizontal pressure gradient.

Glacial–interglacial cycles Phase of the Earth’s history marked by

large changes in continental ice volume and global sea level. See also Ice

age and Interglacials.

Glacial isostatic adjustment (GIA) The deformation of the Earth

and its gravity ﬁeld due to the response of the earth–ocean system to

changes in ice and associated water loads. It is sometimes referred to as

glacio-hydro isostasy. It includes vertical and horizontal deformations of

the Earth’s surface and changes in geoid due to the redistribution of mass

during the ice–ocean mass exchange.

Glacier A perennial mass of land ice that originates from compressed

snow, shows evidence of past or present ﬂow (through internal deforma-

tion and/or sliding at the base) and is constrained by internal stress and

friction at the base and sides. A glacier is maintained by accumulation of

snow at high altitudes, balanced by melting at low altitudes and/or dis-

charge into the sea. An ice mass of the same origin as glaciers, but of

continental size, is called an ice sheet. For the purpose of simplicity in this

Assessment Report, all ice masses other than ice sheets are referred to as

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Glossary Annex III

AIII

glaciers. See also Equilibrium line and Mass balance/budget (of glaciers or

ice sheets).

Global dimming Global dimming refers to a widespread reduction of

solar radiation received at the surface of the Earth from about the year

1961 to around 1990.

Global mean surface temperature An estimate of the global mean

surface air temperature. However, for changes over time, only anomalies,

as departures from a climatology, are used, most commonly based on the

area-weighted global average of the sea surface temperature anomaly and

land surface air temperature anomaly.

Global Warming Potential (GWP) An index, based on radiative

properties of greenhouse gases, measuring the radiative forcing following

a pulse emission of a unit mass of a given greenhouse gas in the present-

day atmosphere integrated over a chosen time horizon, relative to that of

carbon dioxide. The GWP represents the combined effect of the differing

times these gases remain in the atmosphere and their relative effective-

ness in causing radiative forcing. The Kyoto Protocol is based on GWPs

from pulse emissions over a 100-year time frame.

Greenhouse effect The infrared radiative effect of all infrared-absorb-

ing constituents in the atmosphere. Greenhouse gases, clouds, and (to a

small extent) aerosols absorb terrestrial radiation emitted by the Earth’s

surface and elsewhere in the atmosphere. These substances emit infra-

red radiation in all directions, but, everything else being equal, the net

amount emitted to space is normally less than would have been emitted

in the absence of these absorbers because of the decline of temperature

with altitude in the troposphere and the consequent weakening of emis-

sion. An increase in the concentration of greenhouse gases increases the

magnitude of this effect; the difference is sometimes called the enhanced

greenhouse effect. The change in a greenhouse gas concentration because

of anthropogenic emissions contributes to an instantaneous radiative forc-

ing. Surface temperature and troposphere warm in response to this forcing,

gradually restoring the radiative balance at the top of the atmosphere.

Greenhouse gas (GHG) Greenhouse gases are those gaseous con-

stituents of the atmosphere, both natural and anthropogenic, that absorb

and emit radiation at speciﬁc wavelengths within the spectrum of terres-

trial radiation emitted by the Earth’s surface, the atmosphere itself, and by

clouds. This property causes the greenhouse effect. Water vapour (H

O),

carbon dioxide (CO

), nitrous oxide (N

O), methane (CH

) and ozone (O

)

are the primary greenhouse gases in the Earth’s atmosphere. Moreover,

there are a number of entirely human-made greenhouse gases in the

atmosphere, such as the halocarbons and other chlorine- and bromine-

containing substances, dealt with under the Montreal Protocol. Beside CO

O and CH

, the Kyoto Protocol deals with the greenhouse gases sul-

phur hexaﬂuoride (SF

), hydroﬂuorocarbons (HFCs) and perﬂuorocarbons

(PFCs). For a list of well-mixed greenhouse gases, see Table 2.A.1.

Gross Primary Production (GPP) The amount of carbon ﬁxed by the

autotrophs (e.g. plants and algaes).

Grounding line The junction between a glacier or ice sheet and ice

shelf; the place where ice starts to ﬂoat. This junction normally occurs over

a ﬁnite zone, rather than at a line.

Gyre Basin-scale ocean horizontal circulation pattern with slow ﬂow

circulating around the ocean basin, closed by a strong and narrow (100 to

200 km wide) boundary current on the western side. The subtropical gyres

in each ocean are associated with high pressure in the centre of the gyres;

the subpolar gyres are associated with low pressure.

Hadley Circulation A direct, thermally driven overturning cell in the

atmosphere consisting of poleward ﬂow in the upper troposphere, subsid-

ing air into the subtropical anticyclones, return ﬂow as part of the trade

winds near the surface, and with rising air near the equator in the so-called

Inter-Tropical Convergence Zone.

Halocarbons A collective term for the group of partially halogenated

organic species, which includes the chloroﬂuorocarbons (CFCs), hydro-

chloroﬂuorocarbons (HCFCs), hydroﬂuorocarbons (HFCs), halons, methyl

chloride and methyl bromide. Many of the halocarbons have large Global

Warming Potentials. The chlorine and bromine-containing halocarbons are

also involved in the depletion of the ozone layer.

Halocline A layer in the oceanic water column in which salinity changes

rapidly with depth. Generally saltier water is denser and lies below less

salty water. In some high latitude oceans the surface waters may be colder

than the deep waters and the halocline is responsible for maintaining

water column stability and isolating the surface waters from the deep

waters. See also Thermocline.

Halosteric See Sea level change.

HCFC See Halocarbons.

Heat wave A period of abnormally and uncomfortably hot weather. See

also Warm spell.

Heterotrophic respiration The conversion of organic matter to

carbon dioxide by organisms other than autotrophs.

HFC See Halocarbons.

Hindcast or retrospective forecast A forecast made for a period in

the past using only information available before the beginning of the fore-

cast. A sequence of hindcasts can be used to calibrate the forecast system

and/or provide a measure of the average skill that the forecast system has

exhibited in the past as a guide to the skill that might be expected in the

future.

Holocene The Holocene Epoch is the latter of two epochs in the Qua-

ternary System, extending from 11.65 ka (thousand years before 1950) to

the present. It is also known as Marine Isotopic Stage (MIS) 1 or current

interglacial.

Hydroclimate Part of the climate pertaining to the hydrology of a

region.

Hydrological cycle The cycle in which water evaporates from the

oceans and the land surface, is carried over the Earth in atmospheric

circulation as water vapour, condenses to form clouds, precipitates over

ocean and land as rain or snow, which on land can be intercepted by trees

and vegetation, provides runoff on the land surface, inﬁltrates into soils,

recharges groundwater, discharges into streams and ultimately ﬂows out

into the oceans, from which it will eventually evaporate again. The vari-

ous systems involved in the hydrological cycle are usually referred to as

hydrological systems.

Hydrosphere The component of the climate system comprising liquid

surface and subterranean water, such as oceans, seas, rivers, fresh water

lakes, underground water, etc.

Hypsometry The distribution of land or ice surface as a function of

altitude.

Ice age An ice age or glacial period is characterized by a long-term

reduction in the temperature of the Earth’s climate, resulting in growth of

ice sheets and glaciers.

Ice–albedo feedback A climate feedback involving changes in the

Earth’s surface albedo. Snow and ice have an albedo much higher (up to

~0.8) than the average planetary albedo (~0.3). With increasing tempera-

tures, it is anticipated that snow and ice extent will decrease, the Earth’s

overall albedo will decrease and more solar radiation will be absorbed,

warming the Earth further.

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Annex III Glossary

AIII

Ice core A cylinder of ice drilled out of a glacier or ice sheet.

Ice sheet A mass of land ice of continental size that is sufﬁciently thick

to cover most of the underlying bed, so that its shape is mainly determined

by its dynamics (the ﬂow of the ice as it deforms internally and/or slides at

its base). An ice sheet ﬂows outward from a high central ice plateau with

a small average surface slope. The margins usually slope more steeply, and

most ice is discharged through fast ﬂowing ice streams or outlet glaciers,

in some cases into the sea or into ice shelves ﬂoating on the sea. There are

only two ice sheets in the modern world, one on Greenland and one on

Antarctica. During glacial periods there were others.

