Topic 1

Observed Changes and their Causes

Human influence on the climate system is clear, and recent anthropogenic emissions of greenhouse gases are the highest in history. Recent climate changes have had widespread impacts on human and natural systems.


Topic 1 focuses on observational evidence of a changing climate, the impacts caused by this change and the human contributions to it. It discusses observed changes in climate (1.1) and external influences on climate (forcings), differentiating those forcings that are of anthropogenic origin, and their contributions by economic sectors and greenhouse gases (GHGs) (1.2). Section 1.3 attributes observed climate change to its causes and attributes impacts on human and natural systems to climate change, determining the degree to which those impacts can be attributed to climate change. The changing probability of extreme events and their causes are discussed in Section 1.4, followed by an account of exposure and vulnerability within a risk context (1.5) and a section on adaptation and mitigation experience (1.6).

1.1. Observed changes in the climate system

Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, and sea level has risen.


1.1.1. Atmosphere

Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850. The period from 1983 to 2012 was very likely the warmest 30-year period of the last 800 years in the Northern Hemisphere, where such assessment is possible (high confidence) and likely the warmest 30-year period of the last 1400 years (medium confidence). {WGI 2.4.3, 5.3.5}

The globally averaged combined land and ocean surface temperature data as calculated by a linear trend show a warming of 0.85 [0.65 to 1.06] °C1  over the period 1880 to 2012, for which multiple independently produced datasets exist. The total increase between the average of the 1850–1900 period and the 2003–2012 period is 0.78 [0.72 to 0.85] °C, based on the single longest dataset available. For the longest period when calculation of regional trends is sufficiently complete (1901 to 2012), almost the entire globe has experienced surface warming (Figure 1.1). {WGI SPM B.12.4.3}

In addition to robust multi-decadal warming, the globally averaged surface temperature exhibits substantial decadal and interannual variability (Figure 1.1). Due to this natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends. As one example, the rate of warming over the past 15 years (1998–2012; 0.05 [–0.05 to 0.15] °C per decade), which begins with a strong El Niño, is smaller than the rate calculated since 1951 (1951–2012; 0.12 [0.08 to 0.14] °C per decade; see Box 1.1). {WGI SPM B.12.4.3}

Based on multiple independent analyses of measurements, it is virtually certain that globally the troposphere has warmed and the lower stratosphere has cooled since the mid-20th century. There is medium confidence in the rate of change and its vertical structure in the Northern Hemisphere extratropical troposphere. {WGI SPM B.1, 2.4.4}

Confidence in precipitation change averaged over global land areas since 1901 is low prior to 1951 and medium afterwards. Averaged over the mid-latitude land areas of the Northern Hemisphere, precipitation has likely increased since 1901 (medium confidence before and high confidence after 1951). For other latitudes area-averaged long-term positive or negative trends have low confidence  (Figure 1.1). {WGI SPM B.1, Figure SPM.2, 2.5.1}


1.1.2. Oceans

Ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90% of the energy accumulated between 1971 and 2010 (high confidence) with only about 1% stored in the atmosphere (Figure 1.2). On a global scale, the ocean warming is largest near the surface, and the upper 75 m warmed by 0.11 [0.09 to 0.13] °C per decade over the period 1971 to 2010. It is virtually certain that the upper ocean (0−700 m) warmed from 1971 to 2010, and it likely warmed between the 1870s and 1971. It is likely that the ocean warmed from 700 to 2000 m from 1957 to 2009 and from 3000 m to the bottom for the period 1992 to 2005 (Figure 1.2). {WGI SPM B.2, 3.2, Box 3.1}

It is very likely that regions of high surface salinity, where evaporation dominates, have become more saline, while regions of low salinity, where precipitation dominates, have become fresher since the 1950s. These regional trends in ocean salinity provide indirect evidence for changes in evaporation and precipitation over the oceans and thus for changes in the global water cycle (medium confidence). There is no observational evidence of a long-term trend in the Atlantic Meridional Overturning Circulation (AMOC). {WGI SPM B.2, 2.5, 3.3, 3.4.3, 3.5, 3.6.3}

Figure 1.1


Figure 1.1 | Multiple observed indicators of a changing global climate system. (a) Observed globally averaged combined land and ocean surface temperature anomalies (relative to the mean of 1986 to 2005 period, as annual and decadal averages) with an estimate of decadal mean uncertainty included for one data set (grey shading). {WGI Figure SPM.1, WGI Figure 2.20, a listing of data sets and further technical details are given in the WGI Technical Summary Supplementary Material WGI TS.SM.1.1(b) Map of the observed surface temperature change, from 1901 to 2012, derived from temperature trends determined by linear regression from one data set (orange line in Panel a). Trends have been calculated where data availability permitted a robust estimate (i.e., only for grid boxes with greater than 70% complete records and more than 20% data availability in the first and last 10% of the time period), other areas are white. Grid boxes where the trend is significant, at the 10% level, are indicated by a + sign. {WGI Figure SPM.1, WGI Figure 2.21, WGI Figure TS.2, a listing of data sets and further technical details are given in the WGI Technical Summary Supplementary Material WGI TS.SM.1.2(c) Arctic (July to September average) and Antarctic (February) sea ice extent. {WGI Figure SPM.3, Figure 4.3, Figure 4.SM.2, a listing of data sets and further technical details are given in the WGI Technical Summary Supplementary Material WGI TS.SM.3.2(d) Global mean sea level relative to the 1986–2005 mean of the longest running data set, and with all data sets aligned to have the same value in 1993, the first year of satellite altimetry data. All time series (coloured lines indicating different data sets) show annual values, and where assessed, uncertainties are indicated by coloured shading. {WGI Figure SPM.3, WGI Figure 3.13, a listing of data sets and further technical details are given in the WGI Technical Summary Supplementary Material WGI TS.SM.3.4(e) Map of observed precipitation change, from 1951 to 2010; trends in annual accumulation calculated using the same criteria as in Panel b. {WGI SPM. Figure.2, WGI TS TFE.1, Figure 2, WGI Figure 2.29. A listing of data sets and further technical details are given in the WGI Technical Summary Supplementary Material WGI TS.SM.2.1}

Figure 1.2


Figure 1.2 | Energy accumulation within the Earth's climate system. Estimates are in 1021 J, and are given relative to 1971 and from 1971 to 2010, unless otherwise indicated. Components included are upper ocean (above 700 m), deep ocean (below 700 m; including below 2000 m estimates starting from 1992), ice melt (for glaciers and ice caps, Greenland and Antarctic ice sheet estimates starting from 1992, and Arctic sea ice estimate from 1979 to 2008), continental (land) warming, and atmospheric warming (estimate starting from 1979). Uncertainty is estimated as error from all five components at 90% confidence intervals. {WGI Box 3.1, Figure 1}

