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

=== 2.3.2 Cryosphere ===

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This section focuses on large-scale changes in a subset of components of the cryosphere (Cross-Chapter Box 2.2). [[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] undertakes a holistic assessment of past and possible future changes and understanding of key processes in the cryosphere, including those at regional scales, integrating observations, modelling and theoretical understanding, while, here in chapter 2, the focus is on past large-scale, observation-based cryospheric changes.

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==== 2.3.2.1 Sea Ice Coverage and Thickness ====

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===== 2.3.2.1.1 Arctic sea ice =====

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The AR5 reported that the annual mean Arctic sea-ice extent (SIE) ''very likely'' decreased by 3.5–4.1% per decade between 1979 and 2012 with the summer sea-ice minimum (perennial sea ice) ''very likely'' decreasing by 9.4–13.6% per decade. This was confirmed by SROCC reporting the strongest reductions in September (12.8 ± 2.3% per decade; 1979–2018) and stating that these changes were ''likely'' unprecedented in at least 1 kyr ( ''medium confidence'' ). The spatial extent had decreased in all seasons, with the largest decrease for September ( ''high confidence'' ). The AR5 reported also that the average winter sea ice thickness within the Arctic Basin had ''likely'' decreased by between 1.3 m and 2.3 m from 1980 to 2008 ( ''high confidence'' ), consistent with the decline in multi-year and perennial ice extent. The SROCC stated further that it was ''virtually certain'' that Arctic sea ice had thinned, concurrent with a shift to younger ice. Lower sea ice volume in 2010–2012 compared to 2003–2008 was documented in AR5 ( ''medium confidence'' ). There was ''high confidence'' that, where the sea ice thickness had decreased, the sea-ice drift speed had increased.

Proxy records are used in combination with modelling to assess Arctic paleo sea ice conditions to the extent possible. For the Pliocene, ''limited'' proxy ''evidence'' of a reduced sea ice cover compared to ‘modern’ winter conditions ( [[#Knies--2014|Knies et al., 2014]] ; [[#Clotten--2018|Clotten et al., 2018]] ) and model simulations of a largely ice-free Arctic Ocean during summer ( [[#Howell--2016|Howell et al., 2016]] ; [[#Feng--2019|Feng et al., 2019]] ; F. [[#Li--2020|]] [[#Li--2020|Li et al., 2020]] ) imply ''medium confidence'' that the Arctic Ocean was seasonally ice covered. Over the LIG, sparse proxy reconstructions ( [[#Stein--2017|Stein et al., 2017]] ; [[#Kremer--2018|Kremer et al., 2018]] ) and proxy evidence from marine sediments ( [[#Kageyama--2021b|Kageyama et al., 2021b]] ) provide ''medium confidence'' of perennial sea ice cover.

Over the past 13 kyr proxy records suggest extensive sea-ice coverage during the Younger Dryas (at the end of the LDT), followed by a decrease in sea ice coverage during the Early Holocene, and increasing sea-ice coverage from the MH to the mid-15th century ( [[#De%20Vernal--2013|De Vernal et al., 2013]] ; [[#Belt--2015|Belt et al., 2015]] ; [[#Cabedo-Sanz--2016|Cabedo-Sanz et al., 2016]] ; [[#Armand--2017|Armand et al., 2017]] ; [[#Belt--2018|Belt, 2018]] ). There is ''limited evidence'' that the Canadian Arctic had less multiyear sea ice during the Early Holocene than today ( [[#Spolaor--2016|Spolaor et al., 2016]] ). For more regional details on paleo arctic sea ice see Section 9.3.1.1.

Pan-Arctic SIE conditions (annual means and late summer) during the last decade were unprecedented since at least 1850 (Figure 2.20a; [[#Walsh--2017|Walsh et al., 2017]] , 2019; [[#Brennan--2020|Brennan et al., 2020]] ), while, as reported in SROCC, there remains ''medium confidence'' that the September (late summer) Arctic sea ice loss during the last decade was unprecedented during the past 1 kyr. Sea-ice charts since 1850 ( [[#Walsh--2017|Walsh et al., 2017]] , 2019) suggest that there was no significant trend before the 1990s, but the uncertainty of these estimates is large and could mask a trend, a possibility illustrated by [[#Brennan--2020|Brennan et al. (2020)]] , who found a loss of Arctic sea ice between 1910 and 1940 in an estimate based on a data assimilation approach.

