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Changement Climatique - Mise à jour GIEC 2013

2. What have been the observed climate changes in the last centuries?

  • 2.1 How has the global mean temperature on earth evolved?
  • 2.2 How have precipitations and droughts globally changed in the last decades?
  • 2.3 Has the salinity, acidity and oxygen content of oceans changed?
  • 2.4 How has the earth’s ice cover evolved in the last decades?
  • 2.5 Has the level of the oceans changed?
  • 2.6 Have extreme weather events increased in the last decades?
  • 2.7 How have emissions of greenhouse gases evolved in the last centuries?

2.1 How has the global mean temperature on earth evolved?

The source document for this Digest states:

TS.2 Observation of Changes in the Climate System

TS.2.1 Introduction

Observations of the climate system are based on direct physical and biogeochemical measurements, remote sensing from ground stations and satellites; information derived from paleoclimate archives provides a long- term context. Global-scale observations from the instrumental era began in the mid-19th century, and paleoclimate reconstructions extend the record of some quantities back hundreds to millions of years. Together, they provide a comprehensive view of the variability and long-term changes in the atmosphere, the ocean, the cryosphere, and the land surface.

The assessment of observational evidence for climate change is summarized in this section. Substantial advancements in the availability, acquisition, quality and analysis of observational data sets in atmosphere, land surface, ocean, and cryosphere have occurred since the AR4. Many aspects of the climate system are showing evidence of a changing climate.

TS.2.2 Changes in Temperature

TS.2.2.1 Surface

It is certain that Global Mean Surface Temperature (GMST) has increased since the late 19th century (Figure TS.1). Each of the past three decades has been successively warmer at the Earth’s surface than all the previous decades in the instrumental record, and the decade of the 2000’s has been the warmest. The global combined land and ocean temperature data show an increase of about 0.85 [0.65 to 1.06] °C over 1880–2012 and about 0.72°C [0.49- 0.89] over the period 1951–2012 when described by a linear trend and based on three independently-produced data sets 6. The total increase between the average of the 1850–1900 period and the 2003–2012 period is 0.78°C [0.72 to 0.85°C], based on HadCRUT4, the global mean surface temperature dataset with the longest record of the three independently-produced data sets. Both methods used to calculate temperature change were also used in AR4. The first calculates the difference using a best fit linear trend of all points between 1880 and 2012. The second calculates the difference between averages for the two periods 1850 to 1900 and 2003 to 2012. Therefore, the resulting values and their 90% uncertainty intervals are not directly comparable. The warming from 1850–1900 to 1986–2005 (reference period for the modelling chapters and the Atlas in Annex 1) is 0.61°C [0.55 to 0.67], when calculated using HadCRUT4 and its uncertainty estimates. It is also virtually certain that maximum and minimum temperatures over land have increased on a global scale since 1950. {2.4.1, 2.4.3, Supplementary Material 2.SM.3}

Despite the robust multi-decadal timescale warming, there exists substantial interannual to decadal variability in the rate of warming, with several periods exhibiting weaker trends (including the warming hiatus since 1998) (Figure TS.1). The rate of warming over the past 15 years (1998–2012; 0.05°C per decade [–0.05 to +0.15]) is smaller than the trend since 1951 (1951–2012; 0.12°C per decade [0.08 to 0.14])6. Trends for short periods are uncertain and very sensitive to the start and end years. For example, trends for 15-year periods starting in 1995, 1996, and 1997 are 0.13 [0.02 to 0.24], 0.14 [0.03 to 0.24] and 0.07 [-0.02 to 0.18], respectively. Several independently analysed data records of global and regional land surface air temperature obtained from station observations are in broad agreement that land surface air temperature s have increased. Sea surface temperatures have also increased. Intercomparisons of new sea surface temperature data records obtained by different measurement methods, including satellite data, have resulted in better understanding of errors and biases in the records. {2.4.1, 2.4.2, 2.4.3; Box 9.2}

It is unlikely that any uncorrected urban heat-island effects and land use change effects have raised the estimated centennial globally averaged land surface air temperature trends by more than 10% of the reported trend. This is an average value; in some regions that have rapidly developed urban heat island and land use change impacts on regional trends may be substantially larger. {2.4.1}

There is high confidence that annual mean surface warming since the 20th century has reversed long-term cooling trends of the past 5,000 years in mid-to-high latitudes of the Northern Hemisphere. For average annual Northern Hemisphere temperatures, the period 1983–2012 was very likely the warmest 30-year period of the last 800 years (high confidence) and likely the warmest 30-year period of the last 1400 years (medium confidence). This is supported by comparison of instrumental temperatures with multiple reconstructions from a variety of proxy data and statistical methods, and is consistent with AR4. Continental-scale surface temperature reconstructions show, with high confidence, multidecadal periods during the Medieval Climate Anomaly (950 to 1250) that were in some regions as warm as in the mid-20th century and in others as warm as in the late 20th century. With high confidence, these regional warm periods were not as synchronous across regions as the warming since the mid-20th century. Based on the comparison between reconstructions and simulations, there is high confidence that not only external orbital, solar and volcanic forcing, but also internal variability, contributed substantially to the spatial pattern and timing of surface-temperature changes between the Medieval Climate Anomaly and the Little Ice Age (1450 to 1850). {5.3.5, 5.5.1}

TS.2.2.2 Troposphere and Stratosphere

Based upon multiple independent analyses of measurements from radiosondes and satellite sensors, it is virtually certain that globally the troposphere has warmed and the stratosphere has cooled since the mid-20th century (Figure TS.1). Despite unanimous agreement on the sign of the trends, substantial disagreement exists between available estimates as to the rate of temperature changes, particularly outside the Northern Hemisphere extra-tropical troposphere, which has been well sampled by radiosondes. Hence there is only medium confidence in the rate of change and its vertical structure in the Northern Hemisphere extra-tropical troposphere and low confidence elsewhere. {2.4.4}

TS.2.2.3 Ocean

It is virtually certain that the upper ocean (above 700 m) has warmed from 1971 to 2010, and likely that it has warmed from the 1870s to 1971 (Figure TS.1). There is less certainty in changes prior to 1971 because of relatively sparse sampling in earlier time periods. Instrumental biases in historical upper ocean temperature measurements have been identified and mitigated since AR4, reducing artificial decadal variation in temperature and upper ocean heat content, most prominent during the 1970s and 1980s. {3.2.1, 3.2.2, 3.2.3, 3.5.3} It is likely that the ocean warmed between 700 and 2000 m from 1957 to 2010, based on five-year averages. It is likely that the ocean warmed from 3000 m to the bottom from 1992 to 2005, when sufficient observations became available for a global assessment. No significant trends in global average temperature were observed between 2000 and 3000 m depth for either overlapping time period. The largest changes in deep ocean temperature have been observed close to the sources of deep and bottom water in the northern North Atlantic and especially in the Southern Ocean with anomaly amplitudes lessening along the routes through which these waters spread. {3.2.4, 3.5.1}