Since the beginning of the industrial era, oceanic uptake of CO2 has resulted in acidification of the ocean; the pH of ocean surface water has decreased by 0.1 (high confidence), corresponding to a 26% increase in acidity, measured as hydrogen ion concentration. There is medium confidence that, in parallel to warming, oxygen concentrations have decreased in coastal waters and in the open ocean thermocline in many ocean regions since the 1960s, with a likely expansion of tropical oxygen minimum zones in recent decades. {WGI SPM B.5, TS2.8.5, 3.8.1, 3.8.2, 3.8.3, 3.8.5, Figure 3.20}


1.1.3. Cryosphere

Over the last two decades, the Greenland and Antarctic ice sheets have been losing mass (high confidence). Glaciers have continued to shrink almost worldwide (high confidence). Northern Hemisphere spring snow cover has continued to decrease in extent (high confidence). There is high confidence that there are strong regional differences in the trend in Antarctic sea ice extent, with a very likely increase in total extent. {WGI SPM B.3, 4.2–4.7}

Glaciers have lost mass and contributed to sea level rise throughout the 20th century. The rate of ice mass loss from the Greenland ice sheet has very likely substantially increased over the period 1992 to 2011, resulting in a larger mass loss over 2002 to 2011 than over 1992 to 2011. The rate of ice mass loss from the Antarctic ice sheet, mainly from the northern Antarctic Peninsula and the Amundsen Sea sector of West Antarctica, is also likely larger over 2002 to 2011.  {WGI SPM B.3, SPM B.4, 4.3.3, 4.4.2, 4.4.3}

The annual mean Arctic sea ice extent decreased over the period 1979 (when satellite observations commenced) to 2012. The rate of decrease was very likely in the range 3.5 to 4.1% per decade. Arctic sea ice extent has decreased in every season and in every successive decade since 1979, with the most rapid decrease in decadal mean extent in summer (high confidence). For the summer sea ice minimum, the decrease was very likely in the range of 9.4 to 13.6% per decade (range of 0.73 to 1.07 million km2 per decade) (see Figure 1.1).  It is very likely that the annual mean Antarctic sea ice extent increased in the range of 1.2 to 1.8% per decade (range of 0.13 to 0.20 million km2 per decade) between 1979 and 2012. However, there is high confidence that there are strong regional differences in Antarctica, with extent increasing in some regions and decreasing in others. {WGI SPM B.5, 4.2.2, 4.2.3}

There is very high confidence that the extent of Northern Hemisphere snow cover has decreased since the mid-20th century by 1.6 [0.8 to 2.4] % per decade for March and April, and 11.7% per decade for June, over the 1967 to 2012 period. There is high confidence that permafrost temperatures have increased in most regions of the Northern Hemisphere since the early 1980s, with reductions in thickness and areal extent in some regions. The increase in permafrost temperatures has occurred in response to increased surface temperature and changing snow cover. {WGI SPM B.3, 4.5, 4.7.2}


1.1.4. Sea level

Over the period 1901–2010, global mean sea level rose by 0.19 [0.17 to 0.21] m (Figure 1.1). The rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia (high confidence){WGI SPM B.4, 3.7.2, 5.6.3, 13.2}

It is very likely that the mean rate of global averaged sea level rise was 1.7 [1.5 to 1.9] mm/yr between 1901 and 2010 and 3.2 [2.8 to 3.6] mm/yr between 1993 and 2010. Tide gauge and satellite altimeter data are consistent regarding the higher rate during the latter period. It is likely that similarly high rates occurred between 1920 and 1950. {WGI SPM B.43.713.2}

Since the early 1970s, glacier mass loss and ocean thermal expansion from warming together explain about 75% of the observed global mean sea level rise (high confidence). Over the period 1993–2010, global mean sea level rise is, with high confidence, consistent with the sum of the observed contributions from ocean thermal expansion, due to warming, from changes in glaciers, the Greenland ice sheet, the Antarctic ice sheet and land water storage {WGI SPM B.4, 13.3.6}

Rates of sea level rise over broad regions can be several times larger or smaller than the global mean sea level rise for periods of several decades, due to fluctuations in ocean circulation. Since 1993, the regional rates for the Western Pacific are up to three times larger than the global mean, while those for much of the Eastern Pacific are near zero or negative. {WGI 3.7.3, FAQ 13.1}

There is very high confidence that maximum global mean sea level during the last interglacial period (129,000 to 116,000 years ago) was, for several thousand years, at least 5 m higher than present and high confidence that it did not exceed 10 m above present. During the last interglacial period, the Greenland ice sheet very likely contributed between 1.4 and 4.3 m to the higher global mean sea level, implying with medium confidence an additional contribution from the Antarctic ice sheet. This change in sea level occurred in the context of different orbital forcing and with high-latitude surface temperature, averaged over several thousand years, at least 2°C warmer than present (high confidence). {WGI SPM B.4, 5.3.4, 5.6.2, 13.2.1}


Box 1.1 Recent temperature trends and their implications


The observed reduction in surface warming trend over the period 1998 to 2012 as compared to the period 1951 to 2012, is due in roughly equal measure to a reduced trend in radiative forcing and a cooling contribution from natural internal variability, which includes a possible redistribution of heat within the ocean (medium confidence). The rate of warming of the observed global mean surface temperature over the period from 1998 to 2012 is estimated to be around one-third to one-half of the trend over the period from 1951 to 2012 (Box 1.1, Figures 1a and 1c). Even with this reduction in surface warming trend, the climate system has very likely continued to accumulate heat since 1998 (Figure 1.2), and sea level has continued to rise (Figure 1.1). {WGI SPM D.1, Box 9.2}

The radiative forcing of the climate system has continued to increase during the 2000s, as has its largest contributor, the atmospheric concentration of CO2. However, the radiative forcing has been increasing at a lower rate over the period from 1998 to 2011, compared to 1984 to 1998 or 1951 to 2011, due to cooling effects from volcanic eruptions and the cooling phase of the solar cycle over the period from 2000 to 2009. There is, however, low confidence in quantifying the role of the forcing trend in causing the reduction in the rate of surface warming. {WGI 8.5.2, Box 9.2}

For the period from 1998 to 2012, 111 of the 114 available climate-model simulations show a surface warming trend larger than the observations (Box 1.1, Figure 1a). There is medium confidence that this difference between models and observations is to a substantial degree caused by natural internal climate variability, which sometimes enhances and sometimes counteracts the long-term externally forced warming trend (compare Box 1.1, Figures 1a and 1b; during the period from 1984 to 1998, most model simulations show a smaller warming trend than observed). Natural internal variability thus diminishes the relevance of short trends for long-term climate change. The difference between models and observations may also contain contributions from inadequacies in the solar, volcanic and aerosol forcings used by the models and, in some models, from an overestimate of the response to increasing greenhouse gas and other anthropogenic forcing (the latter dominated by the effects of aerosols). {WGI 2.4.3, WGI Box,}

