Editing IPCC:AR6/WGI/Chapter-2 (section)

== Executive Summary ==

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Chapter 2 assesses observed large-scale changes in climate system drivers, key climate indicators and principal modes of variability. [[IPCC:Wg1:Chapter:Chapter-3|Chapter 3]] considers model performance and detection/attribution, and [[IPCC:Wg1:Chapter:Chapter-4|Chapter 4]] covers projections for a subset of these same indicators and modes of variability. Collectively, these chapters provide the basis for later chapters, which focus upon processes and regional changes. Within Chapter 2, changes are assessed from in situ and remotely sensed data and products and from indirect evidence of longer-term changes based upon a diverse range of climate proxies. The time-evolving availability of observations and proxy information dictate the periods that can be assessed. Wherever possible, recent changes are assessed for their significance in a longer-term context, including target proxy periods, both in terms of mean state and rates of change.

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[[File:bca4619a67ecffbd1a49daa56b183201 IPCC_AR6_WGI_Figure_2_1.png]]

Figure 2.1 '''|''' '''Visual guide to Chapter 2.'''

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=== Changes in Climate System Drivers ===

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Climate system drivers lead to climate change by altering the Earth’s energy balance. The influence of a climate driver is described in terms of its effective radiative forcing (ERF), measured in W m <sup>–2</sup> . Positive ERF values exert a warming influence and negative ERF values exert a cooling influence (Chapter 7).

'''Present-day global concentrations of atmospheric carbon dioxide (CO''' <sub>2</sub> ''') are at higher levels than at any time in at least the past two million years''' ( ''high confidence'' '''). Changes in ERF since the late 19th century are dominated by increases in concentrations of greenhouse gases and trends in aerosols; the net ERF is positive and changing at an increasing rate since the 1970s''' ( ''medium confidence'' ''').''' {2.2, 7.2, 7.3}

'''Change in ERF from natural factors since 1750 is negligible in comparison to anthropogenic drivers''' ( ''very high confidence'' ''').''' Solar activity since 1900 was high but not exceptional compared to the past 9000 years ( ''high confidence'' ). The average magnitude and variability of volcanic aerosol forcing since 1900 have not been unusual compared to the past 2500 years ( ''medium confidence'' ). {2.2.1, 2.2.2}

'''In 2019, concentrations of CO''' <sub>2</sub> ''', methane (CH''' <sub>4</sub> ''') and nitrous oxide (N''' <sub>2</sub> '''O) reached levels of 409.9 (±0.4) parts per million (ppm), 1866.3 (±3.3) parts per billion (ppb) and 332.1 (±0.4) ppb, respectively. Since 1850, these well-mixed greenhouse gases (GHGs) have increased at rates that have no precedent on centennial time scales in at least the past 800,000 years.''' Concentrations of CO <sub>2</sub> , CH <sub>4</sub> , and N <sub>2</sub> O increased from 1750 to 2019 by 131.6 ± 2.9 ppm (47.3%), 1137 ± 10 ppb (156%), and 62 ± 6 ppb (23.0%) respectively. These changes are larger than those between glacial and interglacial periods over the last 800,000 years for CO <sub>2</sub> and CH <sub>4</sub> and of comparable magnitude for N <sub>2</sub> O ( ''very high confidence'' ). The best estimate of the total ERF from CO <sub>2</sub> , CH <sub>4</sub> and N <sub>2</sub> O in 2019 relative to 1750 is 2.9 W m <sup>–2</sup> , an increase of 12.5% from 2011. ERF from halogenated components in 2019 was 0.4 W m <sup>–2</sup> , an increase of 3.5% since 2011. {2.2.3, 2.2.4, 7.3.2}

'''Tropospheric aerosol concentrations across the Northern Hemisphere mid-latitudes increased from 1700 to the last quarter of the 20th century, but have subsequently declined''' ( ''high confidence'' ''').''' Aerosol optical depth (AOD) has decreased since 2000 over Northern Hemisphere mid-latitudes and Southern Hemisphere mid-latitude continents, but increased over South Asia and East Africa ( ''high confidence'' ). These trends are even more pronounced in AOD from sub-micrometre aerosols for which the anthropogenic contribution is particularly large. The best-estimate of aerosol ERF in 2019 relative to 1750 is –1.1 W m <sup>–2</sup> . {2.2.6, 7.3.3}