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[[File:4236ee7cfe6bd3c131388169f24b7c4e IPCC_AR6_WGI_Figure_2_20.png]]

'''Figure 2.2''' '''0 |''' '''Changes in Arctic and Antarctic sea ice area. (a)''' Three time series of Arctic sea-ice area (SIA) for March and September from 1979 to 2020 (passive microwave satellite era). In addition, the range of SIA from 1850–1978 is indicated by the vertical bar to the left. '''(b)''' Three time series of Antarctic sea ice area for September and February (1979–2020). In both (a) and (b), decadal means for the three series for the first and most recent decades of observations are shown by horizontal lines in grey (1979–1988) and black (2010–2019). SIA values have been calculated from sea ice concentration fields. Available data for 2020 (OSISAF) is shown in both (a) and (b). Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

There has been a continuing decline in SIE and Arctic sea ice area (SIA) in recent years (Figure 2.20a). To reduce grid-geometry associated biases and uncertainties ( [[#Notz--2014|Notz, 2014]] ; [[#Ivanova--2016|Ivanova et al., 2016]] ; [[#Meier--2019|Meier and Stewart, 2019]] ) SIA is used in addition to, or instead of SIE herein (see also section 9.3.1). A record-low Arctic SIA since the start of the satellite era (1979) occurred in September 2012 (Figure 2.20a). Decadal SIA means based on the average of three different satellite products decreased from 6.23 to 3.76 million km <sup>2</sup> for September and 14.52 to 13.42 million km <sup>2</sup> for March SIA (Figure 2.20a). Initial SIA data for 2020 (OSISAF) are within the range of these recent decadal means or slightly below (Figure 2.20a). SIA has declined since 1979 across the seasonal cycle (Figure 9.13). Most of this decline in SIA has occurred after 2000, and is superimposed by substantial interannual variability. The sharp decline in Arctic summer SIA coincides with earlier surface melt onset ( [[#Mortin--2016|Mortin et al., 2016]] ; [[#Bliss--2017|Bliss et al., 2017]] ), later freeze-up, and thus a longer ice retreat and open water period ( [[#Stammerjohn--2012|Stammerjohn et al., 2012]] ; [[#Parkinson--2014|Parkinson, 2014]] ; [[#Peng--2018|Peng et al., 2018]] ).

Over the past two decades, first-year sea ice has become more dominant and the oldest multiyear ice (older than 4 years) which in March 1985 made up 33% of the Arctic sea-ice cover, has nearly disappeared, making up 1.2% in March 2019 ( [[#Perovich--2020|Perovich et al., 2020]] ). The loss of older ice is indicative of a thinning overall of ice cover ( [[#Tschudi--2016|Tschudi et al., 2016]] ), but also the remaining older ice has become thinner (E. [[#Hansen--2013|]] [[#Hansen--2013|Hansen et al., 2013]] ). Since in situ ice thickness measurements are sparse, information about ice thickness is mainly based on airborne and satellite surveys. Records from a combination of different platforms show for the central and western Arctic Ocean (Arctic Ocean north of Canada and Alaska) negative trends since the mid-1970s ( [[#Lindsay--2015|Lindsay and Schweiger, 2015]] ; [[#Kwok--2018|Kwok, 2018]] ), with a particularly rapid decline during the 2000s, which coincided with a large loss of multiyear sea ice. Direct observations from 2004 and 2017 indicate a decrease of modal ice thickness in the Arctic Ocean north of Greenland by 0.75 m, but with little thinning between 2014 and 2017 ( [[#Haas--2017|Haas et al., 2017]] ). This agrees with data based on satellite altimetry and airborne observations, showing no discernible thickness trend since 2010 ( [[#Kwok--2015|Kwok and Cunningham, 2015]] ; [[#Kwok--2018|Kwok, 2018]] ; [[#Kwok--2018|Kwok and Kacimi, 2018]] ; see Figure 2.21). However, sea-ice thickness derived from airborne and spaceborne data is still subject to uncertainties imposed by snow loading. For radar altimeters, insufficient penetration of radar signal into the snowpack results in overestimation of ice thickness (e.g., [[#Ricker--2015|Ricker et al., 2015]] ; [[#King--2018|King et al., 2018]] ; [[#Nandan--2020|Nandan et al., 2020]] ). Negative trends in ice thickness since the 1990s are also reported from the Fram Strait in the Greenland Sea, and north of Svalbard (E. [[#Hansen--2013|]] [[#Hansen--2013|Hansen et al., 2013]] ; [[#Renner--2014|Renner et al., 2014]] ; [[#King--2018|King et al., 2018]] ; [[#Rösel--2018|Rösel et al., 2018]] ; [[#Spreen--2020|Spreen et al., 2020]] ). Thickness data collected in the Fram Strait originate from ice exported from the interior of the Arctic Basin and are representative of a larger geographical area upstream in the transpolar drift. A reduction of survival rates of sea ice exported from the Siberian shelves by 15% per decade has interrupted the transpolar drift and affected the long-range transport of sea ice ( [[#Krumpen--2019|Krumpen et al., 2019]] ). The thinner and on average younger ice has less resistance to dynamic forcing, resulting in a more dynamic ice cover ( [[#Hakkinen--2008|Hakkinen et al., 2008]] ; [[#Spreen--2011|Spreen et al., 2011]] ; [[#Vihma--2012|Vihma et al., 2012]] ; [[#Kwok--2013|Kwok et al., 2013]] ).