TS.2.3 Changes in Energy Budget and Heat Content

Earth has been in radiative imbalance, with more energy from the sun entering than exiting the top of the atmosphere, since at least circa 1970. It is virtually certain that Earth has gained substantial energy from 1971–2010. The estimated increase in energy inventory between 1971 and 2010 is 274 [196 to 351] x 1021 J), with a heating rate of 213 x 1012 W from a linear fit to the annual values over that time period (see also TFE.4). Ocean warming dominates that total heating rate, with full ocean depth warming accounting for about 93% (and warming of the upper (0–700 m) ocean accounting for about 64%). Melting ice (including Arctic sea ice, ice sheets, and glaciers) and warming of the continents each account for 3 of the total. Warming of the atmosphere makes up the remaining 1%. The 1971–2010 estimated rate of ocean energy gain is 199 x 1012 W from a linear fit to data over that time period, equivalent to 0.42 W m–2 heating applied continuously over Earth's entire surface, and 0.55 W m–2 for the portion owing to ocean warming applied over the ocean's entire surface area. Earth's estimated energy increase from 1993–2010 is 163 [127 to 201]  1021 J with a trend estimate of 275  1015 W. The ocean portion of the trend for 1993–2010 is 257  1012 W, equivalent to a mean heat flux into the ocean of 0.71 W m–2. {3.2.3, 3.2.4; Box 3.1} The rate of ocean warming in some of the 0–700 m estimates was lower from 2003 to 2010 than in the previous decade (Figure TS.1); however, warming in the subsurface layer between 700 and 2000 m likely continued unabated during this period. {3.2.3, 3.2.4; Box 9.2}

TS.2.4 Changes in Circulation and Modes of Variability

Large variability on interannual to decadal time scales hampers robust conclusions on long-term changes in atmospheric circulation in many instances. Confidence is high that the increase of the northern mid-latitude westerly winds and the NAO index from the 1950s to the 1990s, and the weakening of the Pacific Walker circulation from the late 19th century to the 1990s, have been largely offset by recent changes. With high confidence, decadal and multi-decadal changes in the winter North Atlantic Oscillation index (NAO) observed since the 20th century are not unprecedented in the context of the past 500 years. {2.7.2, 2.7.5, 2.7.8, 5.4; Box 2.5; Tables 2.12}

It is likely that circulation features have moved poleward since the 1970s, involving a widening of the tropical belt, a poleward shift of storm tracks and jet streams, and a contraction of the northern polar vortex. Evidence is more robust for the Northern Hemisphere. It is likely that the Southern Annular Mode has become more positive since the 1950s. The increase in the strength of the observed summer Southern Annular Mode since 1950 has been anomalous, with medium confidence, in the context of the past 400 years. {2.7.5, 2.7.6, 2.7.8, 5.4; Box 2.5; Tables 2.2–2.12}

New results from high-resolution coral records document with high confidence that the El Niño-Southern Oscillation (ENSO) system has remained highly variable throughout the past 7,000 years, showing no discernible evidence for an orbital modulation of ENSO. {5.4} Recent observations have strengthened evidence for variability in major ocean circulation systems on time scales from years to decades. It is very likely that the subtropical gyres in the North Pacific and South Pacific have expanded and strengthened since 1993. Based on measurements of the full Atlantic Meridional Overturning Circulation (AMOC) and its individual components at various latitudes and different time periods, there is no evidence of a long-term trend. There is also no evidence for trends in the transports of the Indonesian Throughflow, the Antarctic Circumpolar Current (ACC), or in the transports between the Atlantic Ocean and Nordic Seas. However, a southward shift of the ACC by about 1° of latitude is observed in data spanning the time period 1950 to 2010 with medium confidence. {3.6}

Source & ©: IPCC  Working Group I Contribution To The IPCC Fifth Assessment Report Climate Change 2013: The Physical Science Basis. , TS.2.1-2.4, p.37-40.

Box TS.5: Paleoclimate

Reconstructions from paleoclimate archives allow current changes in atmospheric composition, sea level and climate (including extreme events such as droughts and floods), as well as future projections, to be placed in a broader perspective of past climate variability (see Section TS.2). {5, 5.5.5, 10.5.5}

Past climate information also documents the behaviour of slow components of the climate system including the carbon cycle, ice sheets and the deep ocean for which instrumental records are short compared to their characteristic time scales of responses to perturbations, thus informing on mechanisms of abrupt and irreversible changes. Together with the knowledge of past external climate forcings, syntheses of paleoclimate data have documented polar amplification, characterized by enhanced temperature changes in the Arctic compared to the global mean, in response to high or low CO2 concentrations. {5.2.1, 5.2.2, 5.6, 5.7, 5.8, 6.2, 8.4.2, 13.2.1, 13.4; Box 5.1, 5.2}

Since AR4, the inclusion of paleoclimate simulations in the PMIP3 (Paleoclimate Modelling Intercomparison Project)/CMIP5 framework has enabled paleoclimate information to be more closely linked with future climate projections. Paleoclimate information for the mid-Holocene (6,000 years ago), the Last Glacial Maximum (approximately 21,000 years ago), and last millennium has been used to test the ability of models to simulate realistically the magnitude and large-scale patterns of past changes. Combining information from paleoclimate simulations and reconstructions enables the response of the climate system to radiative perturbations to be quantified, constraints to be placed on the range of equilibrium climate sensitivity, and past patterns of internal climate variability to be documented on inter-annual to multi- centennial scales. {5.3.1, 5.3.2, 5.3.3, 5.3.4, 5.3.5, 5.4, 5.5.1, 9.4.1, 9.4.2, 9.5.3, 9.7.2, 10.7.2, 14.2.2}

Box TS.5, Figure 1 illustrates the comparison between the last millennium PMIP3/CMIP5 simulations and reconstructions, together with the associated solar, volcanic and well-mixed greenhouse gas radiative forcings. For average annual Northern Hemisphere temperatures, the period 1983–2012 was very likely the warmest 30-year period of the last 800 years (high confidence) and likely the warmest 30-year period of the last 1400 years (medium confidence). This is supported by comparison of instrumental temperatures with multiple reconstructions from a variety of proxy data and statistical methods, and is consistent with AR4. In response to solar, volcanic and anthropogenic radiative changes, climate models simulate multi-decadal temperature changes in the last 1200 years in the Northern Hemisphere that are generally consistent in magnitude and timing with reconstructions, within their uncertainty ranges. Continental-scale temperature reconstructions show, with high confidence, multi-decadal intervals during the Medieval Climate Anomaly (ca. 950 to 1250) that were in some regions as warm as the mid-20th century and in others as warm as in the late 20th century. With high confidence, these regional warm periods were not as synchronous across regions as the warming since the mid-20th century. Based on the comparison between reconstructions and simulations, there is high confidence that not only external orbital, solar and volcanic forcing but also internal variability contributed substantially to the spatial pattern and timing of surface temperature changes between the Medieval Climate Anomaly and the Little Ice Age (ca. 1450 to 1850). However, there is only very low confidence in quantitative estimates of their relative contributions. It is very unlikely that northern hemisphere temperature variations from 1400 to 1850 can be explained by internal variability alone. There is medium confidence that external forcing contributed to Northern Hemispheric temperature variability from 850 to 1400 and that external forcing contributed to European temperature variations over the last 5 centuries. {5.3.5, 5.5.1, 10.7.2, 10.7.5; Table 10.1}