Ice shelf A ﬂoating slab of ice of considerable thickness extending from

the coast (usually of great horizontal extent with a very gently sloping

surface), often ﬁlling embayments in the coastline of an ice sheet. Nearly

all ice shelves are in Antarctica, where most of the ice discharged into the

ocean ﬂows via ice shelves.

Ice stream A stream of ice with strongly enhanced ﬂow that is part of

an ice sheet. It is often separated from surrounding ice by strongly sheared,

crevassed margins. See also Outlet glacier.

Incoming solar radiation See Insolation.

Indian Ocean Dipole (IOD) Large–scale mode of interannual variabil-

ity of sea surface temperature in the Indian Ocean. This pattern manifests

through a zonal gradient of tropical sea surface temperature, which in one

extreme phase in boreal autumn shows cooling off Sumatra and warming

off Somalia in the west, combined with anomalous easterlies along the

equator.

Indirect aerosol effect See Aerosol-cloud interaction.

Industrial Revolution A period of rapid industrial growth with far-

reaching social and economic consequences, beginning in Britain during

the second half of the 18th century and spreading to Europe and later to

other countries including the United States. The invention of the steam

engine was an important trigger of this development. The industrial revolu-

tion marks the beginning of a strong increase in the use of fossil fuels and

emission of, in particular, fossil carbon dioxide. In this report the terms pre-

industrial and industrial refer, somewhat arbitrarily, to the periods before

and after 1750, respectively.

Infrared radiation See Terrestrial radiation.

Insolation The amount of solar radiation reaching the Earth by lati-

tude and by season measured in W m

–2

. Usually insolation refers to the

radiation arriving at the top of the atmosphere. Sometimes it is speciﬁed

as referring to the radiation arriving at the Earth’s surface. See also Total

Solar Irradiance.

Interglacials or interglaciations The warm periods between ice age

glaciations. Often deﬁned as the periods at which sea levels were close

to present sea level. For the Last Interglacial (LIG) this occurred between

about 129 and 116 ka (thousand years) before present (deﬁned as 1950)

although the warm period started in some areas a few thousand years

earlier. In terms of the oxygen isotope record interglaciations are deﬁned

as the interval between the midpoint of the preceding termination and

the onset of the next glaciation. The present interglaciation, the Holocene,

started at 11.65 ka before present although globally sea levels did not

approach their present position until about 7 ka before present.

Internal variability See Climate variability.

Inter-Tropical Convergence Zone (ITCZ) The Inter-Tropical Conver-

gence Zone is an equatorial zonal belt of low pressure, strong convection

and heavy precipitation near the equator where the northeast trade winds

meet the southeast trade winds. This band moves seasonally.

Iron fertilization Deliberate introduction of iron to the upper ocean

intended to enhance biological productivity which can sequester addition-

al atmospheric carbon dioxide into the oceans.

Irreversibility A perturbed state of a dynamical system is deﬁned as

irreversible on a given timescale, if the recovery timescale from this state

due to natural processes is signiﬁcantly longer than the time it takes for the

system to reach this perturbed state. In the context of WGI, the time scale

of interest is centennial to millennial. See also Tipping point.

Isostatic or Isostasy Isostasy refers to the response of the earth to

changes in surface load. It includes the deformational and gravitational

response. This response is elastic on short time scales, as in the earth–

ocean response to recent changes in mountain glaciation, or viscoelastic

on longer time scales, as in the response to the last deglaciation following

the Last Glacial Maximum. See also Glacial Isostatic Adjustment (GIA).

Isotopes Atoms of the same chemical element that have the same the

number of protons but differ in the number of neutrons. Some proton–

neutron conﬁgurations are stable (stable isotopes), others are unstable

undergoing spontaneous radioactive decay (radioisotopes). Most elements

have more than one stable isotope. Isotopes can be used to trace transport

processes or to study processes that change the isotopic ratio. Radioiso-

topes provide in addition time information that can be used for radiometric

dating.

Kyoto Protocol The Kyoto Protocol to the United Nations Framework

Convention on Climate Change (UNFCCC) was adopted in 1997 in Kyoto,

Japan, at the Third Session of the Conference of the Parties (COP) to the

UNFCCC. It contains legally binding commitments, in addition to those

included in the UNFCCC. Countries included in Annex B of the Protocol

(most Organisation for Economic Cooperation and Development countries

and countries with economies in transition) agreed to reduce their anthro-

pogenic greenhouse gas emissions (carbon dioxide, methane, nitrous

oxide, hydroﬂuorocarbons, perﬂuorocarbons, and sulphur hexaﬂuoride) by

at least 5% below 1990 levels in the commitment period 2008–2012. The

Kyoto Protocol entered into force on 16 February 2005.

Land surface air temperature The surface air temperature as mea-

sured in well-ventilated screens over land at 1.5 m above the ground.

Land use and Land use change Land use refers to the total of

arrangements, activities and inputs undertaken in a certain land cover type

(a set of human actions). The term land use is also used in the sense of the

social and economic purposes for which land is managed (e.g., grazing,

timber extraction and conservation). Land use change refers to a change

in the use or management of land by humans, which may lead to a change

in land cover. Land cover and land use change may have an impact on

the surface albedo, evapotranspiration, sources and sinks of greenhouse

gases, or other properties of the climate system and may thus give rise to

radiative forcing and/or other impacts on climate, locally or globally. See

also the IPCC Report on Land Use, Land-Use Change, and Forestry (IPCC,

2000).

Land water storage Water stored on land other than in glaciers and

ice sheets (that is water stored in rivers, lakes, wetlands, the vadose zone,

aquifers, reservoirs, snow and permafrost). Changes in land water storage

driven by climate and human activities contribute to sea level change.

La Niña See El Niño-Southern Oscillation.

Lapse rate The rate of change of an atmospheric variable, usually tem-

perature, with height. The lapse rate is considered positive when the vari-

able decreases with height.

Last Glacial Maximum (LGM) The period during the last ice age when

the glaciers and ice sheets reached their maximum extent, approximately

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21 ka ago. This period has been widely studied because the radiative forc-

ings and boundary conditions are relatively well known.

Last Interglacial (LIG) See Interglacials.

Latent heat ﬂux The turbulent ﬂux of heat from the Earth’s surface

to the atmosphere that is associated with evaporation or condensation of

water vapour at the surface; a component of the surface energy budget.

Lifetime Lifetime is a general term used for various time scales char-

acterizing the rate of processes affecting the concentration of trace gases.

The following lifetimes may be distinguished:

Turnover time (T) (also called global atmospheric lifetime) is the

ratio of the mass M of a reservoir (e.g., a gaseous compound in the

atmosphere) and the total rate of removal S from the reservoir: T = M/S.

For each removal process, separate turnover times can be deﬁned. In

soil carbon biology, this is referred to as Mean Residence Time.

Adjustment time or response time (T

) is the time scale character-

izing the decay of an instantaneous pulse input into the reservoir. The

term adjustment time is also used to characterize the adjustment of

the mass of a reservoir following a step change in the source strength.

Half-life or decay constant is used to quantify a ﬁrst-order exponential

decay process. See Response time for a different deﬁnition pertinent to

climate variations.

The term lifetime is sometimes used, for simplicity, as a surrogate for

adjustment time.

In simple cases, where the global removal of the compound is directly

proportional to the total mass of the reservoir, the adjustment time

equals the turnover time: T = T

. An example is CFC-11, which is

removed from the atmosphere only by photochemical processes in the

stratosphere. In more complicated cases, where several reservoirs are

involved or where the removal is not proportional to the total mass,

the equality T = T

no longer holds. Carbon dioxide (CO

) is an extreme

example. Its turnover time is only about 4 years because of the rapid

exchange between the atmosphere and the ocean and terrestrial biota.