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[[File:3d0cd64b5ae223c19653df37531731aa IPCC_AR6_WGI_Figure_2_21.png]]

'''Figure''' '''2.21 |''' '''Arctic sea ice thickness changes (means) for autumn (red/dotted red) and winter (blue/dotted blue).''' Shadings (blue and red) show 1 standard error (S.E.) ranges from the regression analysis of submarine ice thickness and expected uncertainties in satellite ice thickness estimates. Data release area of submarine data ice thickness data is shown in inset. Satellite ice thickness estimates are for the Arctic south of 88°N. Thickness estimates from more localized airborne/ground electromagnetic surveys near the North Pole (diamonds) and from Operation IceBridge (circles) are shown within the context of the larger scale changes in the submarine and satellite records. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

The SROCC noted the lack of continuous records of snow on sea ice. Nevertheless in recent decades, more snow on sea ice has been observed in the Atlantic sector in the Arctic than in the western Arctic Ocean ( [[#Webster--2018|Webster et al., 2018]] ). Previously, [[#Warren--1999|Warren et al. (1999)]] showed that over 1954–1991 there were weak trends towards declining snow depth on sea ice in the Pacific sector. Recent observations indicate a substantial thinning of the spring snowpack in the western Arctic ( [[#Cavalieri--2012|Cavalieri et al., 2012]] ; [[#Brucker--2013|Brucker and Markus, 2013]] ; [[#Kurtz--2013|Kurtz et al., 2013]] ; [[#Laxon--2013|Laxon et al., 2013]] ; [[#Webster--2018|Webster et al., 2018]] ). In contrast, thick snow over Arctic sea ice in the Atlantic sector north of Svalbard (snow thickness around 0.4 m or more) has been observed in the 1970s and since the 1990s ( [[#Rösel--2018|Rösel et al., 2018]] ), but data are too sparse to detect trends.

In summary, over 1979–2019 Arctic SIA has decreased for all months, with the strongest decrease in summer ( ''very high confidence'' ). Decadal means for SIA decreased from the first to the last decade in that period from 6.23 to 3.76 million km <sup>2</sup> for September, and from 14.52 to 13.42 million km <sup>2</sup> for March. Arctic sea ice has become younger, thinner and faster moving ( ''very high confidence'' ). Snow thickness on sea ice has decreased in the western Arctic Ocean ( ''medium confidence'' ). Since the Younger Dryas at the end of the LDT, proxy indicators show that Arctic sea ice has fluctuated on multiple time scales with a decrease in sea ice coverage during the Early Holocene and an increase from the MH to the mid-15th century. Current pan-Arctic sea ice coverage levels (annual mean and late summer) are unprecedentedly low since 1850 ( ''high confidence),'' and with ''medium confidence'' for late summer for at least the past 1 kyr.

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===== 2.3.2.1.2 Antarctic sea ice =====

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The AR5 reported a small but significant increase in the total annual mean Antarctic SIE that was ''very likely'' in the range of 1.2–1.8% per decade between 1979 and 2012 (0.13–0.20 million km <sup>2</sup> per decade) ( ''very high confidence'' ), while SROCC reported that total Antarctic sea ice coverage exhibited no significant trend over the period of satellite observations (1979–2018) ( ''high confidence'' ). The SROCC noted that a significant positive trend in mean annual sea ice cover between 1979 and 2015 had not persisted, due to three consecutive years of below-average sea ice cover (2016–2018). The SROCC stated also that historical Antarctic sea ice data from different sources indicated a decrease in overall Antarctic sea ice cover since the early 1960s, but was too small to be separated from natural variability ( ''high confidence'' ).