Box TS.5, Figure 1: Last-millennium simulations and reconstructions. (a) 850–2000 PMIP3/CMIP5 radiative forcing due to volcanic, solar and well mixed greenhouse gas . Different colors illustrate the two existing datasets for volcanic forcing (CEA and GRA) and the four estimates of solar forcing (DB, MEA, SBF and VSK). For solar forcing, solid (dashed) lines stand for reconstruction variants in which background changes in irradiance are (not) considered; (b) 850–2000 PMIP3/CMIP5 simulated (red) and reconstructed (shading) NH temperature changes. The thick red line depicts the multi-model mean while the thin red lines show the multi-model 90% range. The overlap of reconstructed temperatures is shown by grey shading; all data are expressed as anomalies from their 1500–1850 mean and smoothed with a 30-year filter. Note that some reconstructions represent a smaller spatial domain than the full NH or a specific season, while annual temperatures for the full NH mean are shown for the simulations. (c), (d), (e) and (f) Arctic and North America annual mean temperature, and Europe and Asia June-July-August (JJA) temperature, from 950 to 2000 from reconstructions (black line), and PMIP3/CMIP5 simulations (thick red, multi-model mean; thin red 90% multi- model range). All red curves are expressed as anomalies from their 1500–1850 mean and smoothed with a 30-year filter.The shaded envelope depicts the uncertainties from each reconstruction (Arctic: 90% confidence bands, North American: ±2 standard deviation. Asia: ±2 root mean square error. Europe: 95% confidence bands). For comparison with instrumental record, the CRUTEM4 dataset is shown (yellow line). All lines are smoothed by applying a 30 year moving average. Map shows the individual regions for each reconstruction. {5.3.5; Table 5.A.1; Figures 5.1, 5.8, 5.12}

Source & ©: IPCC  Working Group I Contribution To The IPCC Fifth Assessment Report Climate Change 2013: The Physical Science Basis. , Box TS.5, p.77-78

2.2 How have precipitations and droughts globally changed in the last decades?

The source document for this Digest states:

TS.2.5 Changes in the Water Cycle and Cryosphere

TS.2.5.1 Atmosphere

Confidence in precipitation change averaged over global land areas is low prior to 1951 and medium afterwards because of insufficient data, particularly in the earlier part of the record (for an overview of observed and projected changes in the global water cycle see TFE.1). Further, when virtually all the land area is filled in using a reconstruction method, the resulting time series shows little change in land-based precipitation since 1901. Northern hemisphere mid-latitude land areas do show a likely overall increase in precipitation (medium confidence prior to 1951, but high confidence afterwards). . For other latitudes area-averaged long-term positive or negative trends have low confidence (see Figure SPM.2). {2.5.1} It is very likely that global near surface and tropospheric air specific humidity have increased since the 1970s. However, during recent years the near-surface moistening trend over land has abated (medium confidence) (Figure TS.1). As a result, fairly widespread decreases in relative humidity near the surface are observed over the land in recent years. {2.4.4, 2.5.5, 2.5.6} While trends of cloud cover are consistent between independent data sets in certain regions, substantial ambiguity and therefore low confidence remains in the observations of global-scale cloud variability and trends. {2.5.7}

Source & ©: IPCC  Working Group I Contribution To The IPCC Fifth Assessment Report Climate Change 2013: The Physical Science Basis. , TS.2.5.1, Athmosphere, p. 40.

TFE.1: Water Cycle Change

The water cycle describes the continuous movement of water through the climate system in its liquid, solid and vapour forms, and storage in the reservoirs of ocean, cryosphere, land surface and atmosphere. In the atmosphere, water occurs primarily as a gas, water vapour, but it also occurs as ice and liquid water in clouds. The ocean is primarily liquid water, but the ocean is partly covered by ice in Polar Regions. Terrestrial water in liquid form appears as surface water (lakes, rivers), soil moisture and groundwater. Solid terrestrial water occurs in ice sheets, glaciers, snow and ice on the surface and permafrost. The movement of water in the climate system is essential to life on land, since much of the water that falls on land as precipitation and supplies the soil moisture and river flow has been evaporated from the ocean and transported to land by the atmosphere. Water that falls as snow in winter can provide soil moisture in springtime and river flow in summer and is essential to both natural and human systems. The movement of fresh water between the atmosphere and the ocean can also influence oceanic salinity, which is an important driver of the density and circulation of the ocean. The latent heat contained in water vapour in the atmosphere is critical to driving the circulation of the atmosphere on scales ranging from individual thunderstorms to the global circulation of the atmosphere. {12.4.5; FAQ 3.3, FAQ 12.2}

Observations of Water Cycle Change

Because the saturation vapour pressure of air increases with temperature, it is expected that the amount of water vapour suspended in air will increase with a warming climate. Observations from surface stations, radiosondes, global positioning systems, and satellite measurements indicate increases in tropospheric water vapour at large spatial scales (TFE.1, Figure 1). It is very likely that tropospheric specific humidity has increased since the 1970s. The magnitude of the observed global change in water vapour of about 3.5% in the past 40 years is consistent with the observed temperature change of about 0.5°C during the same period, and the relative humidity has stayed approximately constant. The water vapour change can be attributed to human influence with medium confidence. {2.5.4, 10.3.2}

Changes in precipitation are harder to measure with the existing records, both because of the greater difficulty in sampling precipitation and also because it is expected that precipitation will have a smaller fractional change than the water vapour content of air as the climate warms. When virtually all the land area is filled in using a reconstruction method, the resulting time series shows little change in land-based precipitation since 1900. At present there is medium confidence that there has been a significant human influence on global scale changes in precipitation patterns, including increases in northern hemisphere mid to high latitudes. Changes in the extremes of precipitation, and other climate extremes related to the water cycle are comprehensively discussed in TFE.9. {2.5.1, 10.3.2}

