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

=== 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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