'''Changes in other short-lived gases are associated with an overall positive ERF''' ( ''medium confidence'' ''').''' Stratospheric ozone has declined between 60°S and 60°N by 2.2% from the 1980s to 2014–2017 ( ''high confidence'' ). Since the mid-20th century, tropospheric ozone has increased by 30–70% across the Northern Hemisphere ( ''medium confidence'' ). Since the mid-1990s, free tropospheric ozone increases were 2–7% per decade in the northern mid-latitudes ( ''high confidence'' ), 2–12% per decade in the tropics ( ''high confidence'' ) and &lt;5% per decade in southern mid-latitudes ( ''medium confidence'' ). The best estimate of ozone column ERF (0.5 W m <sup>–2</sup> relative to 1750) is dominated by changes in tropospheric ozone. Due to discrepancies in satellite and in situ records, there is ''low confidence'' in estimates of stratospheric water vapour change. {2.2.5, 7.3.2}

'''Biophysical effects from historical changes in land use have an overall negative ERF''' ( ''medium confidence'' ''').''' The best-estimate ERF from the increase in global albedo is –0.15 W m <sup>–2</sup> since 1700 and –0.12 W m <sup>–2</sup> since 1850 ( ''medium confidence'' ). {2.2.7, 7.3.4}

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=== Changes in Key Indicators of Global Climate Change ===

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'''Observed changes in the atmosphere, oceans, cryosphere and biosphere provide unequivocal evidence of a world that has warmed. Over the past several decades, key indicators of the climate system are increasingly at levels unseen in centuries to millennia, and are changing at rates unprecedented in at least the last 2000 years''' ( ''high confidence'' ''').''' Temperatures as high as during the most recent decade (2011–2020) exceed the warmest centennial-scale range reconstructed for the present interglacial, around 6,500 years ago [0.2°C–1°C relative to 1850–1900] ( ''medium confidence'' ). The next older warm period is the last interglacial when the multi-centennial temperature range about 125,000 years ago [0.5°C–1.5°C relative to 1850–1900] encompassed the recent decade values ( ''medium confidence'' ). {2.3}

'''GMST increased by 0.85 [0.69 to 0.95] °C between 18''' '''50–190''' '''0 and 1995–2014 and by 1.09 [0.95 to 1.20] °C between 18''' '''50–190''' '''0 and 2011–2020. From 1850–1900 to 2011–2020, the temperature increase over land (1.59 [1.34 to 1.83] °C) has been faster than over the oceans (0.88 [0.68 to 1.01] °C).''' GMST in the first two decades of the 21st century (2001–2020) was 0.99 [0.84–1.10] °C higher than 1850–1900. Each of the last four decades has successively been warmer than all preceding decades since 1850. Over the last 50 years, observed GMST has increased at a rate unprecedented in at least the last 2000 years ( ''high confidence'' ). The increase in GMST since the mid-19th century was preceded by a slow decrease that began in the mid-Holocene (around 6500 years ago) ( ''medium confidence'' ). {2.3.1.1, Cross-Chapter Box 2.1}

'''Changes in GMST and global surface air temperature (GSAT) over time differ by at most 10% in either direction''' ( ''high confidence'' '''), and the long-term changes in GMST and GSAT are presently assessed to be identical.''' There is expanded uncertainty in GSAT estimates, with the assessed change from 1850–1900 to 1995–2014 being 0.85 [0.67 to 0.98] °C. {Cross-Chapter Box 2.3}

'''The troposphere has warmed since at least the 1950s, and it is''' ''virtually certain'' '''that the stratosphere has cooled.''' In the Tropics, the upper troposphere has warmed faster than the near-surface since at least 2001, the period over which new observational techniques permit more robust quantification ( ''medium confidence'' ). It is ''virtually certain'' that the tropopause height has risen globally over 1980–2018, but there is ''low confidence'' in the magnitude. {2.3.1.2}

'''Changes in several components of the global hydrological cycle provide evidence for overall strengthening since at least 1980''' ( ''high confidence'' ''')''' '''. However, there is''' ''low confidence'' '''in comparing recent changes with past variations due to limitations in paleoclimate records at continental and global scales.''' Global land precipitation has ''likely'' increased since 1950, with a faster increase since the 1980s ( ''medium confidence'' ). Near-surface specific humidity has increased over both land ( ''very likely'' ) and the oceans ( ''likely'' ) since at least the 1970s. Relative humidity has ''very likely'' decreased over land areas since 2000. Global total column water vapour content has very likely increased during the satellite era. Observational uncertainty leads to ''low confidence'' in global trends in precipitation minus evaporation and river runoff. {2.3.1.3}

'''Several aspects of the large-scale atmospheric circulation have''' ''likely'' '''changed since the mid-20th century, but limited proxy evidence yields''' ''low confidence'' '''in how these changes compare to longer-term climate.''' The Hadley circulation has ''likely'' widened since at least the 1980s, and extratropical storm tracks have ''likely'' shifted poleward in both hemispheres. Global monsoon precipitation has ''likely'' increased since the 1980s, mainly in the Northern Hemisphere ( ''medium confidence'' ). Since the 1970s, near-surface winds have ''likely'' weakened over land. Over the oceans, near-surface winds ''likely'' strengthened over 1980–2000, but divergent estimates lead to ''low confidence'' in the sign (direction) of change thereafter. It is ''likely'' that the northern stratospheric polar vortex has weakened since the 1980s and experienced more frequent excursions toward Eurasia. {2.3.1.4}