Although direct trends in precipitation and evaporation are difficult to measure with the available records, the observed oceanic surface salinity, which is strongly dependent on the difference between evaporation and precipitation, shows significant trends (TFE.1, Figure 1). The spatial patterns of the salinity trends since 1950, the mean salinity and the mean distribution of evaporation–precipitation are similar to each other: regions of high salinity where evaporation dominates have become more saline, while regions of low salinity where rainfall dominates have become fresher (TFE.1, Figure 1). This provides indirect evidence that the pattern of evaporation-precipitation over the oceans has been enhanced since the 1950s (medium confidence). The inferred changes in evaporation minus precipitation are consistent with the observed increased water vapour content of the warmer air. It is very likely that observed changes in surface and subsurface salinity are due in part to anthropogenic increases in forcings. {2.5, 3.3.2, 3.3.3, 3.3.4, 3.4, 3.9, 10.4.2; FAQ 3.3}

TFE.1, Figure 1: Changes in sea surface salinity are related to the atmospheric patterns of Evaporation minus Precipitation (E-P) and trends in total precipitable water: (a) Linear trend (1988 to 2010) in total precipitable water (water vapor integrated from Earth’s surface up through the entire atmosphere) (kg m–2 per decade) from satellite observations. (b) The 1979–2005 climatological mean net evaporation minus precipitation (cm yr–1) from meteorological reanalysis data. (c) Trend (1950 to 2000) in surface salinity (PSS78 per 50years). (d) The climatological-mean surface salinity (PSS78) (blues <35; yellows-reds >>35). (e) Global difference between salinity averaged over regions where the sea surface salinity is greater than the global mean sea surface salinity (“High Salinity”) and salinity averaged over regions values below the global mean (“Low Salinity”). Details of data sources: see Figure 3.21 and FAQ 3.3, Figure 1. {3.9}

In most regions analyzed, it is likely that decreasing numbers of snowfall events are occurring where increased winter temperatures have been observed. Both satellite and in-situ observations show significant reductions in the Northern Hemisphere snow cover extent over the past 90 years, with most of the reduction occurring in the 1980s. Snow cover decreased most in spring when the average extent decreased by around 8% (7 million km2) over the period 1970–2010 compared with the period 1922–1970. Because of earlier spring snowmelt, the duration of the Northern Hemisphere snow season has declined by 5.3 days per decade since the 1972/1973 winter. It is likely that there has been an anthropogenic component to these observed reductions in snow cover since the 1970s. {4.5.2, 10.5.1, 10.5.3} The most recent and most comprehensive analyses of river runoff do not support the AR4 conclusion that global runoff has increased during the 20th century. New results also indicate that the AR4 conclusions regarding global increasing trends in droughts since the 1970s are no longer supported. {2.5.2, 2.6.2}

Projections of Future Changes

Changes in the water cycle are projected to occur in a warming climate (TFE.1, Figure 2, see also TS 4.6, TS 5.6, Annex I). Global-scale precipitation is projected to gradually increase in the 21st century. It is virtually certain, that precipitation increase will be much smaller, approximately 2% K–1, than the rate of lower tropospheric water vapour increase (~7% K–1), due to global energetic constraints. It is virtually certain that changes of average precipitation in a much warmer world will not be uniform, with some regions experiencing increases, and others with decreases or not much change at all. The high latitudes are likely to experience greater amounts of precipitation due to the additional water carrying capacity of the warmer troposphere. Many mid-latitude arid and semi-arid regions will likely experience less precipitation. The largest precipitation changes over northern Eurasia and North America are projected to occur during the winter. {12.4.5, Annex I}

TFE.1, Figure 2: Annual mean changes in precipitation (P), evaporation (E), relative humidity, E-P, runoff and soil moisture, for 2081–2100 relative to 1986–2005 under the RCP8.5 scenario (see Box TS.6). The number of CMIP5 models to calculate the multi-model mean is indicated in the upper right corner of each panel. Hatching indicates regions where the multi model mean is less than one standard deviation of internal variability. Stippling indicates regions where the multi model mean is greater than two standard deviations of internal variability and where 90% of models agree on the sign of change (see Box 12.1). {Figures 12.25–12.27}

Regional to global-scale projections of soil moisture and drought remain relatively uncertain compared to other aspects of the water cycle. Nonetheless, drying in the Mediterranean, southwestern U.S. and south African regions are consistent with projected changes in Hadley circulation, so drying in these regions as global temperatures increase is likely for several degrees of warming under the RCP8.5 scenario. Decreases in runoff are likely in southern Europe and the Middle East. The high northern latitude runoff increases are likely and consistent with the projected precipitation increases. {12.4.5}

Source & ©: IPCC  Working Group I Contribution To The IPCC Fifth Assessment Report Climate Change 2013: The Physical Science Basis. , TFE.1, p.42-45.

2.3 Has the salinity, acidity and oxygen content of oceans changed?

The source document for this Digest states:

TS.2.5.2 Ocean and Surface Fluxes

It is very likely that regional trends have enhanced the mean geographical contrasts in sea surface salinity since the 1950s: saline surface waters in the evaporation-dominated mid-latitudes have become more saline, while relatively fresh surface waters in rainfall-dominated tropical and polar regions have become fresher. The mean contrast between high and low salinity regions increased by 0.13 [0.08 to 0.17] from 1950 to 2008. It is very likely that the inter-basin contrast in freshwater content has increased: the Atlantic has become saltier and the Pacific and Southern Oceans have freshened. While similar conclusions were reached in AR4, recent studies based on expanded data sets and new analysis approaches provide high confidence in this assessment. {3.3.2, 3.3.3, 3.9; FAQ 3.3}

The spatial patterns of the salinity trends, mean salinity and the mean distribution of evaporation minus precipitation are all similar (see also TFE.1, Figure 1). These similarities provide indirect evidence that the pattern of evaporation minus precipitation over the oceans has been enhanced since the 1950s (medium confidence). Uncertainties in currently available surface fluxes prevent the flux products from being reliably used to identify trends in the regional or global distribution of evaporation or precipitation over the oceans on the timescale of the observed salinity changes since the 1950s. {3.3.2, 3.3.3, 3.3.4, 3.4.2, 3.4.3, 3.9; FAQ 3.3}

Source & ©: IPCC  Working Group I Contribution To The IPCC Fifth Assessment Report Climate Change 2013: The Physical Science Basis. , TS.2.5.2 Ocean and Surface Fluxes. p.40.