For the longer period from 1951 to 2012, simulated surface warming trends are consistent with the observed trend (very high confidence(Box 1.1, Figure 1c). Furthermore, the independent estimates of radiative forcing, of surface warming and of observed heat storage (the latter available since 1970) combine to give a heat budget for the Earth that is consistent with the assessed likely range of equilibrium climate sensitivity (1.5–4.5 ºC)2The record of observed climate change has thus allowed characterization of the basic properties of the climate system that have implications for future warming, including the equilibrium climate sensitivity and the transient climate response (see Topic 2). {WGI Box 9.2, 10.8.1, 10.8.2, Box 12.2, Box 13.1}

Box 1.1 Figure 1


Box 1.1, Figure 1 | Trends in the global mean surface temperature over the periods from 1998 to 2012 (a), 1984 to 1998 (b), and 1951 to 2012 (c), from observations (red) and the 114 available simulations with current-generation climate models (grey bars). The height of each grey bar indicates how often a trend of a certain magnitude (in °C per decade) occurs among the 114 simulations. The width of the red-hatched area indicates the statistical uncertainty that arises from constructing a global average from individual station data. This observational uncertainty differs from the one quoted in the text of Section 1.1.1there, an estimate of natural internal variability is also included. Here, by contrast, the magnitude of natural internal variability is characterised by the spread of the model ensemble. {based on WGI Box 9.2, Figure 1}

1.2. Past and recent drivers of climate change

Anthropogenic greenhouse gas emissions have increased since the pre-industrial era driven largely by economic and population growth. From 2000 to 2010 emissions were the highest in history. Historical emissions have driven atmospheric concentrations of carbon dioxide, methane and nitrous oxide to levels that are unprecedented in at least the last 800,000 years, leading to an uptake of energy by the climate system.


Natural and anthropogenic substances and processes that alter the Earth’s energy budget are physical drivers of climate change. Radiative forcing quantifies the perturbation of energy into the Earth system caused by these drivers. Radiative forcings larger than zero lead to a near-surface warming, and radiative forcings smaller than zero lead to a cooling. Radiative forcing is estimated based on in-situ and remote observations, properties of GHGs and aerosols, and calculations using numerical models. The radiative forcing over the 1750–2011 period is shown in Figure 1.4 in major groupings. The ‘Other Anthropogenic’ group is principally comprised of cooling effects from aerosol changes, with smaller contributions from ozone changes, land use reflectance changes and other minor terms. {WGI SPM C, 8.1, 8.5.1}


1.2.1. Natural and anthropogenic radiative forcings

Atmospheric concentrations of GHGs are at levels that are unprecedented in at least 800,000 years. Concentrations of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) have all shown large increases since 1750 (40%, 150% and 20%, respectively) (Figure 1.3). CO2 concentrations are increasing at the fastest observed decadal rate of change (2.0 ± 0.1 ppm/yr) for 2002– 2011. After almost one decade of stable CH4 concentrations since the late 1990s, atmospheric measurements have shown renewed increases since 2007. N2O concentrations have steadily increased at a rate of 0.73 ± 0.03 ppb/yr over the last three decades. {WGI SPM B5, 2.2.1, 6.1.2, 6.1.3, 6.3}

The total anthropogenic radiative forcing over 1750–2011 is calculated to be a warming effect of 2.3 [1.1 to 3.3] W/m2 (Figure 1.4), and it has increased more rapidly since 1970 than during prior decades. Carbon dioxide is the largest single contributor to radiative forcing over 1750–2011 and its trend since 1970. The total anthropogenic radiative forcing estimate for 2011 is substantially higher (43%) than the estimate reported in the IPCC Fourth Assessment Report (AR4) for the year 2005. This is caused by a combination of continued growth in most GHG concentrations and an improved estimate of radiative forcing from aerosols. {WGI SPM C, 8.5.1}

Figure 1.3


Figure 1.3 | Observed changes in atmospheric greenhouse gas concentrations. Atmospheric concentrations of carbon dioxide (CO2, green), methane (CH4, orange), and nitrous oxide (N2O, red). Data from ice cores (symbols) and direct atmospheric measurements (lines) are overlaid. {WGI 2.2, 6.2, 6.3, Figure 6.11}

The radiative forcing from aerosols, which includes cloud adjustments, is better understood and indicates a weaker cooling effect than in AR4. The aerosol radiative forcing over 1750–2011 is estimated as –0.9 [–1.9 to −0.1] W/m2 (medium confidence). Radiative forcing from aerosols has two competing components: a dominant cooling effect from most aerosols and their cloud adjustments and a partially offsetting warming contribution from black carbon absorption of solar radiation. There is high confidence that the global mean total aerosol radiative forcing has counteracted a substantial portion of radiative forcing from wellmixed GHGs. Aerosols continue to contribute the largest uncertainty to the total radiative forcing estimate. {WGI SPM C, 7.5, 8.3, 8.5.1}

Changes in solar irradiance and volcanic aerosols cause natural radiative forcing (Figure 1.4). The radiative forcing from stratospheric volcanic aerosols can have a large cooling effect on the climate system for some years after major volcanic eruptions. Changes in total solar irradiance are calculated to have contributed only around 2% of the total radiative forcing in 2011, relative to 1750. {WGI SPM C, Figure SPM.58.4}


Figure 1.4


Figure 1.4 | Radiative forcing of climate change during the industrial era (1750-2011). Bars show radiative forcing from well-mixed greenhouse gases (WMGHG), other anthropogenic forcings, total anthropogenic forcings and natural forcings. The error bars indicate the 5 to 95% uncertainty. Other anthropogenic forcings include aerosol, land use surface reflectance and ozone changes. Natural forcings include solar and volcanic effects. The total anthropogenic radiative forcing for 2011 relative to 1750 is 2.3 W/m2 (uncertainty range 1.1 to 3.3 W/m2). This corresponds to a CO2-equivalent concentration (see Glossary) of 430 ppm (uncertainty range 340 to 520 ppm). {Data from WGI 7.5 and Table 8.6}