'''Current Arctic sea ice coverage levels are the lowest since at least 1850 for both annual mean and late-summer values''' ( ''high confidence'' ''') and for the past 1000 years for late-summer values''' ( ''medium confidence'' ''').''' Between 1979 and 2019, Arctic sea ice area has decreased in both summer and winter, with sea ice becoming younger, thinner and more dynamic ( ''very high confidence'' ). Decadal means for Arctic sea ice area decreased from 6.23 million km <sup>2</sup> in 1979–1988 to 3.76 million km <sup>2</sup> in 2010–2019 for September and from 14.52 to 13.42 million km <sup>2</sup> for March. Antarctic sea ice area has experienced little net change since 1979 ( ''high confidence'' ), with only minor differences between sea ice area decadal means for 1979–1988 (2.04 million km <sup>2</sup> for February, 15.39 million km <sup>2</sup> for September) and 2010–2019 (2.17 million km <sup>2</sup> for February, 15.75 million km <sup>2</sup> for September). {2.3.2.1}

'''Changes across the terrestrial cryosphere are widespread, with several indicators now in states unprecedented in centuries to millennia''' ( ''high confidence'' ''').''' Reductions in spring snow cover extent have occurred across the Northern Hemisphere since at least 1978 ( ''very high confidence'' ). With few exceptions, glaciers have retreated since the second half of the 19th century and have continued to retreat at increased rates since the 1990s ( ''very high confidence'' ); this behaviour is unprecedented in at least the last 2000 years ( ''medium confidence'' ). Greenland Ice Sheet (GrIS) mass loss has increased substantially since 2000 ( ''high confidence'' ). The Greenland Ice Sheet was smaller than at present during the Last Interglacial period ( ''high confidence'' ) and the mid-Holocene ( ''high confidence'' ). The Antarctic Ice Sheet (AIS) lost mass between 1992 and 2020 ( ''very high confidence'' ), with an increasing rate of mass loss over this period ( ''medium confidence'' ). Although permafrost persists in areas of the Northern Hemisphere where it was absent prior to 3000 years ago, increases in temperatures in the upper 30 m over the past three to four decades have been widespread ( ''high confidence'' ). {2.3.2}

'''Global mean sea level (GMSL) is rising, and the rate of GMSL rise since the 20th century is faster than over any preceding century in at least the last three millennia''' ( ''high confidence'' '''). Since 1901, GMSL has risen by 0.20 [0.15 to 0.25] m, and the rate of rise is accelerating.''' The average rate of sea level rise was 1.3 [0.6 to 2.1] mm yr <sup>–1</sup> between 1901 and 1971, increasing to 1.9 [0.8 to 2.9] mm yr <sup>–1</sup> between 1971 and 2006, and further increasing to 3.7 [3.2 to 4.2] mm yr <sup>–1</sup> between 2006 and 2018 ( ''high'' ''confidence'' ). Further back in time, there is ''medium confidence'' that GMSL was within –3.5 to +0.5 m ( ''very likely'' ) of present during the mid-Holocene (6000 years ago), 5 to 10 m ( ''likely'' ) higher during the Last Interglacial (125,000 years ago), and 5 to 25 m ( ''very likely'' ) higher during the mid-Pliocene Warm Period (MPWP) (3.3 million years ago). {2.3.3.3}

'''Recent ocean changes are widespread, and key ocean indicators are in states unprecedented for centuries to millennia''' ( ''high confidence'' ''').''' Since 1971, it is ''virtually certain'' that global ocean heat content has increased for the upper (0–700 m) layer, ''very likely'' for the intermediate (700–2000 m) layer and ''likely'' below 2000 m, and is currently increasing faster than at any point since at least the last deglacial transition (18 to 11 thousand years ago) ( ''medium confidence'' ). It is ''virtually certain'' that large-scale near-surface salinity contrasts have intensified since at least 1950. The Atlantic Meridional Overturning Circulation (AMOC) was relatively stable during the past 8000 years ( ''medium confidence'' ) but declined during the 20th century ( ''low confidence'' ). Ocean pH has declined globally at the surface over the past four decades ( ''virtually certain'' ) and in all ocean basins in the ocean interior ( ''high confidence'' ) over the past 2–3 decades. A long-term increase in surface open ocean pH occurred over the past 50 million years ( ''high confidence'' ), and surface ocean pH as low as recent times is uncommon in the last 2 million years ( ''medium confidence'' ). Deoxygenation has occurred in most open ocean regions during the mid 20th to early 21st centuries ( ''high confidence'' ), with decadal variability ( ''medium confidence'' ). Oxygen minimum zones are expanding at many locations ( ''high confidence'' ). {2.3.3}