2.4 How has the earth’s ice cover evolved in the last decades?

The source document for this Digest states:

TS.2.5.3 Sea Ice

Continuing the trends reported in AR4, there is very high confidence that the Arctic sea ice extent (annual, multiyear and perennial) decreased over the period 1979–2012 (Figure TS.1). The rate of the annual decrease was very likely between 3.5 and 4.1% per decade. The average decrease in decadal extent of annual Arctic sea ice has been most rapid in summer and autumn (high confidence), but the extent has decreased in every season, and in every successive decade since 1979 (high confidence). The extent of Arctic perennial and multiyear ice decreased between 1979 and 2012 (very high confidence}. The rates are very likely 11.5 [9.4 to 13.6] % per decade for the perennial sea ice area at summer minimum and very likely 13.5 [11 to 16] % per decade for multiyear ice. There is medium confidence from reconstructions that the current (1980 to 2012) Arctic summer sea-ice retreat was unprecedented and sea-surface temperatures were anomalously high in the perspective of at least the last 1,450 years. {4.2.2, 5.5.2} It is likely that the annual period of surface melt on Arctic perennial sea ice lengthened by 5.7 [4.8 to 6.6] days per decade over the period 1979–2012. Over this period, in the region between the East Siberian Sea and the western Beaufort Sea, the duration of ice-free conditions increased by nearly 2 months. {4.2.2}

There is high confidence that the average winter sea ice thickness within the Arctic Basin decreased between 1980 and 2008. The average decrease was likely between 1.3 m and 2.3 m. High confidence in this assessment is based on observations from multiple sources: submarine, electromagnetic probes, and satellite altimetry; and is consistent with the decline in multiyear and perennial ice extent. Satellite measurements made in the period 2010–2012 show a decrease in sea ice volume compared to those made over the period 2003–2008 (medium confidence). There is high confidence that in the Arctic, where the sea ice thickness has decreased, the sea ice drift speed has increased. {4.2.2}

It is very likely that the annual Antarctic sea ice extent increased at a rate of between 1.2 and 1.8% per decade between 1979 and 2012. There was a greater increase in sea ice area, due to a decrease in the percentage of open water within the ice pack. There is high confidence that there are strong regional differences in this annual rate, with some regions increasing in extent/area and some decreasing. {4.2.3}

TS.2.5.4 Glaciers and Ice Sheets

There is very high confidence—with a very few regional exceptions—that, since AR4, overall glaciers world-wide have continued to shrink as revealed by the time series of measured changes in glacier length, area, volume and mass (Figure TS.1, Figure TS.3). Measurements of glacier change have increased substantially in number since AR4. Most of the new datasets, along with a globally complete glacier inventory, have been derived from satellite remote sensing {4.3.1, 4.3.3}

There is very high confidence that, during the last decade, most ice was lost from glaciers in Alaska, the Canadian Arctic, the periphery of the Greenland ice sheet, the Southern Andes and the Asian mountains. Together these areas account for more than 80% of the total ice loss. Total mass loss from all glaciers in the world, excluding those on the periphery of the ice sheets, was very likely 226 [91 to 361] Gt yr–1 (sea-level equivalent, 0.62 [0.25 to 0.99] mm yr–1) in the period 1971–2009, 275 [140 to 410] Gt yr–1 (0.76 [0.39 to 1.13] mm yr–1) in the period 1993–2009, and 301 [166 to 436] Gt yr–1 (0.83 [0.46 to 1.20] mm yr–1) between 2005 and 20097. {4.3.3; Tables 4.4, 4.5}

There is high confidence that current glacier extents are out of balance with current climatic conditions, indicating that glaciers will continue to shrink in the future even without further temperature increase. {4.3.3}

There is very high confidence that the Greenland Ice Sheet has lost ice during the last two decades. Combinations of satellite and airborne remote sensing together with field data indicate with high confidence that the ice loss has occurred in several sectors and that large rates of mass loss have spread to wider regions than reported in AR4 (Figure TS.3). There is high confidence that the mass loss of the Greenland Ice Sheet has accelerated since 1992: the average rate has very likely increased from 34 [–6 to 74] Gt yr–1 over the period 1992–2001 (sea-level equivalent, 0.09 [–0.02 to 0.20] mm yr–1), to 215 [157 to 274] Gt yr–1 over the period 2002–2011 (0.59 [0.43 to 0.76] mm yr–1). There is high confidence that ice loss from Greenland resulted from increased surface melt and runoff, and increased outlet glacier discharge, and these occurred in similar amounts. There is high confidence that the area subject to summer melt has increased over the last two decades. {4.4.2, 4.4.3}

There is high confidence that the Antarctic Ice Sheet has been losing ice during the last two decades (Figure TS.3). There is very high confidence that these losses are mainly from the northern Antarctic Peninsula and the Amundsen Sea sector of West Antarctica, and high confidence that they result from the acceleration of outlet glaciers. The average rate of ice loss from Antarctica likely increased from 30 [–37 to 97] Gt yr–1 (sea level equivalent, 0.08 [–0.10 to 0.27] mm yr–1) over the period 1992–2001, to 147 [72 to 221] Gt yr–1 over the period 2002–2011 (0.40 [0.20 to 0.61] mm yr–1). {4.4.2, 4.4.3}

There is high confidence that in parts of Antarctica floating ice shelves are undergoing substantial changes. There is medium confidence that ice shelves are thinning in the Amundsen Sea region of West Antarctica, and low confidence that this is due to high ocean heat flux. There is high confidence that ice shelves around the Antarctic Peninsula continue a long-term trend of retreat and partial collapse that began decades ago. {4.4.2, 4.4.5}

TS.2.5.5 Snow Cover, Freshwater Ice and Frozen Ground

There is very high confidence that snow cover extent has decreased in the Northern Hemisphere, especially in spring (Figure TS.1). Satellite records indicate that over the period 1967–2012, snow cover extent very likely decreased; the largest change, –53% [–40% to –66%], occurred in June. No month had statistically significant increases. Over the longer period, 1922–2012, data are only available for March and April, but these show very likely a 7% [4.5% to 9.5%] decline and a negative correlation (–0.76) with March to April 40°N–60°N land temperature. In the Southern Hemisphere, evidence is too limited to conclude whether changes have occurred. {4.5.2, 4.5.3}

Permafrost temperatures have increased in most regions around the world since the early 1980s (high confidence). These increases were in response to increased air temperature, and changes in the timing and thickness of snow cover (high confidence). The temperature increase for colder permafrost was generally greater than for warmer permafrost (high confidence). {4.7.2, Table 4.8}

Source & ©: IPCC  Working Group I Contribution To The IPCC Fifth Assessment Report Climate Change 2013: The Physical Science Basis. , TS.2.5.3-2.5.5, p.40-46.