1.2.2. Human activities affecting emission drivers

About half of the cumulative anthropogenic CO2 emissions between 1750 and 2011 have occurred in the last 40 years (high confidence). Cumulative anthropogenic CO2 emissions of 2040 ± 310 GtCO2 were added to the atmosphere between 1750 and 2011. Since 1970, cumulative CO2 emissions from fossil fuel combustion, cement production and flaring have tripled, and cumulative CO2 emissions from forestry and other land use (FOLU)3 have increased by about 40% (Figure 1.5)4. In 2011 annual CO2 emissions from fossil fuel combustion, cement production and flaring were 34.8 ± 2.9 GtCO2/yr. For 2002-2011 average annual emissions from forestry and other land use were 3.3 ± 2.9 GtCO2/yr. {WGI 6.3.1, 6.3.2, WGIII SPM.3}

About 40% of these anthropogenic CO2 emissions have remained in the atmosphere (880 ± 35 GtCO2) since 1750. The rest was removed from the atmosphere by sinks, and stored in natural carbon cycle reservoirs. Sinks from ocean uptake and vegetation with soils account, in roughly equal measures, for the remainder of the cumulative CO2 emissions. The ocean has absorbed about 30% of the emitted anthropogenic CO2, causing ocean acidification. {WG1 3.8.1, 6.3.1

Total annual anthropogenic GHG emissions have continued to increase over 1970 to 2010 with larger absolute increases between 2000 and 2010 (high confidence). Despite a growing number of climate change mitigation policies, annual GHG emissions grew on average by 1.0 GtCO2-eq (2.2%) per year, from 2000 to 2010, compared to 0.4 GtCO2-eq (1.3%) per year, from 1970 to 2000 (Figure 1.6)5Total anthropogenic GHG emissions from 2000 to 2010 were the highest in human history and reached 49 (±4.5) GtCO2-eq/yr in 2010. The global economic crisis of 2007/2008 reduced emissions only temporarily. {WGIII SPM.3, 1.3, 5.2, 13.3, 15.2.2, Box TS.5, Figure 15.1}

Figure 1.5


Figure 1.5 | Annual global anthropogenic carbon dioxide (CO2) emissions (gigatonne of CO2-equivalent per year, GtCO2/yr) from fossil fuel combustion, cement production and flaring, and forestry and other land use (FOLU), 1750–2011. Cumulative emissions and their uncertainties are shown as bars and whiskers, respectively, on the right-hand side. The global effects of the accumulation of methane (CH4) and nitrous oxide (N2O) emissions are shown in Figure 1.3. Greenhouse gas emission data from 1970 to 2010 are shown in Figure 1.6. {modified from WGI Figure TS.4 and WGIII Figure TS.2}

CO2 emissions from fossil fuel combustion and industrial processes contributed about 78% to the total GHG emission increase between 1970 and 2010, with a contribution of similar percentage over the 2000–2010 period (high confidence). Fossil-fuel-related CO2 emissions reached 32 (±2.7) GtCO2/yr, in 2010, and grew further by about 3% between 2010 and 2011, and by about 1 to 2% between 2011 and 2012. CO2 remains the major anthropogenic GHG, accounting for 76% of total anthropogenic GHG emissions in 2010. Of the total, 16% comes from CH4, 6.2% from N2O, and 2.0% from fluorinated gases (F-gases) (Figure 1.6)6Annually, since 1970, about 25% of anthropogenic GHG emissions have been in the form of non-CO2 gases.7 {WGIII SPM.3, 1.2, 5.2}

Figure 1.6


Figure 1.6 | Total annual anthropogenic greenhouse gas (GHG) emissions (gigatonne of CO2-equivalent per year, GtCO2-eq/yr) for the period 1970 to 2010, by gases: CO2 from fossil fuel combustion and industrial processes; CO2 from Forestry and Other Land Use (FOLU); methane (CH4); nitrous oxide (N2O); fluorinated gases covered under the Kyoto Protocol (F-gases). Right hand side shows 2010 emissions, using alternatively CO2-equivalent emission weightings based on IPCC Second Assessment Report (SAR) and AR5 values. Unless otherwise stated, CO2-equivalent emissions in this report include the basket of Kyoto gases (CO2, CH4, N2O as well as F-gases) calculated based on 100-year Global Warming Potential (GWP100) values from the SAR (see Glossary). Using the most recent GWP100 values from the AR5 (right-hand bars) would result in higher total annual GHG emissions (52 GtCO2-eq/yr) from an increased contribution of methane, but does not change the long-term trend significantly. Other metric choices would change the contributions of different gases (see Box 3.2). The 2010 values are shown again broken down into their components with the associated uncertainties (90% confidence interval) indicated by the error bars. Global CO2 emissions from fossil fuel combustion are known with an 8% uncertainty margin (90% confidence interval). There are very large uncertainties (of the order of ±50%) attached to the CO2 emissions from FOLU. Uncertainty about the global emissions of CH4, N2O and the F-gases has been estimated at 20%, 60% and 20%, respectively. 2010 was the most recent year for which emission statistics on all gases as well as assessments of uncertainties were essentially complete at the time of data cut off for this report. The uncertainty estimates only account for uncertainty in emissions, not in the GWPs (as given in WGI 8.7). {WGIII Figure SPM.1}

Total annual anthropogenic GHG emissions have increased by about 10 GtCO2-eq between 2000 and 2010. This increase directly came from the energy (47%), industry (30%), transport (11%) and building (3%) sectors (medium confidence). Accounting for indirect emissions raises the contributions by the building and industry sectors (high confidence). Since 2000, GHG emissions have been growing in all sectors, except in agriculture, forestry and other land use (AFOLU)8. In 2010, 35% of GHG emissions were released by the energy sector, 24% (net emissions) from AFOLU, 21% by industry, 14% by transport and 6.4% by the building sector. When emissions from electricity and heat production are attributed to the sectors that use the final energy (i.e., indirect emissions), the shares of the industry and building sectors in global GHG emissions are increased to 31% and 19%, respectively (Figure 1.7). {WGIII SPM.3, 7.3, 8.1, 9.2, 10.3, 11.2} See also Box 3.2 for contributions from various sectors, based on metrics other than 100-year Global Warming Potential (GWP100).