There is only ''limited evidence'' from predominantly regional paleo proxies for the evolution of Southern Ocean sea ice before the instrumental record and estimates are not available for all proxy target periods (Section 9.3.2). Proxies from marine sediments for intervals preceding and following the MPWP indicate open water conditions with less sea ice than modern conditions ( [[#Taylor-Silva--2018|Taylor-Silva and Riesselman, 2018]] ; [[#Ishino--2020|Ishino and Suto, 2020]] ). During the LGM, proxies indicate that austral winter sea ice coverage reached the polar ocean front (e.g., [[#Nair--2019|Nair et al., 2019]] ). More recently, sea ice coverage appears to have fluctuated substantially throughout the Holocene (e.g., for the western Amundsen Sea, [[#Lamping--2020|Lamping et al., 2020]] ). At the beginning of the CE, regional summer sea ice coverage in the north-western Ross Sea was lower than today ( [[#Tesi--2020|Tesi et al., 2020]] ). [[#Crosta--2021|Crosta et al. (2021)]] suggest, based on different proxies, four different phases with 7–10 months periods of sea ice occurrence per year in the Antarctic region off Adelie Land during the CE, where each phase was several hundred years long.

More recent sea ice reconstructions are based on diverse sources including whaling records ( [[#de%20La%20Mare--1997|de La Mare, 1997]] , 2009; [[#Cotté--2007|Cotté and Guinet, 2007]] ), old ship logbooks ( [[#Ackley--2003|Ackley et al., 2003]] ; [[#Edinburgh--2016|Edinburgh and Day, 2016]] ), and ice core records ( [[#Curran--2003|Curran et al., 2003]] ; [[#Abram--2010|Abram et al., 2010]] ; [[#Sinclair--2014|Sinclair et al., 2014]] ), amongst other methods (e.g., [[#Murphy--2014|Murphy et al., 2014]] ). These reconstructions, in combination with recent satellite-based observations indicate: (i) a decrease in summer SIE across all Antarctic sectors since the early- to mid-20th century; (ii) a decrease in winter SIE in the East Antarctic and Amundsen-Bellingshausen Seas sectors starting in the 1960s; and (iii) small fluctuations in winter SIE in the Weddell Sea over the 20th century ( [[#Hobbs--2016a|Hobbs et al., 2016a]] , b). There are also ice-core indications that the pronounced Ross Sea increase dates back to the mid-1960s ( [[#Sinclair--2014|Sinclair et al., 2014]] ; [[#Thomas--2016|Thomas and Abram, 2016]] ). While there is reasonable broad-scale concurrence across these estimates, the uncertainties are large, there is considerable interannual variability, and reconstructions require further validation ( [[#Hobbs--2016a|Hobbs et al., 2016a]] , b). New reconstructions ( [[#Thomas--2019|Thomas et al., 2019]] ) from Antarctic land ice cores show that SIE in the Ross Sea had increased between 1900 and 1990, while the Bellingshausen Sea had experienced a decline in SIE; this dipole pattern is consistent with satellite-based observations from 1979 to 2019 ( [[#Parkinson--2019|Parkinson, 2019]] ), but the recent rate of change then has been larger. Records of Antarctic SIE for the late 19th and early 20th centuries ( [[#Edinburgh--2016|Edinburgh and Day, 2016]] ), show SIE comparable with the satellite era, although with marked spatial heterogeneity (e.g., [[#Thomas--2019|Thomas et al., 2019]] ).