However, a large part of that CO

is returned to the atmosphere within

a few years. Thus, the adjustment time of CO

in the atmosphere is

actually determined by the rate of removal of carbon from the surface

layer of the oceans into its deeper layers. Although an approximate

value of 100 years may be given for the adjustment time of CO

in the

atmosphere, the actual adjustment is faster initially and slower later

on. In the case of methane (CH

), the adjustment time is different from

the turnover time because the removal is mainly through a chemical

reaction with the hydroxyl radical (OH), the concentration of which

itself depends on the CH

concentration. Therefore, the CH

removal

rate S is not proportional to its total mass M.

Likelihood The chance of a speciﬁc outcome occurring, where this

might be estimated probabilistically. This is expressed in this report using a

standard terminology, deﬁned in Table 1.1. See also Conﬁdence and Uncer-

tainty.

Lithosphere The upper layer of the solid Earth, both continental and

oceanic, which comprises all crustal rocks and the cold, mainly elastic

part of the uppermost mantle. Volcanic activity, although part of the litho-

sphere, is not considered as part of the climate system, but acts as an

external forcing factor. See also Isostatic.

Little Ice Age (LIA) An interval during the last millennium charac-

terized by a number of extensive expansions of mountain glaciers and

moderate retreats in between them, both in the Northern and Southern

Hemispheres. The timing of glacial advances differs between regions and

the LIA is, therefore, not clearly deﬁned in time. Most deﬁnitions lie in the

period 1400 CE and 1900 CE. Currently available reconstructions of aver-

age Northern Hemisphere temperature indicate that the coolest periods at

the hemispheric scale may have occurred from 1450 to 1850 CE.

Longwave radiation See Terrestrial radiation.

Madden–Julian Oscillation (MJO) The largest single component of

tropical atmospheric intraseasonal variability (periods from 30 to 90 days).

The MJO propagates eastwards at around 5m s

–1

in the form of a large-

scale coupling between atmospheric circulation and deep convection.As it

progresses, it is associated with large regions of both enhanced and sup-

pressed rainfall, mainly over theIndianand western Paciﬁc Oceans. Each

MJO event lasts approximately 30 to 60 days, hence the MJO is also known

as the30- to 60-day wave, or the intraseasonal oscillation.

Marine-based ice sheet An ice sheet containing a substantial region

that rests on a bed lying below sea level and whose perimeter is in contact

with the ocean. The best known example is the West Antarctic ice sheet.

Mass balance/budget (of glaciers or ice sheets) The balance

between the mass input to the ice body (accumulation) and the mass loss

(ablation and iceberg calving) over a stated period of time, which is often

a year or a season. Point mass balance refers to the mass balance at a

particular location on the glacier or ice sheet. Surface mass balance is the

difference between surface accumulation and surface ablation. The input

and output terms for mass balance are:

Accumulation All processes that add to the mass of a glacier. The

main contribution to accumulation is snowfall. Accumulation also

includes deposition of hoar, freezing rain, other types of solid precipita-

tion, gain of wind-blown snow, and avalanching.

Ablation Surface processes that reduce the mass of a glacier. The

main contributor to ablation is melting with runoff but on some gla-

ciers sublimation, loss of wind-blown snow and avalanching are also

signiﬁcant processes of ablation.

Discharge/outﬂow Mass loss by iceberg calving or ice discharge

across the grounding line of a ﬂoating ice shelf. Although often treated

as an ablation term, in this report iceberg calving and discharge is con-

sidered separately from surface ablation.

Mean sea level The surface level of the ocean at a particular point

averaged over an extended period of time such as a month or year. Mean

sea level is often used as a national datum to which heights on land are

referred.

Medieval Climate Anomaly (MCA) See Medieval Warm Period.

Medieval Warm Period (MWP) An interval of relatively warm con-

ditions and other notable climate anomalies such as more extensive

drought in some continental regions. The timing of this interval is not

clearly deﬁned, with different records showing onset and termination of

the warmth at different times, and some showing intermittent warmth.

Most deﬁnitions lie within the period 900 to 1400 CE. Currently available

reconstructions of average Northern Hemisphere temperature indicate that

the warmest period at the hemispheric scale may have occurred from 950

to 1250 CE. Currently available records and temperature reconstructions

indicate that average temperatures during parts of the MWP were indeed

warmer in the context of the last 2 kyr, though the warmth may not have

been as ubiquitous across seasons and geographical regions as the 20th

century warming. It is also called Medieval Climate Anomaly.

Meridional Overturning Circulation (MOC) Meridional (north–

south) overturning circulation in the ocean quantiﬁed by zonal (east–west)

sums of mass transports in depth or density layers. In the North Atlantic,

away from the subpolar regions, the MOC (which is in principle an observ-

able quantity) is often identiﬁed with the thermohaline circulation (THC),

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which is a conceptual and incomplete interpretation. It must be borne in

mind that the MOC is also driven by wind, and can also include shallower

overturning cells such as occur in the upper ocean in the tropics and sub-

tropics, in which warm (light) waters moving poleward are transformed to

slightly denser waters and subducted equatorward at deeper levels.

Metadata Information about meteorological and climatological data

concerning how and when they were measured, their quality, known prob-

lems and other characteristics.

Methane (CH

) Methane is one of the six greenhouse gases to be miti-

gated under the Kyoto Protocol and is the major component of natural gas

and associated with all hydrocarbon fuels, animal husbandry and agricul-

ture.

Metric A consistent measurement of a characteristic of an object or

activity that is otherwise difﬁcult to quantify. Within the context of the

evaluation of climate models, this is a quantitative measure of agreement

between a simulated and observed quantity which can be used to assess

the performance of individual models.

Microwave Sounding Unit (MSU) A microwave sounder on National

Oceanic and Atmospheric Administration (NOAA) polar orbiter satellites,

that estimates the temperature of thick layers of the atmosphere by mea-

suring the thermal emission of oxygen molecules from a complex of emis-

sion lines near 60 GHz. A series of nine MSUs began making this kind

of measurement in late 1978. Beginning in mid 1998, a follow-on series

of instruments, the Advanced Microwave Sounding Units (AMSUs), began

operation.

Mineralization/Remineralization The conversion of an element

from its organic form to an inorganic form as a result of microbial decom-

position. In nitrogen mineralization, organic nitrogen from decaying plant

and animal residues (proteins, nucleic acids, amino sugars and urea) is

converted to ammonia (NH

) and ammonium (NH

) by biological activity.

Mitigation A human intervention to reduce the sources or enhance the

sinks of greenhouse gases.

Mixing ratio See Mole fraction.

Model drift Since model climate differs to some extent from observed

climate, climate forecasts will typically ‘drift’ from the initial observation-

based state towards the model’s climate. This drift occurs at different time

scales for different variables, can obscure the initial-condition forecast

information and is usually removed a posteriori by an empirical, usually

linear, adjustment.

Model hierarchy See Climate model (spectrum or hierarchy).

Model initialization A climate forecast typically proceeds by integrat-

ing a climate model forward in time from an initial state that is intended

to reﬂect the actual state of the climate system. Available observations of

the climate system are ‘assimilated’ into the model. Initialization is a com-

plex process that is limited by available observations, observational errors

and, depending on the procedure used, may be affected by uncertainty in

the history of climate forcing. The initial conditions will contain errors that

grow as the forecast progresses, thereby limiting the time for which the

forecast will be useful. See also Climate prediction.

Model spread The range or spread in results from climate models,

such as those assembled for Coupled Model Intercomparison Project

Phase 5 (CMIP5). Does not necessarily provide an exhaustive and formal

estimate of the uncertainty in feedbacks, forcing or projections even when

expressed numerically, for example, by computing a standard deviation of

the models’ responses. In order to quantify uncertainty, information from

observations, physical constraints and expert judgement must be com-

bined, using a statistical framework.

Mode of climate variability Underlying space–time structure with

preferred spatial pattern and temporal variation that helps account for the

gross features in variance and for teleconnections. A mode of variability

is often considered to be the product of a spatial climate pattern and an

associated climate index time series.

Mole fraction Mole fraction, or mixing ratio, is the ratio of the number

of moles of a constituent in a given volume to the total number of moles

of all constituents in that volume. It is usually reported for dry air. Typical

values for well-mixed greenhouse gases are in the order of μmol mol

–1

(parts per million: ppm), nmol mol

–1

(parts per billion: ppb), and fmol mol

–1

(parts per trillion: ppt). Mole fraction differs from volume mixing ratio,

often expressed in ppmv etc., by the corrections for non-ideality of gases.