Figure 1.7


Figure 1.7 | Total anthropogenic greenhouse gas (GHG) emissions (gigatonne of CO2-equivalent per year, GtCO2-eq/yr) from economic sectors in 2010. The circle shows the shares of direct GHG emissions (in % of total anthropogenic GHG emissions) from five economic sectors in 2010. The pull-out shows how shares of indirect CO2 emissions (in % of total anthropogenic GHG emissions) from electricity and heat production are attributed to sectors of final energy use. ‘Other energy’ refers to all GHG emission sources in the energy sector as defined in WGIII Annex II, other than electricity and heat production {WGIII Annex II.9.1}. The emission data on agriculture, forestry and other land use (AFOLU) includes land-based CO2 emissions from forest fires, peat fires and peat decay that approximate to net CO2 flux from the sub-sectors of forestry and other land use (FOLU) as described in Chapter 11 of the WGIII report. Emissions are converted into CO2-equivalents based on 100-year Global Warming Potential (GWP100), taken from the IPCC Second Assessment Report (SAR). Sector definitions are provided in WGIII Annex II.9{WGIII Figure SPM.2}

Globally, economic and population growth continue to be the most important drivers of increases in CO2 emissions from fossil fuel combustion. The contribution of population growth between 2000 and 2010 remained roughly identical to that of the previous three decades, while the contribution of economic growth has risen sharply (high confidence). Between 2000 and  2010, both drivers outpaced emission reductions from improvements in energy intensity of gross domestic product (GDP) (Figure 1.8). Increased use of coal relative to other energy sources has reversed the long-standing trend in gradual decarbonization (i.e., reducing the carbon intensity of energy) of the world’s energy supply. {WGIII SPM.3, TS.2.2, 1.3, 5.3, 7.2, 7.3, 14.3}


1.3. Attribution of climate changes and impacts

The evidence for human influence on the climate system has grown since AR4. Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, and in global mean sea level rise; and it is extremely likely to have been the dominant cause of the observed warming since the mid-20th century. In recent decades, changes in climate have caused impacts on natural and human systems on all continents and across the oceans. Impacts are due to observed climate change, irrespective of its cause, indicating the sensitivity of natural and human systems to changing climate. 



The causes of observed changes in the climate system, as well as in any natural or human system impacted by climate, are established following a consistent set of methods. Detection addresses the question of whether climate or a natural or human system affected by climate has actually changed in a statistical sense, while attribution evaluates the relative contributions of multiple causal factors to an observed change or event with an assignment of statistical confidence9. Attribution of climate change to causes quantifies the links between observed climate change and human activity, as well as other, natural, climate drivers. In contrast, attribution of observed impacts to climate change considers the links between observed changes in natural or human systems and observed climate change, regardless of its cause. Results from studies attributing climate change to causes provide estimates of the magnitude of warming in response to changes in radiative forcing and hence support projections of future climate change (Topic 2). Results from studies attributing impacts to climate change provide strong indications for the sensitivity of natural or human systems to future climate change. {WGI 10.8, WGII SPM A-1, WGI/II/III/SYR Glossaries}


Figure 1.8


Figure 1.8 | Decomposition of the change in total annual carbon dioxide (CO2) emissions from fossil fuel combustion by decade and four driving factors: population, income (gross domestic product, GDP) per capita, energy intensity of GDP and carbon intensity of energy. The bar segments show the changes associated with each individual factor, holding the respective other factors constant. Total emission changes are indicated by a triangle. The change in emissions over each decade is measured in gigatonnes of CO2 per year (GtCO2/yr); income is converted into common units, using purchasing power parities. {WGIII SPM.3}

1.3.1. Attribution of climate changes to human and natural influences on the climate system

It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in GHG concentrations and other anthropogenic forcings together (Figure 1.9).  The best estimate of the human induced contribution to warming is similar to the observed warming over this period. GHGs contributed a global mean surface warming likely to be in the range of 0.5°C to 1.3°C over the period 1951 to 2010, with further contributions from other anthropogenic forcings, including the cooling effect of aerosols, from natural forcings, and from natural internal variability (see Figure 1.9). Together these assessed contributions are consistent with the observed warming of approximately 0.6°C to 0.7°C over this period. {WGI SPM D.3, 10.3.1}

Figure 1.9


Figure 1.9 | Assessed likely ranges (whiskers) and their mid-points (bars) for warming trends over the 1951–2010 period from well-mixed greenhouse gases, other anthropogenic forcings (including the cooling effect of aerosols and the effect of land use change), combined anthropogenic forcings, natural forcings, and natural internal climate variability (which is the element of climate variability that arises spontaneously within the climate system, even in the absence of forcings). The observed surface temperature change is shown in black, with the 5%– 95% uncertainty range due to observational uncertainty. The attributed warming ranges (colours) are based on observations combined with climate model simulations, in order to estimate the contribution by an individual external forcing to the observed warming. The contribution from the combined anthropogenic forcings can be estimated with less uncertainty than the separate contributions from greenhouse gases and other anthropogenic forcings separately. This is because these two contributions are partially compensational, resulting in a signal that is better constrained by observations. {Based on Figure WGI TS.10}

It is very likely that anthropogenic influence, particularly GHGs and stratospheric ozone depletion, has led to a detectable observed pattern of tropospheric warming and a corresponding cooling in the lower stratosphere since 1961. {WGI SPM D.3, 2.4.4, 9.4.1, 10.3.1}

Over every continental region except Antarctica, anthropogenic forcings have likely made a substantial contribution to surface temperature increases since the mid-20th century (Figure 1.10). For Antarctica, large observational uncertainties result in low confidence that anthropogenic forcings have contributed to the observed warming averaged over available stations. In contrast, it is likely that there has been an anthropogenic contribution to the very substantial Arctic warming since the mid-20th century. Human influence has likely contributed to temperature increases in many sub-continental regions. {WGI SPM D.3TS.4.8, 10.3.1}

Anthropogenic influences have very likely contributed to Arctic sea ice loss since 1979 (Figure 1.10). There is low confidence in the scientific understanding of the small observed increase in Antarctic sea ice extent due to the incomplete and competing scientific explanations for the causes of change and low confidence in estimates of natural internal variability in that region. {WGI SPM D.3, 10.5.1, Figure 10.16}

Anthropogenic influences likely contributed to the retreat of glaciers since the 1960s and to the increased surface melting of the Greenland ice sheet since 1993. Due to a low level of scientific understanding, however, there is low confidence in attributing the causes of the observed loss of mass from the Antarctic ice sheet over the past two decades. It is likely that there has been an anthropogenic contribution to observed reductions in Northern Hemisphere spring snow cover since 1970. {WGI 4.3.3, 10.5.2, 10.5.3}

It is likely that anthropogenic influences have affected the global water cycle since 1960. Anthropogenic influences have contributed to observed increases in atmospheric moisture content (medium confidence), to global-scale changes in precipitation patterns over land (medium confidence), to intensification of heavy precipitation over land regions where data are sufficient (medium confidence) (see 1.4), and to changes in surface and subsurface ocean salinity (very likely). {WGI SPM D.3, 2.5.1, 2.6.2, 3.3.2, 3.3.3, 7.6.2, 10.3.2, 10.4.2, 10.6}