2.5 Has the level of the oceans changed?

The source document for this Digest states:

TS.2.6 Changes in Sea Level

The primary contributions to changes in the volume of water in the ocean are the expansion of the ocean water as it warms and the transfer to the ocean of water currently stored on land, particularly from glaciers and ice sheets. Water impoundment in reservoirs and ground water depletion (and its subsequent run-off to the ocean) also affects sea level. Change in sea level relative to the land (relative sea level) can be significantly different from the global mean sea level (GMSL) change because of changes in the distribution of water in the ocean and vertical movement of the land. For an overview on the scientific understanding and uncertainties associated with recent (and projected) sea level change see TFE.2. {3.7.3, 13.1} During the warm intervals of the middle Pliocene (3.3 to 3.0 million years ago), when there is medium confidence that global mean surface temperatures were 2°C to 3.5°C warmer than for pre-industrial climate and CO2 levels were between 250 and 450 ppm, sedimentary records suggest periodic deglaciation of West Antarctica and parts of the East Antarctica. Ice-sheet models suggest near-complete deglaciation of the Greenland, West Antarctica and partial deglaciation of East Antarctica. Together, the evidence suggests that GMSL was above present levels at that time, but did not exceed 20 m above present (high confidence). {5.6.1, 13.2} 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, implying substantial contributions from the Greenland and Antarctic ice sheets. [5.6.2, 13.2.1] 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). [5.3.4] Based on ice-sheet model simulations consistent with elevation changes derived from a new Greenland ice core, the Greenland ice sheet very likely contributed between 1.4 m and 4.3 m sea level equivalent, implying with medium confidence a contribution from the Antarctic ice sheet to the global mean sea level during the last interglacial period. {5.6.2, 13.2} Based on proxy data, the magnitude of centennial-scale global mean sea level variations did not exceed 0.25 m over the past few millennia (medium confidence). The current rate of global mean sea level change, starting in the late 19th-early 20th century, is, with medium confidence, unusually high in the context of centennial-scale variations of the last two millennia. Tide gauge data also indicate a likely acceleration during the last two centuries. Based on proxy and instrumental data, it is virtually certain that the rate of global mean sea level rise has accelerated during the last two centuries, marking the transition from relatively low rates of change during the late Holocene (order tenths of mm yr–1) to modern rates (order mm yr–1). {3.7, 5.6.3, 13.2} Global mean sea level has risen by 0.19 [0.17 to 0.21] m, estimated from a linear trend over the period 1901– 2010, based on tide gauge records and additionally on satellite data since 1993. It is very likely that the mean rate of sea level rise was 1.7 [1.5 to 1.9] mm yr–1 between 1901 and 2010. Between 1993 and 2010, the rate was very likely higher at 3.2 [2.8 to 3.6] mm yr–1; similarly high rates likely occurred between 1930 and 1950. The rate of global mean sea level rise has likely increased since the early 1900 with estimates ranging from 0.000 to 0.013 [–0.002 to 0.019] mm yr–2. {3.7, 5.6.3, 13.2}

Source & ©: IPCC  Working Group I Contribution To The IPCC Fifth Assessment Report Climate Change 2013: The Physical Science Basis. , TS.2.6 Changes in Sea Level, p.46.

TFE.2: Sea Level Change: Scientific Understanding and Uncertainties

After the Last Glacial Maximum, global mean sea levels reached close to present-day values several thousand years ago. Since then, it is virtually certain that the rate of global mean sea level rise has increased from low rates of sea level change during the late Holocene (order tenths of mm yr–1) to 20th century rates (order mm yr–1, Figure TS1). {3.7, 5.6, 13.2} Ocean thermal expansion and glacier mass loss are very likely the dominant contributors to global mean sea level rise during the 20th century. It is very likely that warming of the ocean has contributed 0.8 [0.5 to 1.1] mm yr–1 of sea level change during 1971–2010, with the majority of the contribution coming from the upper 700 m. The model-mean rate of ocean thermal expansion for 1971–2010 is close to observations. {3.7, 13.3}

Observations, combined with improved methods of analysis, indicate that the global glacier contribution (excluding the peripheral glaciers around Greenland and Antarctica) to sea level was 0.25 to 1.00 mm yr–1 sea level equivalent during 1971–2010. Medium confidence in global glacier mass balance models used for projections of global changes arises from the ability of the models of the well-observed glaciers to reproduce time series of historical changes of those glaciers using observed climate input. A simulation using observed climate data shows a larger rate of glacier mass loss during the 1930s than the simulations using AOGCM input, possibly a result of an episode of warming in Greenland associated with unforced regional climate variability. {4.3, 13.3}

Observations indicate that the Greenland Ice Sheet has very likely experienced a net loss of mass due to both increased surface melting and run off, and increased ice discharge over the last two decades (Figure TS.3). Regional climate models indicate that Greenland ice-sheet surface mass balance showed no significant trend from the 1960s to the 1980s, but melting and consequent runoff has increased since the early 1990s. This tendency is related to pronounced regional warming, which may be attributed to a combination of anomalous regional variability in recent years and anthropogenic climate change. High confidence in projections of future warming in Greenland and increased surface melting is based on the qualitative agreements of models in projecting amplified warming at high northern latitudes for well-understood physical reasons. {4.4, 13.3}

There is high confidence that the Antarctic Ice Sheet is in a state of net mass loss and its contribution to sea level is also likely to have increased in the last two decades. Acceleration in ice outflow has been observed since the 1990s, especially in the Amundsen Sea sector of West Antarctica. Interannual variability in accumulation is large and as a result no significant trend is present in accumulation since 1979 in either models or observations. Surface melting is currently negligible in Antarctica. {4.4, 13.3}

Model-based estimates of climate-related changes in water storage on land (as snow cover, surface water, soil moisture and ground water) do not show significant long-term contributions to sea level change for recent decades. However, human-induced changes (reservoir impoundment and groundwater depletion) have each contributed at least several tenths of mm yr–1 to sea level change. Reservoir impoundment exceeded groundwater depletion for the majority of the 20th century but the rate of groundwater depletion has increased and now exceeds the rate of impoundment. Their combined net contribution for the 20th century is estimated to be small. {13.3}

The observed global mean sea level (GMSL) rise for 1993–2010 is consistent with the sum of the observationally estimated contributions (TFE.2, Figure 1e). The closure of the observational budget for recent periods within uncertainties represents a significant advance since the AR4 in physical understanding of the causes of past GMSL change, and provides an improved basis for critical evaluation of models of these contributions in order to assess their reliability for making projections. {13.3}

The sum of modelled ocean thermal expansion and glacier contributions and the estimated change in land water storage (which is relatively small) accounts for about 65% of the observed GMSL rise for 1901–1990, and 90% for 1971–2010 and 1993–2010 (TFE.2, Figure 1). After inclusion of small long-term contributions from ice sheets and the possible greater mass loss from glaciers during the 1930s due to unforced climate variability, the sum of the modelled contribution is close to the observed rise. The addition of the observed ice sheet contribution since 1993 improves the agreement further between the observed and modelled sea level rise (TFE.2, Figure 1). The evidence now available gives a clearer account than in previous IPCC assessments of 20th century sea level change. {13.3}

When calibrated appropriately, recently improved dynamical ice-sheet models can reproduce the observed rapid changes in ice-sheet outflow for individual glacier systems (e.g., Pine Island Glacier in Antarctica; medium confidence). However, models of ice sheet response to global warming and particularly ice sheet- ocean interactions are incomplete and the omission of ice-sheet models, especially of dynamics, from the model budget of the past means that they have not been as critically evaluated as other contributions. {13.3, 13.4}