This correction is signiﬁcant relative to measurement precision for many

greenhouse gases (Schwartz and Warneck, 1995).

Monsoon A monsoon is a tropical and subtropical seasonal reversal in

both the surface winds and associated precipitation, caused by differential

heating between a continental-scale land mass and the adjacent ocean.

Monsoon rains occur mainly over land in summer.

Montreal Protocol The Montreal Protocol on Substances that Deplete

the Ozone Layer was adopted in Montreal in 1987, and subsequently

adjusted and amended in London (1990), Copenhagen (1992), Vienna

(1995), Montreal (1997) and Beijing (1999). It controls the consump-

tion and production of chlorine- and bromine-containing chemicals that

destroy stratospheric ozone, such as chloroﬂuorocarbons, methyl chloro-

form, carbon tetrachloride and many others.

Near-surface permafrost A term frequentlyused in climate model

applications to refer to permafrost at depths close to the ground surface

(typically down to 3.5 m). In modelling studies, near-surface permafrost

is usually diagnosed from 20 or 30 year climate averages, whichis dif-

ferent from the conventional deﬁnition of permafrost. Disappearance of

near-surface permafrost in a location does not preclude the longer-term

persistence of permafrost at greater depth. See also Active layer, Frozen

ground and Thermokarst.

Near-term climate forcers (NTCF) Near-term climate forcers (NTCF)

refer to those compounds whose impact on climate occurs primarily within

the ﬁrst decade after their emission. This set of compounds is primarily

composed of those with short lifetimes in the atmosphere compared to

well-mixed greenhouse gases, and has been sometimes referred to as

short lived climate forcers or short-lived climate pollutants. However, the

common property that is of greatest interest to a climate assessment is

the timescale over which their impact on climate is felt. This set of com-

pounds includes methane, which is also a well-mixed greenhouse gas, as

well as ozone and aerosols, or their precursors, and some halogenated

species that are not well-mixed greenhouse gases. These compounds do

not accumulate in the atmosphere at decadal to centennial timescales,

and so their effect on climate is predominantly in the near term following

their emission.

Nitrogen deposition Nitrogen deposition is deﬁned as the nitrogen

transferred from the atmosphere to the Earth’s surface by the processes of

wet deposition and dry deposition.

Nitrous oxide (N

O) One of the six greenhouse gases to be mitigat-

ed under the Kyoto Protocol. The main anthropogenic source of nitrous

oxide is agriculture (soil and animal manure management), but important

contributions also come from sewage treatment, combustion of fossil fuel,

and chemical industrial processes. Nitrous oxide is also produced naturally

from a wide variety of biological sources in soil and water, particularly

microbial action in wet tropical forests.

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Nonlinearity A process is called nonlinear when there is no simple pro-

portional relation between cause and effect. The climate system contains

many such nonlinear processes, resulting in a system with potentially very

complex behaviour. Such complexity may lead to abrupt climate change.

See also Chaotic and Predictability.

North Atlantic Oscillation (NAO) The North Atlantic Oscillation con-

sists of opposing variations of surface pressure near Iceland and near the

Azores. It therefore corresponds to ﬂuctuations in the strength of the main

westerly winds across the Atlantic into Europe, and thus to ﬂuctuations in

the embedded extratropical cyclones with their associated frontal systems.

See NAO Index, Box 2.5.

Northern Annular Mode (NAM) A winter ﬂuctuation in the ampli-

tude of a pattern characterized by low surface pressure in the Arctic and

strong mid-latitude westerlies. The NAM has links with the northern polar

vortex into the stratosphere. Its pattern has a bias to the North Atlantic and

its index has a large correlation with the North Atlantic Oscillation index.

See NAM Index, Box 2.5.

Ocean acidiﬁcation Ocean acidiﬁcation refers to a reduction in the pH

of the ocean over an extended period, typically decades or longer, which

is caused primarily by uptake of carbon dioxide from the atmosphere, but

can also be caused by other chemical additions or subtractions from the

ocean. Anthropogenic ocean acidiﬁcation refers to the component of pH

reduction that is caused by human activity (IPCC, 2011, p. 37).

Ocean heat uptake efﬁciency This is a measure (W m

–2

°C

–1

) of

the rate at which heat storage by the global ocean increases as global

mean surface temperature rises. It is a useful parameter for climate change

experiments in which the radiative forcing is changing monotonically,

when it can be compared with the Climate Feedback Parameter to gauge

the relative importance of climate response and ocean heat uptake in

determining the rate of climate change. It can be estimated from such an

experiment as the ratio of the rate of increase of ocean heat content to the

global mean surface air temperature change.

Organic aerosol Component of the aerosol that consists of organic

compounds, mainly carbon, hydrogen, oxygen and lesser amounts of other

elements. See also Carbonaceous aerosol.

Outgoing longwave radiation Net outgoing radiation in the infra-

red part of the spectrum at the top of the atmosphere. See also Terrestrial

radiation.

Outlet glacier A glacier, usually between rock walls, that is part of, and

drains an ice sheet. See also Ice stream.

Ozone Ozone, the triatomic form of oxygen (O

), is a gaseous atmospher-

ic constituent. In the troposphere, it is created both naturally and by photo-

chemical reactions involving gases resulting from human activities (smog).

Tropospheric ozone acts as a greenhouse gas. In the stratosphere, it is cre-

ated by the interaction between solar ultraviolet radiation and molecular

oxygen (O

). Stratospheric ozone plays a dominant role in the stratospheric

radiative balance. Its concentration is highest in the ozone layer.

Ozone hole See Ozone layer.

Ozone layer The stratosphere contains a layer in which the concentra-

tion of ozone is greatest, the so-called ozone layer. The layer extends from

about 12 to 40 km above the Earth’s surface. The ozone concentration

reaches a maximum between about 20 and 25 km. This layer has been

depleted by human emissions of chlorine and bromine compounds. Every

year, during the Southern Hemisphere spring, a very strong depletion of

the ozone layer takes place over the Antarctic, caused by anthropogenic

chlorine and bromine compounds in combination with the speciﬁc meteo-

rological conditions of that region. This phenomenon is called the Ozone

hole. See also Montreal Protocol.

Paciﬁc Decadal Oscillation (PDO) The pattern and time series of

the ﬁrst empirical orthogonal function of sea surface temperature over the

North Paciﬁc north of 20°N. The PDO broadened to cover the whole Paciﬁc

Basin is known as the Inter-decadal Paciﬁc Oscillation. The PDO and IPO

exhibit similar temporal evolution. See also Paciﬁc Decadal Variability.

Paciﬁc decadal variability Coupled decadal-to-inter-decadal vari-

ability of the atmospheric circulation and underlying ocean in the Paciﬁc

Basin. It is most prominent in the North Paciﬁc, where ﬂuctuations in the

strength of the winter Aleutian Low pressure system co-vary with North

Paciﬁc sea surface temperatures, and are linked to decadal variations in

atmospheric circulation, sea surface temperatures and ocean circulation

throughout the whole Paciﬁc Basin. Such ﬂuctuations have the effect of

modulating the El Niño-Southern Oscillation cycle. Key measures of Paciﬁc

decadal variability are the North Paciﬁc Index (NPI), the Paciﬁc Decadal

Oscillation (PDO) index and the Inter-decadal Paciﬁc Oscillation (IPO)

index, all deﬁned in Box 2.5.

Paciﬁc–North American (PNA) pattern An atmospheric large-scale

wave pattern featuring a sequence of tropospheric high and low pressure

anomalies stretching from the subtropical west Paciﬁc to the east coast of

North America. See PNA pattern index, Box 2.5.

Paleoclimate Climate during periods prior to the development of mea-

suring instruments, including historic and geologic time, for which only

proxy climate records are available.

Parameterization In climate models, this term refers to the technique

of representing processes that cannot be explicitly resolved at the spatial

or temporal resolution of the model (sub-grid scale processes) by relation-

ships between model-resolved larger-scale variables and the area- or time-

averaged effect of such subgrid scale processes.