It is very likely that anthropogenic forcings have made a substantial contribution to increases in global upper ocean heat content (0–700 m) observed since the 1970s (Figure 1.10). There is evidence for human influence in some individual ocean basins. It is very likely that there is a substantial anthropogenic contribution to the global mean sea level rise since the 1970s. This is based on the high confidence in an anthropogenic influence on the two largest contributions to sea level rise: thermal expansion and glacier mass loss. Oceanic uptake of anthropogenic carbon dioxide has resulted in gradual acidification of ocean surface waters (high confidence). {WGI SPM D.3, 3.2.3, 3.8.2, 10.4.1, 10.4.3, 10.4.4, 10.5.2, 13.3, Box 3.2, TS.4.4, WGII CC-OA}

Figure 1.10


Figure 1.10 | Comparison of observed and simulated change in continental surface temperatures on land (yellow panels), Arctic and Antarctic September sea ice extent (white panels), and upper ocean heat content in the major ocean basins (blue panels). Global average changes are also given. Anomalies are given relative to 1880–1919 for surface temperatures, to 1960–1980 for ocean heat content, and to 1979–1999 for sea ice. All time series are decadal averages, plotted at the centre of the decade. For temperature panels, observations are dashed lines if the spatial coverage of areas being examined is below 50%. For ocean heat content and sea ice panels, the solid lines are where the coverage of data is good and higher in quality, and the dashed lines are where the data coverage is only adequate, and, thus, uncertainty is larger (note that different lines indicate different data sets; for details, see WGI Figure SPM.6). Model results shown are Coupled Model Intercomparison Project Phase 5 (CMIP5) multi-model ensemble ranges, with shaded bands indicating the 5 to 95% confidence intervals. {WGI Figure SPM 6; for detail, see WGI Figure TS.12}


1.3.2. Observed impacts attributed to climate change

In recent decades, changes in climate have caused impacts on natural and human systems on all continents and across the oceans. Impacts are due to observed climate change, irrespective of its cause, indicating the sensitivity of natural and human systems to changing climate. Evidence of observed climate change impacts is strongest and most comprehensive for natural systems. Some impacts on human systems have also been attributed to climate change, with a major or minor contribution of climate change distinguishable from other influences (Figure 1.11).  Impacts on human systems are often geographically heterogeneous because they depend not only on changes in climate variables but also on social and economic factors. Hence, the changes are more easily observed at local levels, while attribution can remain difficult. {WGII SPM A-1A-3, 18.1, 18.3-6}

Figure 1.11


Figure 1.11 | Widespread impacts in a changing world: (a) Based on the available scientific literature since the IPCC Fourth Assessment Report (AR4), there are substantially more impacts in recent decades now attributed to climate change. Attribution requires defined scientific evidence on the role of climate change. Absence from the map of additional impacts attributed to climate change does not imply that such impacts have not occurred. The publications supporting attributed impacts reflect a growing knowledge base, but publications are still limited for many regions, systems and processes, highlighting gaps in data and studies. Symbols indicate categories of attributed impacts, the relative contribution of climate change (major or minor) to the observed impact and confidence in attribution. Each symbol refers to one or more entries in WGII Table SPM.A1, grouping related regional-scale impacts. Numbers in ovals indicate regional totals of climate change publications from 2001 to 2010, based on the Scopus bibliographic database for publications in English with individual countries mentioned in title, abstract or key words (as of July 2011). These numbers provide an overall measure of the available scientific literature on climate change across regions; they do not indicate the number of publications supporting attribution of climate change impacts in each region. Studies for polar regions and small islands are grouped with neighbouring continental regions. The inclusion of publications for assessment of attribution followed IPCC scientific evidence criteria defined in WGII Chapter 18. Publications considered in the attribution analyses come from a broader range of literature assessed in the WGII AR5. See WGII Table SPM.A1 for descriptions of the attributed impacts. (b) Average rates of change in distribution (km per decade) for marine taxonomic groups based on observations over 1900–2010. Positive distribution changes are consistent with warming (moving into previously cooler waters, generally poleward). The number of responses analysed is given for each category. (c) Summary of estimated impacts of observed climate changes on yields over 1960–2013 for four major crops in temperate and tropical regions, with the number of data points analysed given within parentheses for each category. {WGII Figure SPM.2, Box TS.1 Figure 1}

In many regions, changing precipitation or melting snow and ice are altering hydrological systems, affecting water resources in terms of quantity and quality (medium confidence). Glaciers continue to shrink almost worldwide due to climate change (high confidence), affecting runoff and water resources downstream (medium confidence). Climate change is causing permafrost warming and thawing in high-latitude regions and in high-elevation regions (high confidence). {WGII SPM A-1}

Many terrestrial, freshwater and marine species have shifted their geographic ranges, seasonal activities, migration patterns, abundances and species interactions in response to ongoing climate change (high confidence). While only a few recent species extinctions have been attributed as yet to climate change (high confidence), natural global climate change at rates slower than current anthropogenic climate change caused significant ecosystem shifts and species extinctions during the past millions of years (high confidence). Increased tree mortality, observed in many places worldwide, has been attributed to climate change in some regions. Increases in the frequency or intensity of ecosystem disturbances such as droughts, windstorms, fires and pest outbreaks have been detected in many parts of the world and in some cases are attributed to climate change (medium confidence). Numerous observations over the last decades in all ocean basins show changes in abundance, distribution shifts poleward and/ or to deeper, cooler waters for marine fishes, invertebrates and phytoplankton (very high confidence), and altered ecosystem composition (high confidence), tracking climate trends. Some warm-water corals and their reefs have responded to warming with species replacement, bleaching, and decreased coral cover causing habitat loss (high confidence). Some impacts of ocean acidification on marine organisms have been attributed to human influence, from the thinning of pteropod and foraminiferan shells (medium confidence) to the declining growth rates of corals (low confidence). Oxygen minimum zones are progressively expanding in the tropical Pacific, Atlantic and Indian Oceans, due to reduced ventilation and O2 solubility in warmer, more stratified oceans, and are constraining fish habitat (medium confidence). {WGII SPM A-1, Table SPM.A1TS A-1,, 6.3.3, 18.3-18.4,, Box CC-OA, Box CC-CR}

Assessment of many studies covering a wide range of regions and crops shows that negative impacts of climate change on crop yields have been more common than positive impacts (high confidence). The smaller number of studies showing positive impacts relate mainly to high-latitude regions, though it is not yet clear whether the balance of impacts has been negative or positive in these regions (high confidence). Climate change has negatively affected wheat and maize yields for many regions and in the global aggregate (medium confidence). Effects on rice and soybean yield have been smaller in major production regions and globally, with a median change of zero across all available data which are fewer for soy compared to the other crops. (See Figure 1.11c) Observed impacts relate mainly to production aspects of food security rather than access or other components of food security. Since AR4, several periods of rapid food and cereal price increases following climate extremes in key producing regions indicate a sensitivity of current markets to climate extremes among other factors (medium confidence). {WGII SPM A-1}