Early Nimbus satellite visible and infrared imagery from the 1960s ( [[#Meier--2013|Meier et al., 2013]] ; [[#Gallaher--2014|Gallaher et al., 2014]] ) indicate higher overall SIE compared to 1979–2013 ( [[#Hobbs--2016a|Hobbs et al., 2016a]] , b), but with large uncertainties and poorly quantified biases ( [[#NA%20SEM--2017|NA SEM, 2017]] ). The continuous satellite passive-microwave record shows that there was a modest increase in overall Antarctic SIA of 2.5% ± 0.2% per decade (1 standard error over 1979–2015; [[#Comiso--2017|Comiso et al., 2017]] ). For overall ice coverage and for this period, positive long-term trends were most pronounced during austral autumn advance ( [[#Maksym--2019|Maksym, 2019]] ), being moderate in summer and winter, and lowest in spring ( [[#Holland--2014|Holland, 2014]] ; [[#Turner--2015|Turner et al., 2015]] ; [[#Hobbs--2016a|Hobbs et al., 2016a]] , b; [[#Comiso--2017|Comiso et al., 2017]] ). Since 2014, overall Antarctic SIE (and SIA) has exhibited major fluctuations from record-high to record-low satellite era extents ( [[#Massonnet--2015|Massonnet et al., 2015]] ; [[#Reid--2015|Reid and Massom, 2015]] ; [[#Reid--2015|Reid et al., 2015]] ; [[#Comiso--2017|Comiso et al., 2017]] ; [[#Parkinson--2019|Parkinson, 2019]] ). After setting record-high extents each September from 2012 through 2014, Antarctic SIE (and SIA) dipped rapidly in mid-2016 and remained predominantly below average through 2019 ( [[#Reid--2020|Reid et al., 2020]] ). For the most recent decade of observations (2010–2019), the decadal means of three SIA products (Figure 2.20b) were 2.17 million km <sup>2</sup> for February and 15.75 million km <sup>2</sup> for September, respectively. The corresponding levels for the means for the first decade of recordings (1979–1988) were 2.04 million km <sup>2</sup> for February and 15.39 million km <sup>2</sup> for September indicating little overall change. Initial SIA data for 2020 (OSISAF) show SIA for September above, and for February slightly below the recent decadal means (Figure 2.20b). The 2020 September level (OSISAF) remains below the levels observed over 2012–2014.

In summary, Antarctic sea ice has experienced both increases and decreases in SIA over 1979–2019, and substantively lower levels since 2016, with only minor differences between decadal means of SIA for the first (for February 2.04 million km <sup>2</sup> , for September 15.39 million km <sup>2</sup> ) and last decades (for February 2.17 million km <sup>2</sup> , for September 15.75 million km <sup>2</sup> ) of satellite observations ( ''high confidence'' ). There remains ''low confidence'' in all aspects of Antarctic sea ice prior to the satellite era owing to a paucity of records that are highly regional in nature and often seemingly contradictory.

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==== 2.3.2.2 Terrestrial Snow Cover ====

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The AR5 concluded that snow cover extent (SCE) had decreased in the NH, especially in spring ( ''very high confidence'' ). For 1967–2012, the largest change was in June and March–April SCE ''very likely'' declined. No trends were identified for the SH due to limited records and large variability. The SROCC concluded with ''high confidence'' that Arctic June SCE declined between 1967 and 2018 and in nearly all mountain regions, snow cover declined in recent decades.

Analysis of the combined in situ observations ( [[#Brown--2002|Brown, 2002]] ) and the multi-observation product (Mudryk et al. 2020) indicates that since 1922, April SCE in the NH has declined by 0.29 million km <sup>2</sup> per decade, with significant interannual variability (Figure 2.22) and regional differences (Section 9.5.3.1). The limited pre-satellite era data does not allow for a similar assessment for the entire spring-summer period. Assessment of SCE trends in the NH since 1978 indicates that for the October to February period there is substantial uncertainty in trends with the sign dependent on the observational product. Analysis using the NOAA Climate Data Record shows an increase in October to February SCE ( [[#Hernández-Henríquez--2015|Hernández-Henríquez et al., 2015]] ; [[#Kunkel--2016|Kunkel et al., 2016]] ) while analyses based on satellite borne optical sensors ( [[#Hori--2017|Hori et al., 2017]] ) or multi-observation products ( [[#Mudryk--2020|Mudryk et al., 2020]] ) show a negative trend for all seasons (Section 9.5.3.1 and Figure 9.23). The greatest declines in SCE have occurred during boreal spring and summer, although the estimated magnitude is dataset dependent ( [[#Rupp--2013|Rupp et al., 2013]] ; [[#Estilow--2015|Estilow et al., 2015]] ; [[#Bokhorst--2016|Bokhorst et al., 2016]] ; [[#Thackeray--2016|Thackeray et al., 2016]] ; [[#Connolly--2019|Connolly et al., 2019]] ).