Percentiles The set of partition values which divides the total popula-

tion of a distribution into 100 equal parts, the 50th percentile correspond-

ing to the median of the population.

Permafrost Ground (soil or rock and included ice and organic material)

that remains at or below 0°C for at least two consecutive years. See also

Near-surface permafrost.

pH pH is a dimensionless measure of the acidity of water (or any solu-

tion) given by its concentration of hydrogen ions (H

). pH is measured on

a logarithmic scale where pH = –log

). Thus, a pH decrease of 1 unit

corresponds to a 10-fold increase in the concentration of H

, or acidity.

Photosynthesis The process by which plants take carbon dioxide from

the air (or bicarbonate in water) to build carbohydrates, releasing oxygen

in the process. There are several pathways of photosynthesis with different

responses to atmospheric carbon dioxide concentrations. See also Carbon

dioxide fertilization.

Plankton Microorganisms living in the upper layers of aquatic systems.

A distinction is made between phytoplankton, which depend on photo-

synthesis for their energy supply, and zooplankton, which feed on phyto-

plankton.

Pleistocene The Pleistocene Epoch is the earlier of two epochs in the

Quaternary System, extending from 2.59 Ma to the beginning of the Holo-

cene at 11.65 ka.

Pliocene The Plionece Epoch is the last epoch of the Neogene System

and extends from 5.33 Ma to the beginning of the Pleistocene at 2.59 Ma.

Pollen analysis A technique of both relative dating and environmental

reconstruction, consisting of the identiﬁcation and counting of pollen types

preserved in peat, lake sediments and other deposits. See also Proxy.

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Precipitable water The total amount of atmospheric water vapour in

a vertical column of unit cross-sectional area. It is commonly expressed in

terms of the height of the water if completely condensed and collected in

a vessel of the same unit cross section.

Precursors Atmospheric compounds that are not greenhouse gases or

aerosols, but that have an effect on greenhouse gas or aerosol concen-

trations by taking part in physical or chemical processes regulating their

production or destruction rates.

Predictability The extent to which future states of a system may be

predicted based on knowledge of current and past states of the system.

Because knowledge of the climate system’s past and current states is gen-

erally imperfect, as are the models that utilize this knowledge to produce a

climate prediction, and because the climate system is inherently nonlinear

and chaotic, predictability of the climate system is inherently limited. Even

with arbitrarily accurate models and observations, there may still be limits

to the predictability of such a nonlinear system (AMS, 2000).

Prediction quality/skill Measures of the success of a prediction

against observationally based information. No single measure can sum-

marize all aspects of forecast quality and a suite of metrics is considered.

Metrics will differ for forecasts given in deterministic and probabilistic

form. See also Climate prediction.

Pre-industrial See Industrial Revolution.

Probability Density Function (PDF) A probability density function

is a function that indicates the relative chances of occurrence of different

outcomes of a variable. The function integrates to unity over the domain

for which it is deﬁned and has the property that the integral over a sub-

domain equals the probability that the outcome of the variable lies within

that sub-domain. For example, the probability that a temperature anomaly

deﬁned in a particular way is greater than zero is obtained from its PDF

by integrating the PDF over all possible temperature anomalies greater

than zero. Probability density functions that describe two or more variables

simultaneously are similarly deﬁned.

Process-based Model Theoretical concepts and computational meth-

ods that represent and simulate the behaviour of real-world systems

derived from a set of functional components and their interactions with

each other and the system environment, through physical and mechanistic

processes occurring over time. See also Climate model.

Projection A projection is a potential future evolution of a quantity or

set of quantities, often computed with the aid of a model. Unlike predic-

tions, projections are conditional on assumptions concerning, for example,

future socioeconomic and technological developments that may or may

not be realized. See also Climate prediction and Climate projection.

Proxy A proxy climate indicator is a record that is interpreted, using

physical and biophysical principles, to represent some combination of

climate-related variations back in time. Climate-related data derived in

this way are referred to as proxy data. Examples of proxies include pollen

analysis, tree ring records, speleothems, characteristics of corals and vari-

ous data derived from marine sediments and ice cores. Proxy-data can be

calibrated to provide quantitative climate information.

Quasi-Biennal Oscillation (QBO) A near-periodic oscillation of the

equatorial zonal wind between easterlies and westerlies in the tropical

stratosphere with a mean period of around 28 months. The alternating

wind maxima descend from the base of the mesosphere down to the tro-

popause, and are driven by wave energy that propagates up from the tro-

posphere.

Quaternary The Quaternary System is the latter of three systems that

make up the Cenozoic Era (65 Ma to present), extending from 2.59 Ma to

the present, and includes the Pleistocene and Holocene epochs.

Radiative effect The impact on a radiation ﬂux or heating rate (most

commonly, on the downward ﬂux at the top of atmosphere) caused by

the interaction of a particular constituent with either the infrared or solar

radiation ﬁelds through absorption, scattering and emission, relative to

an otherwise identical atmosphere free of that constituent. This quanti-

ﬁes the impact of the constituent on the climate system. Examples include

the aerosol–radiation interactions, cloud radiative effect, and greenhouse

effect. In this report, the portion of any top-of-atmosphere radiative effect

that is due to anthropogenic or other external inﬂuences (e.g., volcanic

eruptions or changes in the sun) is termed the instantaneous radiative forc-

ing.

Radiative forcing Radiative forcing is the change in the net, down-

ward minus upward, radiative ﬂux (expressed in W m

–2

) at the tropopause

or top of atmosphere due to a change in an external driver of climate

change, such as, for example, a change in the concentration of carbon diox-

ide or the output of the Sun. Sometimes internal drivers are still treated as

forcings even though they result from the alteration in climate, for example

aerosol or greenhouse gas changes in paleoclimates. The traditional radia-

tive forcing is computed with all tropospheric properties held ﬁxed at their

unperturbed values, and after allowing for stratospheric temperatures, if

perturbed, to readjust to radiative-dynamical equilibrium. Radiative forc-

ing is called instantaneous if no change in stratospheric temperature is

accounted for. The radiative forcing once rapid adjustments are accounted

for is termed the effective radiative forcing. For the purposes of this report,

radiative forcing is further deﬁned as the change relative to the year 1750

and, unless otherwise noted, refers to a global and annual average value.

Radiative forcing is not to be confused with cloud radiative forcing, which

describes an unrelated measure of the impact of clouds on the radiative

ﬂux at the top of the atmosphere.

Rapid adjustment The response to an agent perturbing the climate

system that is driven directly by the agent, independently of any change

in the global mean surface temperature. For example, carbon dioxide and

aerosols, by altering internal heating and cooling rates within the atmo-

sphere, can each cause changes to cloud cover and other variables thereby

producing a radiative effect even in the absence of any surface warming or

cooling. Adjustments are rapid in the sense that they begin to occur right

away, before climate feedbacks which are driven by warming (although

some adjustments may still take signiﬁcant time to proceed to completion,

for example those involving vegetation or ice sheets). It is also called the

rapid response or fast adjustment. For further explanation on the concept,

see Sections 7.1 and 8.1.

Rapid climate change See Abrupt climate change.

Rapid dynamical change (of glaciers or ice sheets) Changes in

glacier or ice sheet mass controlled by changes in ﬂow speed and dis-

charge rather than by accumulation or ablation. This can result in a rate

of mass change larger than that due to any imbalance between accumula-

tion and ablation. Rapid dynamical change may be initiated by a climatic

trigger, such as incursion of warm ocean water beneath an ice shelf, or

thinning of a grounded tidewater terminus, which may lead to reactions

within the glacier system, that may result in rapid ice loss. See also Mass

balance/budget (of glaciers or ice sheets).

Reanalysis Reanalyses are estimates of historical atmospheric tem-

perature and wind or oceanographic temperature and current, and other

quantities, created by processing past meteorological or oceanographic

data using ﬁxed state-of-the-art weather forecasting or ocean circulation

models with data assimilation techniques. Using ﬁxed data assimilation

avoids effects from the changing analysis system that occur in operational

analyses. Although continuity is improved, global reanalyses still suffer

from changing coverage and biases in the observing systems.