At present the worldwide burden of human ill-health from climate change is relatively small compared with effects of other stressors and is not well quantified. However, there has been increased heat-related mortality and decreased cold-related mortality in some regions as a result of warming (medium confidence). Local changes in temperature and rainfall have altered the distribution of some water-borne illnesses and disease vectors (medium confidence). {WGII SPM A-1}

‘Cascading’ impacts of climate change can now be attributed along chains of evidence from physical climate through to intermediate systems and then to people. (Figure 1.12) The changes in climate feeding into the cascade, in some cases, are linked to human drivers (e.g., a decreasing amount of water in spring snowpack in western North America), while, in other cases, assessments of the causes of observed climate change leading into the cascade are not available. In all cases, confidence in detection and attribution to observed climate change decreases for effects further down each impact chain. {WGII 18.6.3}


Figure 1.12


Figure 1.12 | Major systems where new evidence indicates interconnected, ‘cascading’ impacts from recent climate change through several natural and human subsystems. Bracketed text indicates confidence in the detection of a climate change effect and the attribution of observed impacts to climate change. The role of climate change can be major (solid arrow) or minor (dashed arrow). Initial evidence indicates that ocean acidification is following similar trends with respect to impact on human systems as ocean warming. {WGII Figure 18-4}

1.4. Extreme events

Changes in many extreme weather and climate events have been observed since about 1950. Some of these changes have been linked to human influences, including a decrease in cold temperature extremes, an increase in warm temperature extremes, an increase in extreme high sea levels and an increase in the number of heavy precipitation events in a number of regions.


It is very likely that the number of cold days and nights has decreased and the number of warm days and nights has increased on the global scale. It is likely that the frequency of heat waves has increased in large parts of Europe, Asia and Australia. It is very likely that human influence has contributed to the observed global scale changes in the frequency and intensity of daily temperature extremes since the mid-20th century. It is likely that human influence has more than doubled the probability of occurrence of heat waves in some locations. {WGI SPM B.1, SPM D.3, Table SPM.1, FAQ 2.2, 2.6.1, 10.6}

There is medium confidence that the observed warming has increased heat-related human mortality and decreased cold-related human mortality in some regions. Extreme heat events currently result in increases in mortality and morbidity in North America (very high confidence), and in Europe with impacts that vary according to people’s age, location and socio-economic factors (high confidence). {WGII SPM A-111.4.1, Table 23-1,

There are likely more land regions where the number of heavy precipitation events has increased than where it has decreased. The frequency and intensity of heavy precipitation events has likely increased in North America and Europe. In other continents, confidence in trends is at most medium. It is very likely that global near-surface and tropospheric air specific humidity has increased since the 1970s. In land regions where observational coverage is sufficient for assessment, there is medium confidence that anthropogenic forcing has contributed to a global-scale intensification of heavy precipitation over the second half of the 20th century. {WGI SPM B-1, 2.5.1, 2.5.4-2.5.5, 2.6.2, 10.6, Table SPM.1, FAQ 2.2, SREX Table 3-1, 3.2}

There is low confidence that anthropogenic climate change has affected the frequency and magnitude of fluvial floods on a global scale. The strength of the evidence is limited mainly by a lack of long-term records from unmanaged catchments. Moreover, floods are strongly influenced by many human activities impacting catchments, making the attribution of detected changes to climate change difficult. However, recent detection of increasing trends in extreme precipitation and discharges in some catchments implies greater risks of flooding on a regional scale (medium confidence). Costs related to flood damage, worldwide, have been increasing since the 1970s, although this is partly due to the increasing exposure of people and assets. {WGI 2.6.2, WGII 3.2.7, SREX SPM B}

There is low confidence in observed global-scale trends in droughts, due to lack of direct observations, dependencies of inferred trends on the choice of the definition for drought, and due to geographical inconsistencies in drought trends. There is also low confidence in the attribution of changes in drought over global land areas since the mid-20th century, due to the same observational uncertainties and difficulties in distinguishing decadal scale variability in drought from long-term trends. {WGI Table SPM.1,, 10.6, Figure 2.33, WGII 3.ES, 3.2.7}

There is low confidence that long-term changes in tropical cyclone activity are robust, and there is low confidence in the attribution of global changes to any particular cause. However, it is virtually certain that intense tropical cyclone activity has increased in the North Atlantic since 1970. {WGI Table SPM.1, 2.6.3, 10.6}

It is likely that extreme sea levels (for example, as experienced in storm surges) have increased since 1970, being mainly the result of mean sea level rise. Due to a shortage of studies and the difficulty of distinguishing any such impacts from other modifications to coastal systems, limited evidence is available on the impacts of sea level rise.{WGI 3.7.4-3.7.6Figure 3.15, WGII, 18.3}

Impacts from recent climate-related extremes, such as heat waves, droughts, floods, cyclones and wildfires, reveal significant vulnerability and exposure of some ecosystems and many human systems to current climate variability (very high confidence). Impacts of such climate-related extremes include alteration of ecosystems, disruption of food production and water supply, damage to infrastructure and settlements, human morbidity and mortality and consequences for mental health and human well-being. For countries at all levels of development, these impacts are consistent with a significant lack of preparedness for current climate variability in some sectors. {WGII SPM A-1, 3.2, 4.2-3, 8.1, 9.3, 10.7, 11.3, 11.7, 13.2, 14.1, 18.6, 22.2.3, 22.3,,, 25.6-8, 26.6-7, 30.5, Table 18-3, Table 23-1, Figure 26-2, Box 4-3, Box 4-4, Box 25-5, Box 25-6, Box 25-8, and Box CC-CR}

Direct and insured losses from weather-related disasters have increased substantially in recent decades, both globally and regionally. Increasing exposure of people and economic assets has been the major cause of long-term increases in economic losses from weather- and climate-related disasters (high confidence). {WGII 10.7.3, SREX SPM B4.5.3.3}


1.5. Exposure and vulnerability

The character and severity of impacts from climate change and extreme events emerge from risk that depends not only on climate-related hazards but also on exposure (people and assets at risk) and vulnerability (susceptibility to harm) of human and natural systems.