'''Changes in the marine biosphere are consistent with large-scale warming and changes in ocean geochemistry''' ( ''high confidence'' ''').''' The ranges of many marine organisms are shifting towards the poles and towards greater depths ( ''high confidence'' ), but a minority of organisms are shifting in the opposite directions. This mismatch in responses across species means that the species composition of ecosystems is changing ( ''medium confidence'' ). At multiple locations, various phenological metrics for marine organisms have changed in the last 50 years, with the nature of the changes varying with location and with species ( ''high confidence'' ). In the last two decades, the concentration of phytoplankton at the base of the marine food web, as indexed by chlorophyll concentration, has shown weak and variable trends in low and mid-latitudes and an increase in high latitudes ( ''medium confidence'' ). Global marine primary production decreased slightly from 1998–2018, with increasing production in the Arctic ( ''medium confidence'' ). {2.3.4.2}

'''Changes in key global aspects of the terrestrial biosphere are consistent with large-scale warming''' ( ''high confidence'' ''').''' Over the last century, there have been poleward and upslope shifts in the distributions of many land species ( ''very high confidence'' ) as well as increases in species turnover within many ecosystems ( ''high confidence'' ). Over the past half century, climate zones have shifted poleward, accompanied by an increase in the length of the growing season in the Northern Hemisphere extratropics and an increase in the amplitude of the seasonal cycle of atmospheric CO <sub>2</sub> above 45°N ( ''high confidence'' ). Since the early 1980s, there has been a global-scale increase in the greenness of the terrestrial surface ( ''high confidence'' ). {2.3.4.1, 2.3.4.3}

'''During the mid-Pliocene warm period (MPWP, 3.3 to 3.0 million years ago) slowly changing large-scale indicators reflect a world that was warmer than present, with CO''' <sub>2</sub> '''similar to current levels.''' CO <sub>2</sub> levels during the MPWP were similar to present for a sustained period, within a range of 360–420 ppm ( ''medium confidence'' ). Relative to the present, GMST, GMSL and precipitation rate were all higher, the Northern Hemisphere latitudinal temperature gradient was lower, and major terrestrial biomes were shifted northward ( ''very high confidence'' ). There is ''high confidence'' that cryospheric indicators were diminished and ''medium confidence'' that the Pacific longitudinal temperature gradient weakened and monsoon systems strengthened. {2.3, Cross-Chapter Box 2.4, 9.6.2}

'''Inferences from past climate states based on proxy records can be compared with climate projections over coming centuries to place the range of possible futures into a longer-term context.''' There is ''medium confidence'' in the following mappings between selected paleo periods and future projections: during the Last Interglacial, GMST is estimated to have been 0.5°C–1.5°C warmer than the 1850–1900 reference for a sustained period, which overlaps the low end of the range of warming projected under SSP1-2.6, including its negative-emissions extension to the end of the 23rd century [1.0°C to 2.2°C]. During the mid-Pliocene Warm Period, the GMST estimate [2.5°C to 4.0°C] is similar to the range projected under SSP2-4.5 for the end of the 23rd century [2.3°C to 4.6°C]. GMST estimates for the Miocene Climatic Optimum [5°C to 10°C] and Early Eocene Climatic Optimum [10°C to 18°C], about 15 and 50 million years ago, respectively, overlap with the range projected for the end of the 23rd century under SSP5-8.5 [6.6°C to 14.1°C]. {Cross-Chapter Box 2.1, 2.3.1, 4.3.1.1, 4.7.1.1}

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=== Changes in Modes of Variability ===

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'''Since the late 19th century, major modes of climate variability show no sustained trends but do exhibit fluctuations in frequency and magnitude at inter-decadal time scales, with the notable exception of the Southern Annular Mode, which has become systematically more positive''' ( ''high confidence'' '''). There is''' ''high confidence'' '''that these modes of variability have existed for millennia or longer, but''' ''low confidence'' '''in detailed reconstructions of most modes prior to direct instrumental records.''' Both polar annular modes have exhibited strong positive trends toward increased zonality of mid-latitude circulation over multi-decadal periods, but these trends have not been sustained for the Northern Annular Mode since the early 1990s ( ''high confidence'' ). For tropical ocean modes, a sustained shift beyond multi-centennial variability has not been observed for El Niño–Southern Oscillation ( ''medium confidence'' ), but there is ''limited evidence'' and ''low agreement'' about the long-term behaviour of other tropical ocean modes. Modes of decadal and multi-decadal variability over the Pacific and Atlantic oceans exhibit no significant trends over the period of observational records ( ''high confidence'' ). {2.4}

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