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Rebound effect When CO

is removed from the atmosphere, the CO

concentration gradient between atmospheric and land/ocean carbon reser-

voirs is reduced. This leads to a reduction or reversal in subsequent inher-

ent rate of removal of CO

from the atmosphere by natural carbon cycle

processes on land and ocean.

Reconstruction (of climate variable) Approach to reconstructing

the past temporal and spatial characteristics of a climate variable from

predictors. The predictors can be instrumental data if the reconstruction is

used to inﬁll missing data or proxy data if it is used to develop paleoclimate

reconstructions. Various techniques have been developed for this purpose:

linear multivariate regression based methods and nonlinear Bayesian and

analog methods.

Reforestation Planting of forests on lands that have previously

contained forests but that have been converted to some other use. For

a discussion of the term forest and related terms such as afforestation,

reforestation and deforestation, see the IPCC Report on Land Use, Land-

Use Change and Forestry (IPCC, 2000). See also the Report on Deﬁnitions

and Methodological Options to Inventory Emissions from Direct Human-

induced Degradation of Forests and Devegetation of Other Vegetation

Types (IPCC, 2003).

Region A region is a territory characterized by speciﬁc geographical

and climatological features. The climate of a region is affected by regional

and local scale features like topography, land use characteristics and lakes,

as well as remote inﬂuences from other regions. See also Teleconnection.

Regional Climate Model (RCM) A climate model at higher resolu-

tion over a limited area. Such models are used in downscaling global cli-

mate results over speciﬁc regional domains.

Relative humidity The relative humidity speciﬁes the ratio of actual

water vapour pressure to that at saturation with respect to liquid water or

ice at the same temperature. See also Speciﬁc humidity.

Relative sea level Sea level measured by a tide gauge with respect to

the land upon which it is situated. See also Mean sea level and Sea level

change.

Representative Concentration Pathways (RCPs) Scenarios that

include time series of emissions and concentrations of the full suite of

greenhouse gases and aerosols and chemically active gases, as well as land

use/land cover (Moss et al., 2008). The word representative signiﬁes that

each RCP provides only one of many possible scenarios that would lead to

the speciﬁc radiative forcing characteristics. The term pathway emphasizes

that not only the long-term concentration levels are of interest, but also

the trajectory taken over time to reach that outcome. (Moss et al., 2010).

RCPs usually refer to the portion of the concentration pathway extend-

ing up to 2100, for which Integrated Assessment Models produced

corresponding emission scenarios. Extended Concentration Pathways

(ECPs) describe extensions of the RCPs from 2100 to 2500 that were

calculated using simple rules generated by stakeholder consultations,

and do not represent fully consistent scenarios.

Four RCPs produced from Integrated Assessment Models were selected

from the published literature and are used in the present IPCC Assess-

ment as a basis for the climate predictions and projections presented

in Chapters 11 to 14:

RCP2.6 One pathway where radiative forcing peaks at approxi-

mately 3 W m

–2

before 2100 and then declines (the corresponding ECP

assuming constant emissions after 2100)

RCP4.5 and RCP6.0 Two intermediate stabilization pathways in

which radiative forcing is stabilized at approximately 4.5 W m

–2

and

6.0 W m

–2

after 2100 (the corresponding ECPs assuming constant con-

centrations after 2150)

RCP8.5 One high pathway for which radiative forcing reaches great-

er than 8.5 W m

–2

by 2100 and continues to rise for some amount of

time (the corresponding ECP assuming constant emissions after 2100

and constant concentrations after 2250)

For further description of future scenarios, see Box 1.1.

Reservoir A component of the climate system, other than the atmo-

sphere, which has the capacity to store, accumulate or release a substance

of concern, for example, carbon, a greenhouse gas or a precursor. Oceans,

soils and forests are examples of reservoirs of carbon. Pool is an equivalent

term (note that the deﬁnition of pool often includes the atmosphere). The

absolute quantity of the substance of concern held within a reservoir at a

speciﬁed time is called the stock.

Resolution In climate models, this term refers to the physical distance

(metres or degrees) between each point on the grid used to compute the

equations. Temporal resolution refers to the time step or time elapsed

between each model computation of the equations.

Respiration The process whereby living organisms convert organic

matter to carbon dioxide, releasing energy and consuming molecular

oxygen.

Response time The response time or adjustment time is the time

needed for the climate system or its components to re-equilibrate to a

new state, following a forcing resulting from external processes. It is very

different for various components of the climate system. The response time

of the troposphere is relatively short, from days to weeks, whereas the

stratosphere reaches equilibrium on a time scale of typically a few months.

Due to their large heat capacity, the oceans have a much longer response

time: typically decades, but up to centuries or millennia. The response

time of the strongly coupled surface–troposphere system is, therefore,

slow compared to that of the stratosphere, and mainly determined by the

oceans. The biosphere may respond quickly (e.g., to droughts), but also

very slowly to imposed changes. See lifetime for a different deﬁnition of

response time pertinent to the rate of processes affecting the concentra-

tion of trace gases.

Return period An estimate of the average time interval between

occurrences of an event (e.g., ﬂood or extreme rainfall) of (or below/above)

a deﬁned size or intensity. See also Return value.

Return value The highest (or, alternatively, lowest) value of a given

variable, on average occurring once in a given period of time (e.g., in 10

years). See also Return period.

River discharge See Streamﬂow.

Runoff That part of precipitation that does not evaporate and is not

transpired, but ﬂows through the ground or over the ground surface and

returns to bodies of water. See also Hydrological cycle.

Scenario A plausible description of how the future may develop based

on a coherent and internally consistent set of assumptions about key driv-

ing forces (e.g., rate of technological change, prices) and relationships.

Note that scenarios are neither predictions nor forecasts, but are useful to

provide a view of the implications of developments and actions. See also

Climate scenario, Emission scenario, Representative Concentration Path-

ways and SRES scenarios.

Sea ice Ice found at the sea surface that has originated from the freez-

ing of seawater. Sea ice may be discontinuous pieces (ice ﬂoes) moved on

the ocean surface by wind and currents (pack ice), or a motionless sheet

attached to the coast (land-fast ice). Sea ice concentration is the fraction

of the ocean covered by ice. Sea ice less than one year old is called ﬁrst-

year ice. Perennial ice is sea ice that survives at least one summer. It may

be subdivided into second-year ice and multi-year ice, where multiyear ice

has survived at least two summers.

1462

Annex III Glossary

AIII

Sea level change Sea level can change, both globally and locally due

to (1) changes in the shape of the ocean basins, (2) a change in ocean

volume as a result of a change in the mass of water in the ocean, and (3)

changes in ocean volume as a result of changes in ocean water density.

Global mean sea level change resulting from change in the mass of the

ocean is called barystatic. The amount of barystatic sea level change due to

the addition or removal of a mass of water is called its sea level equivalent

(SLE). Sea level changes, both globally and locally, resulting from changes

in water density are called steric. Density changes induced by tempera-

ture changes only are called thermosteric, while density changes induced

by salinity changes are called halosteric. Barystatic and steric sea level

changes do not include the effect of changes in the shape of ocean basins

induced by the change in the ocean mass and its distribution. See also

Relative Sea Level and Thermal expansion.

Sea level equivalent (SLE) The sea level equivalent of a mass of

water (ice, liquid or vapour) is that mass, converted to a volume using a

density of 1000 kg m

–3

, and divided by the present-day ocean surface area

of 3.625 × 10

. Thus, 362.5 Gt of water mass added to the ocean will

cause 1 mm of global mean sea level rise. See also Sea level change.

Seasonally frozen ground See Frozen ground.

Sea surface temperature (SST) The sea surface temperature is the

subsurface bulk temperature in the top few metres of the ocean, measured

by ships, buoys and drifters. From ships, measurements of water samples in

buckets were mostly switched in the 1940s to samples from engine intake

water. Satellite measurements of skin temperature (uppermost layer; a

fraction of a millimetre thick) in the infrared or the top centimetre or so in

the microwave are also used, but must be adjusted to be compatible with

the bulk temperature.