Exposure and vulnerability are influenced by a wide range of social, economic and cultural factors and processes that have been incompletely considered to date and that make quantitative assessments of their future trends difficult (high confidence). These factors include wealth and its distribution across society, demographics, migration, access to technology and information, employment patterns, the quality of adaptive responses, societal values, governance structures and institutions to resolve conflict. {WGII SPM A-3SREX SPM B}

Differences in vulnerability and exposure arise from non-climatic factors and from multidimensional inequalities often produced by uneven development processes (very high confidence). These differences shape differential risks from climate change. People who are socially, economically, culturally, politically, institutionally or otherwise marginalized are especially vulnerable to climate change and also to some adaptation and mitigation responses (medium evidence, high agreement). This heightened vulnerability is rarely due to a single cause. Rather, it is the product of intersecting social processes that result in inequalities in socio-economic status and income, as well as in exposure. Such social processes include, for example, discrimination on the basis of gender, class, ethnicity, age and (dis)ability. {WGII SPM A-1, Figure SPM.1, 8.1-8.2, 9.3-9.4, 10.9, 11.1, 11.3-11.5, 12.2-12.5, 13.1-13.3, 14.1-14.3, 18.4, 19.6, 23.5, 25.8, 26.6, 26.8, 28.4, Box CC-GC}

Climate-related hazards exacerbate other stressors, often with negative outcomes for livelihoods, especially for people living in poverty (high confidence). Climate-related hazards affect poor people’s lives directly through impacts on livelihoods, reductions in crop yields or the destruction of homes, and indirectly through, for example, increased food prices and food insecurity. Observed positive effects for poor and marginalized people, which are limited and often indirect, include examples such as diversification of social networks and of agricultural practices. {WGII SPM A-1, 8.2-8.3, 9.3, 11.3, 13.1-13.3, 22.3, 24.4, 26.8}

Violent conflict increases vulnerability to climate change (medium evidence, high agreement). Large-scale violent conflict harms assets that facilitate adaptation, including infrastructure, institutions, natural resources, social capital and livelihood opportunities.  {WGII SPM A-1, 12.5, 19.2, 19.6}


1.6. Human responses to climate change: adaptation and mitigation

Adaptation and mitigation experience is accumulating across regions and scales, even while global anthropogenic greenhouse gas emissions have continued to increase. 


Throughout history, people and societies have adjusted to and coped with climate, climate variability and extremes, with varying degrees of success. In today’s changing climate, accumulating experience with adaptation and mitigation efforts can provide opportunities for learning and refinement (see Topic 3 and Topic 4{WGII SPM A-2}

Adaptation is becoming embedded in some planning processes, with more limited implementation of responses (high confidence). Engineered and technological options are commonly implemented adaptive responses, often integrated within existing programmes, such as disaster risk management and water management. There is increasing recognition of the value of social, institutional and ecosystem-based measures and of the extent of constraints to adaptation. {WGII SPM A-2, 4.4, 5.5, 6.4, 8.3, 9.4, 11.7, 14.1, 14.3-14.4, 15.2-15.5, 17.2-17.3, 21.3, 21.5, 22.4, 23.7, 25.4, 26.8-26.9, 30.6, Box 25-1, Box 25-2, Box 25-9, Box CC-EA}

Governments at various levels have begun to develop adaptation plans and policies and integrate climate change considerations into broader development plans. Examples of adaptation are now available from all regions of the world (see Topic 4 for details on adaptation options and policies to support their implementation). {WGII SPM A-2, 22.4, 23.7, 24.4-24.6, 24.9, 25.4, 25.10, 26.7-26.9, 27.3, 28.2, 28.4, 29.3, 29.6, 30.6, Table 25-2, Table 29-3, Figure 29-1, Boxes 5-1, Box 23-3, Box 25-1, Box 25-2, Box 25-9, and Box CC-TC}

Global increases in anthropogenic emissions and climate impacts have occurred, even while mitigation activities have taken place in many parts of the world. Though various mitigation initiatives between the sub-national and global scales have been developed or implemented, a full assessment of their impact may be premature.  {WG III SPM.3, SPM.5}



  1. Ranges in square brackets indicate a 90% uncertainty interval unless otherwise stated. The 90% uncertainty interval is expected to have a 90% likelihood of covering the value that is being estimated. Uncertainty intervals are not necessarily symmetric about the corresponding best estimate. A best estimate of that value is also given where available.
  2. The connection between the heat budget and equilibrium climate sensitivity, which is the long-term surface warming under an assumed doubling of the atmospheric CO2 concentration, arises because a warmer surface causes enhanced radiation to space, which counteracts the increase in the Earth’s heat content. How much the radiation to space increases for a given increase in surface temperature, depends on the same feedback processes (e.g., cloud feedback, water vapour feedback) that determine equilibrium climate sensitivity.
  3. Forestry and other land use (FOLU)—also referred to as LULUCF (land use, land-use change and forestry)—is the subset of agriculture, forestry and other land use (AFOLU) emissions and removals of GHGs related to direct human-induced LULUCF activities, excluding agricultural emissions and removals (see WGIII AR5 Glossary).
  4. Numbers from WGI 6.3 converted into GtCO2 units. Small differences in cumulative emissions from Working Group 3 {WGIII SPM.3, TS.2.1} are due to different approaches to rounding, different end years and the use of different data sets for emissions from FOLU. Estimates remain extremely close, given their uncertainties.
  5.  CO2-equivalent emission is a common scale for comparing emissions of different GHGs. Throughout the SYR, when historical emissions of GHGs are provided in GtCO2-eq, they are weighted by Global Warming Potentials with a 100-year time horizon (GWP100), taken from the IPCC Second Assessment Report (SAR) unless otherwise stated. A unit abbreviation of GtCO2-eq is used. {Box 3.2, Glossary}
  6. Using the most recent 100-year Global Warming Potential (GWP100) values from the AR5 {WGI 8.7} instead of GWP100 values from the IPCC Second Assessment Report, global GHG emission totals would be slightly higher (52 GtCO2-eq/yr) and non-CO2 emission shares would be 20% for CH4, 5% for N2O and 2.2% for F-gases.
  7.  For this report, data on non-CO2 GHGs, including F-gases, were taken from the Electronic Data Gathering, Analysis, and Retrieval (EDGAR) database {WGIII Annex II.9}, which covers substances included in the Kyoto Protocol in its first commitment period.
  8.   Forestry and other land use (FOLU)—also referred to as LULUCF (land use, land-use change and forestry)—is the subset of agriculture, forestry and other land use (AFOLU) emissions and removals of GHGs related to direct human-induced LULUCF activities, excluding agricultural emissions and removals (see WGIII AR5 Glossary).
  9. Definitions were taken from the ‘Good Practice Guidance Paper on Detection and Attribution, the agreed product of the IPCC Expert Meeting on Detection and Attribution Related to Anthropogenic Climate Change’; see glossary

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Referencing this report

IPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel o Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland, 151 pp. in IPCC AR5 Synthesis Report website


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