Global mean sea level rise for 2081–2100 (relative to 1986–2005) for the RCPs will likely be in the 5–95% ranges derived from CMIP5 climate projections in combination with process-based models (medium confidence), i.e., 0.26–0.54 m (RCP2.6), 0.32–0.62 m (RCP4.5), 0.33–0.62 m (RCP6.0), 0.45–0.81 (RCP8.5) m (see Table TS.1 and Figure TS.15 for RCP forcing). For RCP8.5 the range at 2100 is 0.53–0.97 m. Confidence in the projected likely ranges comes from the consistency of process-based models with observations and physical understanding. It is assessed that there is currently insufficient evidence to evaluate the probability of specific levels above the likely range. Based on current understanding, only the collapse of marine-based sectors of the Antarctic ice sheet, if initiated, could cause GMSL to rise substantially above the likely range during the 21st century. There is a lack of consensus on the probability for such a collapse, and the potential additional contribution to GMSL rise cannot be precisely quantified, but there is medium confidence that it would not exceed several tenths of a meter of sea level rise during the 21st century. It is virtually certain that global mean sea level rise will continue beyond 2100. {13.5.1, 13.5.3}

Many semi-empirical models projections of global mean sea level rise are higher than process-based model projections, but there is low agreement in semi-empirical model projections, and no consensus about their reliability. {13.5.2, 13.5.3}

TFE.2, Figure 2 combines the paleo, tide-gauge, and altimeter observations of sea level rise from 1700 withthe projected global mean sea level change to 2100. {13.5, 13.7, 13.8}

Source & ©: IPCC  Working Group I Contribution To The IPCC Fifth Assessment Report Climate Change 2013: The Physical Science Basis., TFE.2 Sea Level Change: Scientific Understanding and Uncertainties, p.46.

2.6 Have extreme weather events increased in the last decades?

The source document for this Digest states:

TS.2.7 Changes In Extremes

TS.2.7.1 Atmosphere

Recent analyses of extreme events generally support the AR4 and SREX conclusions (see TFE.9 and in particular TFE.9, Table 1, for a synthesis). 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 between 1951 and 2010. Globally, there is medium confidence that the length and frequency of warm spells, including heat waves, has increased since the middle of the 20th century mostly due to lack of data or studies in Africa and South America. However it is likely that heat wave frequency has increased over this period in large parts of Europe, Asia, and Australia. {2.6.1; Table 2.12, 2.13}

It is likely that since 1950 the number of heavy precipitation events over land has increased in more regions than it has decreased. Regional trends vary but confidence is high for central North America with very likely trends towards heavier precipitation events. {2.6.2; Table 2.13} There is low confidence in a global-scale observed trend in drought or dryness (lack of rainfall), due to lack of direct observations, dependencies of inferred trends on the index choice and geographical inconsistencies in the trends. However this masks important regional changes and, for example, the frequency and intensity of drought has likely increased in the Mediterranean and West Africa and likely decreased in Central North America and North-West Australia since 1950. {2.6.2; Table 2.13}

During the last millennium, there is high confidence for the occurrence of droughts of greater magnitude and longer duration than observed since 1900 in many regions. There is medium confidence that more megadroughts occurred in monsoon Asia and wetter conditions prevailed in arid Central Asia and the South American monsoon region during the Little Ice Age (1450–1850) compared to the Medieval Climate Anomaly (950–1250). {5.5.4, 5.5.5} Confidence remains low for long-term (centennial) changes in tropical cyclone activity, after accounting for past changes in observing capabilities. However since the 1970s, it is virtually certain that the frequency and intensity of storms in the North Atlantic has increased although the reasons for this increase are debated (see TFE.9). There is low confidence of large-scale trends in storminess over the last century and there is still insufficient evidence to determine whether robust trends exist in small-scale severe weather events such as hail or thunder storms. {2.6.2, 2.6.3, 2.6.4}

With high confidence, past floods larger than recorded since the 20th century occurred during the past five centuries in northern and central Europe, the western Mediterranean region, and eastern Asia. There is medium confidence that in the Near East, India, central North America, modern large floods are comparable or surpass historical floods in magnitude and/or frequency. {5.5.5}

It is likely that the magnitude of extreme high sea level events has increased since 1970 (see TFE.9, Table 1). Most of the increase in extreme sea level can be explained by the mean sea level rise: changes in extreme high sea levels are reduced to less than 5 mm yr–1 at 94% of tide gauges once the rise in mean sea level is accounted for. There is medium confidence based on reanalysis forced model hindcasts and ship observations that mean significant wave height has increased since the 1950s over much of the North Atlantic north of 45°N, with typical winter season trends of up to 20 cm per decade. {3.4.5, 3.7.5}

TS.2.8 Changes in Carbon and Other Biogeochemical Cycles Concentrations of the atmospheric greenhouse gases carbon dioxide (CO2 ), methane (CH4 ), and nitrous oxide (N2O ) in 2011, exceed the range of concentrations recorded in ice cores during the past 800,000 years. Past changes in atmospheric greenhouse-gas concentrations are determined with very high confidence from polar ice cores. Since AR4 these records have been extended from 650,000 years to 800,000 years ago. {5.2.2}

With very high confidence, the current rates of CO2 , CH4 and N2O rise in atmospheric concentrations and the associated increases in radiative forcing are unprecedented with respect to the “highest resolution” ice core records of the last 22,000 years. There is medium confidence that the rate of change of the observed greenhouse gas rise is also unprecedented compared with the lower resolution records of the past 800,000 years. {2.2.1, 5.2.2}

In several periods characterized by high atmospheric CO2 concentrations, there is medium confidence that global mean temperature was significantly above pre-industrial level. During the mid-Pliocene (3.3 to 3.0 million years ago), atmospheric CO2 concentration between 350 ppm and 450 ppm (medium confidence) occurred when global mean surface temperature was approximately 2°C to 3.5°C warmer (medium confidence) than for pre-industrial climate. During the Early Eocene (52 to 48 million years ago), atmospheric CO2 concentration exceeded ~1000 ppm when global mean surface temperature was 9°C to 14°C higher (medium confidence) than for pre-industrial conditions. {5.3.1}

Source & ©: IPCC  Working Group I Contribution To The IPCC Fifth Assessment Report Climate Change 2013: The Physical Science Basis. , TS.2.7 Changes in Extremes, p.46-50

2.7 How have emissions of greenhouse gases evolved in the last centuries?

The source document for this Digest states:

TS.2.8 Changes in Carbon and Other Biogeochemical Cycles

TS.2.8.1 CO2

Between 1750 and 2011, CO2 emissions from fossil fuel combustion and cement production are estimated from energy and fuel use statistics to have released 365 [335 to 395] PgC8. In 2000–2009, average fossil fuel and cement manufacturing emissions were 7.8 [7.2 to 8.4] PgC yr–1, with an average growth rate of 3.2% yr– 1 (Figure TS.4). This rate of increase of fossil fuel emissions is higher than during the 1990’s (1.0% yr–1). In 2011, fossil fuel emissions were 9.5 [8.7 to 10.3] PgC. {2.2.1, 6.3.1; Table 6.1}

Between 1750 and 2011, land use change (mainly deforestation), derived from land cover data and modelling, is estimated to have released 180 [100 to 260] PgC. Land use change emissions between 2000 and 2009 are dominated by tropical deforestation, and are estimated at 1.1 [0.3 to 1.9] PgC yr–1, with possibly a small decrease from the 1990s due to lower reported forest loss during this decade. This estimate includes gross deforestation emissions of around 3 PgC yr–1 compensated by around 2 PgC yr–1 of forest regrowth in some regions, mainly abandoned agricultural land. {6.3.2; Table 6.2}

Of the 545 [460 to 630] PgC released to the atmosphere from fossil fuel and land use emissions from 1750 to 2011, 240 [230 to 250] PgC accumulated in the atmosphere, as estimated with very high accuracy from the observed increase of atmospheric CO2 concentration from 278 [275 to 281] ppm9 in 1750 to 390.5 ppm in 2011. The amount CO2 in the atmosphere grew by 4.0 [3.8 to 4.2] PgC yr–1 in the first decade of the 21st century. The distribution of observed atmospheric CO2 increases with latitude clearly shows that the increases are driven by anthropogenic emissions which primarily occur in the industrialized countries north of the equator. Based on annual average concentrations, stations in the Northern Hemisphere show slightly higher concentrations than stations in the Southern Hemisphere. An independent line of evidence for the anthropogenic origin of the observed atmospheric CO2 increase comes from the observed consistent decrease in atmospheric O2 content and a decrease in the stable isotopic ratio of CO2 (13C/12C) in the atmosphere (Figure TS.5). {2.2.1, 6.1.3}

The remaining amount of carbon released by fossil fuel and land-use emissions has been re-absorbed by the ocean and terrestrial ecosystems. Based on high agreement between independent estimates using different methods and data sets (e.g., oceanic carbon, oxygen, and transient tracer data), it is very likely that the global ocean inventory of anthropogenic carbon increased from 1994 to 2010. In 2011, it is estimated to be 155 [125 to 185] PgC. The annual global oceanic uptake rates calculated from independent data sets (from oceanic Cant inventory changes, from atmospheric O2/N2 measurements or from pCO2 data) and for different time periods agree with each other within their uncertainties, and very likely are in the range of 1.0–3.2 PgC yr–1. Regional observations of the storage rate of anthropogenic carbon in the ocean are in broad agreement with the expected rate resulting from the increase in atmospheric CO2 concentrations, but with significant spatial and temporal variations. {3.8.1, 6.3}

Natural terrestrial ecosystems (those not affected by land use change) are estimated by difference from changes in other reservoirs to have accumulated 150 [60 to 240] PgC between 1750 and 2010. The gain of carbon by natural terrestrial ecosystems is estimated to take place mainly through the uptake of CO2 by enhanced photosynthesis at higher CO2 levels and nitrogen deposition, longer growing seasons in mid and high latitudes. Natural carbon sinks vary regionally due to physical, biological and chemical processes acting on different time scales. An excess of atmospheric CO2 absorbed by land ecosystems gets stored as organic matter in diverse carbon pools, from short lived (leaves, fine roots) to long-lived (stems, soil carbon). {6.3; Table 6.1}

TS.2.8.2 Carbon and Ocean Acidification

It is very likely that oceanic uptake of anthropogenic CO2 results in gradual acidification of the ocean. The pH10 of seawater has decreased by 0.1 since the beginning of the industrial era, corresponding to a 26% increase in hydrogen ion concentration. The observed pH trends range between –0.0014 and –0.0024 per year in surface waters. In the ocean interior, natural physical and biological processes, as well as uptake of anthropogenic CO2 can cause changes in pH over decadal and longer time scales. {3.8.2; Box 3.2; Table 3.2; FAQ 3.2}

TS.2.8.3 CH4

The concentration of CH4 has increased by a factor of 2.5 since preindustrial times, from 720 [695 to 745] ppb in 1750 to 1803 [1799 to 1807] ppb in 2011 (Figure TS.5). There is very high confidence that the atmospheric CH4 increase during the Industrial Era is caused by anthropogenic activities. The massive increase in the number of ruminants, the emissions from fossil fuel extraction and use, the expansion of rice paddy agriculture and the emissions from landfills and waste, are the dominant anthropogenic CH4 sources. Anthropogenic emissions account for 50% to 65% of total emissions. By including natural geological CH4 emissions that were not accounted for in previous budgets, the fossil component of the total CH4 emissions (i.e., anthropogenic emissions related to leaks in the fossil fuel industry and natural geological leaks) is now estimated to amount to about 30% of the total CH4 emissions (medium confidence). {2.2.1, 6.1, 6.3.3}

In recent decades, CH4 growth in the atmosphere has been variable. CH4 concentrations were relatively stable for about a decade in the 1990s, but then started growing again starting in 2007. The exact drivers of this renewed growth are still debated. Climate driven fluctuations of CH4 emissions from natural wetlands (177 to 284  1012 g (CH4 ) yr–1 for 2000–2009 based on bottom-up estimates) are the main drivers of the global inter-annual variability of CH4 emissions (high confidence), with a smaller contribution from biomass burning emissions during high fire years {2.2.1, 6.3.3; Table 6.8}.

TS.2.8.4 Nitrogen

Since preindustrial times, the concentration of N2O in the atmosphere has increased by a factor of 1.2 (Figure TS.5). Changes in the nitrogen cycle, in addition to interactions with CO2 sources and sinks, affect emissions of N2O both on land and from the ocean. {2.2.1, 6.4.6}

TS.2.8.5 Oceanic Oxygen

High agreement among analyses provides medium confidence that oxygen concentrations have decreased in the open ocean thermocline in many ocean regions since the 1960s. The general decline is consistent with the expectation that warming-induced stratification leads to a decrease in the supply of oxygen to the thermocline from near surface waters, that warmer waters can hold less oxygen, and that changes in wind- driven circulation affect oxygen concentrations. It is likely that the tropical oxygen minimum zones have expanded in recent decades. {3.8.3}

Source & ©: IPCC  Working Group I Contribution To The IPCC Fifth Assessment Report Climate Change 2013: The Physical Science Basis. , TS.2.8 Changes in Carbon and Other Biogeochemical Cycles, p.50-52


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