Semi-direct (aerosol) effect See Aerosol–radiation interaction.

Semi-empirical model Model in which calculations are based on a

combination of observed associations between variables and theoretical

considerations relating variables through fundamental principles (e.g.,

conservation of energy). For example, in sea level studies, semi-empirical

models refer speciﬁcally to transfer functions formulated to project future

global mean sea level change, or contributions to it, from future global

mean surface temperature change or radiative forcing.

Sensible heat ﬂux The turbulent or conductive ﬂux of heat from the

Earth’s surface to the atmosphere that is not associated with phase chang-

es of water; a component of the surface energy budget.

Sequestration See Uptake.

Shortwave radiation See Solar radiation.

Signiﬁcant wave height The average trough-to-crest height of the

highest one third of the wave heights (sea and swell) occurring in a par-

ticular time period.

Sink Any process, activity or mechanism that removes a greenhouse

gas, an aerosol or a precursor of a greenhouse gas or aerosol from the

atmosphere.

Slab-ocean model A simpliﬁed representation in a climate model of

the ocean as a motionless layer of water with a depth of 50 to 100 m.

Climate models with a slab ocean can be used only for estimating the equi-

librium response of climate to a given forcing, not the transient evolution

of climate. See also Equilibrium and transient climate experiment.

Snow cover extent The areal extent of snow covered ground.

Snow water equivalent (SWE) The depth of liquid water that would

result if a mass of snow melted completely.

Soil moisture Water stored in the soil in liquid or frozen form.

Soil temperature The temperature of the soil. This can be measured or

modelled at multiple levels within the depth of the soil.

Solar activity General term describing a variety of magnetic phenome-

na on the Sun such as sunspots, faculae (bright areas), and ﬂares (emission

of high-energy particles). It varies on time scales from minutes to millions

of years. See also Solar cycle.

Solar (‘11-year’) cycle A quasi-regular modulation of solar activity

with varying amplitude and a period of between 8 and 14 years.

Solar radiation Electromagnetic radiation emitted by the Sun with a

spectrum close to the one of a black body with a temperature of 5770 K.

The radiation peaks in visible wavelengths. When compared to the ter-

restrial radiation it is often referred to as shortwave radiation. See also

Insolation and Total solar irradiance (TSI).

Solar Radiation Management (SRM) Solar Radiation Management

refers to the intentional modiﬁcation of the Earth’s shortwave radiative

budget with the aim to reduce climate change according to a given metric

(e.g., surface temperature, precipitation, regional impacts, etc). Artiﬁcial

injection of stratospheric aerosols and cloud brightening are two examples

of SRM techniques. Methods to modify some fast-responding elements of

the longwave radiative budget (such as cirrus clouds), although not strictly

speaking SRM, can be related to SRM. SRM techniques do not fall within

the usual deﬁnitions of mitigation and adaptation (IPCC, 2012, p. 2). See

also Solar radiation, Carbon Dioxide Removal (CDR) and Geoengineering.

Solubility pump Solubility pump is an important physicochemical pro-

cess that transports dissolved inorganic carbon from the ocean’s surface

to its interior. This process controls the inventory of carbon in the ocean.

The solubility of gaseous carbon dioxide can alter carbon dioxide concen-

trations in the oceans and the overlying atmosphere. See also Biological

pump.

Source Any process, activity or mechanism that releases a greenhouse

gas, an aerosol or a precursor of a greenhouse gas or aerosol into the

atmosphere.

Southern Annular Mode (SAM) The leading mode of variability of

Southern Hemisphere geopotential height, which is associated with shifts

in the latitude of the midlatitude jet. See SAM Index, Box 2.5.

Southern Oscillation See El Niño-Southern Oscillation (ENSO).

South Paciﬁc Convergence Zone (SPCZ) A band of low-level con-

vergence, cloudiness and precipitation ranging from the west Paciﬁc warm

pool south-eastwards towards French Polynesia, which is one of the most

signiﬁcant features of subtropical Southern Hemisphere climate. It shares

some characteristics with the ITCZ, but is more extratropical in nature,

especially east of the Dateline.

Spatial and temporal scales Climate may vary on a large range of

spatial and temporal scales. Spatial scales may range from local (less than

100 000 km

), through regional (100 000 to 10 million km

) to continental

(10 to 100 million km

). Temporal scales may range from seasonal to geo-

logical (up to hundreds of millions of years).

Speciﬁc humidity The speciﬁc humidity speciﬁes the ratio of the mass

of water vapour to the total mass of moist air. See also Relative humidity.

SRES scenarios SRES scenarios are emission scenarios developed by

Nakićenović and Swart (2000) and used, among others, as a basis for some

of the climate projections shown in Chapters 9 to 11 of IPCC (2001) and

Chapters 10 and 11 of IPCC (2007). The following terms are relevant for a

better understanding of the structure and use of the set of SRES scenarios:

Scenario family Scenarios that have a similar demographic, soci-

etal, economic and technical change storyline. Four scenario families

comprise the SRES scenario set: A1, A2, B1 and B2.

1463

Glossary Annex III

AIII

Illustrative Scenario A scenario that is illustrative for each of

the six scenario groups reﬂected in the Summary for Policymakers of

Nakićenović and Swart (2000). They include four revised marker scenar-

ios for the scenario groups A1B, A2, B1, B2 and two additional scenar-

ios for the A1FI and A1T groups. All scenario groups are equally sound.

Marker Scenario A scenario that was originally posted in draft form

on the SRES website to represent a given scenario family. The choice of

markers was based on which of the initial quantiﬁcations best reﬂected

the storyline, and the features of speciﬁc models. Markers are no more

likely than other scenarios, but are considered by the SRES writing team

as illustrative of a particular storyline. They are included in revised form

in Nakićenović and Swart (2000). These scenarios received the closest

scrutiny of the entire writing team and via the SRES open process. Sce-

narios were also selected to illustrate the other two scenario groups.

Storyline A narrative description of a scenario (or family of sce-

narios), highlighting the main scenario characteristics, relationships

between key driving forces and the dynamics of their evolution.

Steric See Sea level change.

Stock See Reservoir.

Storm surge The temporary increase, at a particular locality, in the

height of the sea due to extreme meteorological conditions (low atmo-

spheric pressure and/or strong winds). The storm surge is deﬁned as being

the excess above the level expected from the tidal variation alone at that

time and place.

Storm tracks Originally, a term referring to the tracks of individual

cyclonic weather systems, but now often generalized to refer to the main

regions where the tracks of extratropical disturbances occur as sequences

of low (cyclonic) and high (anticyclonic) pressure systems.

Stratosphere The highly stratiﬁed region of the atmosphere above the

troposphere extending from about 10 km (ranging from 9 km at high lati-

tudes to 16 km in the tropics on average) to about 50 km altitude.

Streamﬂow Water ﬂow within a river channel, for example expressed

in m

–1

. A synonym for river discharge.

Subduction Ocean process in which surface waters enter the ocean

interior from the surface mixed layer through Ekman pumping and lateral

advection. The latter occurs when surface waters are advected to a region

where the local surface layer is less dense and therefore must slide below

the surface layer, usually with no change in density.

Sunspots Dark areas on the Sun where strong magnetic ﬁelds reduce

the convection causing a temperature reduction of about 1500 K com-

pared to the surrounding regions. The number of sunspots is higher during

periods of higher solar activity, and varies in particular with the solar cycle.

Surface layer See Atmospheric boundary layer.

Surface temperature See Global mean surface temperature, Land

surface air temperature and Sea surface temperature.

Talik A layer of year-round unfrozen ground that lies in permafrost

areas.

Teleconnection A statistical association between climate variables at

widely separated, geographically-ﬁxed spatial locations. Teleconnections

are caused by large spatial structures such as basin-wide coupled modes

of ocean–atmosphere variability, Rossby wave-trains, mid-latitude jets and

storm tracks, etc. See also Teleconnection pattern.

Teleconnection pattern A correlation map obtained by calculating

the correlation between variables at different spatial locations and a cli-

mate index. It is the special case of a climate pattern obtained for stan-

dardized variables and a standardized climate index, that is, the variables

and index are each centred and scaled to have zero mean and unit vari-

ance. One-point teleconnection maps are made by choosing a variable at

one of the locations to be the climate index. See also Teleconnection.

Terrestrial radiation Radiation emitted by the Earth’s surface, the

atmosphere and the clouds. It is also known as thermal infrared or long-

wave radiation, and is to be distinguished from the near-infrared radia-

tion that is part of the solar spectrum. Infrared radiation, in general, has a

distinctive range of wavelengths (spectrum) longer than the wavelength of

the red light in the visible part of the spectrum. The spectrum of terrestrial

radiation is almost entirely distinct from that of shortwave or solar radia-

tion because of the difference in temperature between the Sun and the

Earth–atmosphere system. See also Outgoing longwave radiation.

Thermal expansion In connection with sea level, this refers to the

increase in volume (and decrease in density) that results from warming

water. A warming of the ocean leads to an expansion of the ocean volume

and hence an increase in sea level. See also Sea level change.

Thermocline The layer of maximum vertical temperature gradient in

the ocean, lying between the surface ocean and the abyssal ocean. In sub-

tropical regions, its source waters are typically surface waters at higher

latitudes that have subducted (see Subduction) and moved equatorward.

At high latitudes, it is sometimes absent, replaced by a halocline, which is

a layer of maximum vertical salinity gradient.

Thermohaline circulation (THC) Large-scale circulation in the ocean

that transforms low-density upper ocean waters to higher-density interme-

diate and deep waters and returns those waters back to the upper ocean.

The circulation is asymmetric, with conversion to dense waters in restrict-

ed regions at high latitudes and the return to the surface involving slow

upwelling and diffusive processes over much larger geographic regions.

The THC is driven by high densities at or near the surface, caused by cold

temperatures and/or high salinities, but despite its suggestive though

common name, is also driven by mechanical forces such as wind and tides.

Frequently, the name THC has been used synonymously with Meridional

Overturning Circulation.

Thermokarst The process by which characteristic landforms result from

the thawing of ice-rich permafrost or the melting of massive ground ice.

Thermosteric See Sea level change.

Tide gauge A device at a coastal or deep-sea location that continu-

ously measures the level of the sea with respect to the adjacent land. Time

averaging of the sea level so recorded gives the observed secular changes

of the relative sea level.

Tipping point In climate, a hypothesized critical threshold when global

or regional climate changes from one stable state to another stable state.

The tipping point event may be irreversible. See also Irreversibility.

Total solar irradiance (TSI) The total amount of solar radiation in

watts per square metre received outside the Earth’s atmosphere on a

surface normal to the incident radiation, and at the Earth’s mean distance

from the Sun.

Reliable measurements of solar radiation can only be made from space

and the precise record extends back only to 1978. The generally accept-

ed value is 1368 W m

−2

with an accuracy of about 0.2%. It has recently

been estimated to 1360.8 ± 0.5 W m

–2

for the solar minimum of 2008.

Variations of a few tenths of a percent are common, usually associ-

ated with the passage of sunspots across the solar disk. The solar cycle

variation of TSI is of the order of 0.1% (AMS, 2000). Changes in the

ultraviolet part of the spectrum during a solar cycle are comparatively

larger (percent) than in TSI. See also Insolation.

1464

Annex III Glossary

AIII

Transient climate response See Climate sensitivity.

Transient climate response to cumulative CO

emissions (TCRE)

The transient global average surface temperature change per unit cumu-

lated CO

emissions, usually 1000 PgC. TCRE combines both information

on the airborne fraction of cumulated CO

emissions (the fraction of the

total CO

emitted that remains in the atmosphere), and on the transient

climate response (TCR).

Tree rings Concentric rings of secondary wood evident in a cross sec-

tion of the stem of a woody plant. The difference between the dense, small-

celled late wood of one season and the wide-celled early wood of the

following spring enables the age of a tree to be estimated, and the ring

widths or density can be related to climate parameters such as tempera-

ture and precipitation. See also Proxy.

Trend In this report, the word trend designates a change, generally

monotonic in time, in the value of a variable.

Tropopause The boundary between the troposphere and the strato-

sphere.

Troposphere The lowest part of the atmosphere, from the surface to

about 10 km in altitude at mid-latitudes (ranging from 9 km at high lati-

tudes to 16 km in the tropics on average), where clouds and weather phe-

nomena occur. In the troposphere, temperatures generally decrease with

height. See also Stratosphere.

Turnover time See Lifetime.

Uncertainty A state of incomplete knowledge that can result from a

lack of information or from disagreement about what is known or even

knowable. It may have many types of sources, from imprecision in the data

to ambiguously deﬁned concepts or terminology, or uncertain projections

of human behaviour. Uncertainty can therefore be represented by quantita-

tive measures (e.g., a probability density function) or by qualitative state-

ments (e.g., reﬂecting the judgment of a team of experts) (see Moss and

Schneider, 2000; Manning et al., 2004; Mastrandrea et al., 2010). See also

Conﬁdence and Likelihood.

United Nations Framework Convention on Climate Change

(UNFCCC) The Convention was adopted on 9 May 1992 in New York

and signed at the 1992 Earth Summit in Rio de Janeiro by more than 150

countries and the European Community. Its ultimate objective is the ‘sta-

bilisation of greenhouse gas concentrations in the atmosphere at a level

that would prevent dangerous anthropogenic interference with the climate

system’. It contains commitments for all Parties. Under the Convention,

Parties included in Annex I (all OECD countries and countries with econo-

mies in transition) aim to return greenhouse gas emissions not controlled

by the Montreal Protocol to 1990 levels by the year 2000. The convention

entered in force in March 1994. In 1997, the UNFCCC adopted the Kyoto

Protocol.

Uptake The addition of a substance of concern to a reservoir. The

uptake of carbon containing substances, in particular carbon dioxide, is

often called (carbon) sequestration.

Urban heat island (UHI) The relative warmth of a city compared with

surrounding rural areas, associated with changes in runoff, effects on heat

retention, and changes in surface albedo.

Ventilation The exchange of ocean properties with the atmospheric

surface layer such that property concentrations are brought closer to equi-

librium values with the atmosphere (AMS, 2000), and the processes that

propagate these properties into the ocean interior.

Volatile Organic Compounds (VOC) Important class of organic

chemical air pollutants that are volatile at ambient air conditions. Other

terms used to represent VOCs are hydrocarbons (HCs), reactive organic

gases (ROGs) and non-methane volatile organic compounds (NMVOCs).

NMVOCs are major contributors (together with NO

and CO) to the forma-

tion of photochemical oxidants such as ozone.

Walker Circulation Direct thermally driven zonal overturning circula-

tion in the atmosphere over the tropical Paciﬁc Ocean, with rising air in the

western and sinking air in the eastern Paciﬁc.

Warm days/warm nights Days where maximum temperature, or

nights where minimum temperature, exceeds the 90th percentile, where

the respective temperature distributions are generally deﬁned with respect

to the 1961–1990 reference period. For the corresponding indices, see Box

2.4.

Warm spell A period of abnormally hot weather. For the corresponding

indices, see Box 2.4. See also Heat wave.

Water cycle See Hydrological cycle.

Water mass A body of ocean water with identiﬁable properties (tem-

perature, salinity, density, chemical tracers) resulting from its unique for-

mation process. Water masses are often identiﬁed through a vertical or

horizontal extremum of a property such as salinity. North Paciﬁc Intermedi-

ate Water (NPIW) and Antarctic Intermediate Water (AAIW) are examples

of water masses.

Weathering The gradual removal of atmospheric CO

through disso-

lution of silicate and carbonate rocks. Weathering may involve physical

processes (mechanical weathering) or chemical activity (chemical weath-

ering).

Well-mixed greenhouse gas See Greenhouse gas.

Younger Dryas A period 12.85 to 11.65 ka (thousand years before

1950), during the deglaciation, characterized by a temporary return to

colder conditions in many locations, especially around the North Atlantic.

1465

Glossary Annex III

AIII

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