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[[File:ce87e8bb066a3b89434858a8b908ff2d IPCC_AR6_WGI_Figure_2_22.png]]

'''Figure 2.22''' '''|''' '''April snow cover extent (SCE) for the Northern Hemisphere (1922–2018).''' Shading shows ''very likely'' range. The trend over the entire 1922–2018 period (black line) is –0.29 (± 0.07) million km <sup>2</sup> per decade. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

There has been a commensurate decrease in the snow-cover duration and persistence, particularly in higher latitudes due to earlier spring melt and, in some cases, later autumn onset of snow cover ( [[#Chen--2015|Chen et al., 2015]] ; [[#Derksen--2015|Derksen et al., 2015]] ; [[#Hori--2017|Hori et al., 2017]] ; [[#Hammond--2018|Hammond et al., 2018]] ). Arctic snow-cover duration has decreased by 2–4 days per decade since the 1970s ( [[#Brown--2017|Brown et al., 2017]] ). Significant decreases in snow-cover duration have been documented over western Eurasia since 1978 ( [[#Hori--2017|Hori et al., 2017]] ).

For the NH, maximum snow depth has generally decreased since the 1960s, with more robust trends for North America and greater uncertainty for Eurasia ( [[#Kunkel--2016|Kunkel et al., 2016]] ). Several satellite-based passive microwave and other products indicate general declines in pre-melt snow water equivalent since 1981 although there is regional and inter-dataset variability ( [[#Brown--2017|Brown et al., 2017]] ; [[#Jeong--2017|Jeong et al., 2017]] ; [[#Marty--2017|Marty et al., 2017]] ; [[#Mortimer--2020|Mortimer et al., 2020]] ; [[#Mudryk--2020|Mudryk et al., 2020]] ; [[#Pulliainen--2020|Pulliainen et al., 2020]] , Section 9.5.3).

In summary, substantial reductions in spring snow cover extent have occurred in the NH since 1978 ( ''very high confidence'' ) with ''limited evidence'' that this decline extends back to the early 20th century. Since 1981 there has been a general decline in NH spring snow water equivalent ( ''high confidence'' ).

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==== 2.3.2.3 Glacier Mass ====

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The AR5 concluded with ''high confidence'' that, during the Holocene, glaciers were at times smaller than at the end of the 20th century. The AR5 stated further with ''very high confidence'' that most glaciers had been shrinking since the mid-1800s, and the mass loss from all glaciers worldwide ''very likely'' increased from 1970 to 2009. The SROCC reported a globally coherent picture of continued glacier recession in recent decades ( ''very high confidence'' ) based on in situ and satellite observations of changes in glacier area, length and mass, although there were considerable inter-annual and regional variations. Between 2006 and 2015 the global glacier mass change assessed by SROCC was –278 ± 113 Gt yr <sup>–1</sup> .

Two recent global reviews on glaciers over the Holocene ( [[#Solomina--2015|Solomina et al., 2015]] ) and the past 2 kyr ( [[#Solomina--2016|Solomina et al., 2016]] ) summarize the chronologies of respectively 189 and 275 glaciers. The former shows that glaciers retreated during the LDT and retracted to their minimum extent between 8 ka and 6 ka. Except for some glaciers in the SH and tropics, glaciers expanded thereafter, reaching their maximum extent beyond their present-day margins during the mid-15th to late 19th centuries CE. With few exceptions, glacier margins worldwide have retreated since the 19th century, with the rate of retreat and its global character since the late 20th century being unusual in the context of the Holocene ( [[#Solomina--2016|Solomina et al., 2016]] , Figure 2.23a). However, the areal extents of modern glaciers in most places in the NH are still larger than those of the early and/or middle Holocene ( [[#Solomina--2015|Solomina et al., 2015]] ). When considering Holocene and present glaciers extents, it is important to account for the relatively long adjustment time of glaciers (often referred to as response time; Section 9.5.1.3); the majority of modern glaciers are currently out of equilibrium with current climate, even without further global warming ( [[#Mernild--2013|Mernild et al., 2013]] ; [[#Christian--2018|Christian et al., 2018]] ; [[#Marzeion--2018|Marzeion et al., 2018]] ; [[#Zekollari--2020|Zekollari et al., 2020]] ). The size of glaciers during other periods warmer than the Early to Mid-Holocene, such as the MPWP and LIG, is largely unknown because the deposits marking previous extents were in almost all cases over-ridden by later glaciations. For Arctic glaciers, different regional studies consistently indicate that in many places glaciers are now smaller than they have been in millennia ( [[#Lowell--2013|Lowell et al., 2013]] ; [[#Miller--2013|Miller et al., 2013]] , 2017; [[#Harning--2016|Harning et al., 2016]] , 2018; [[#Schweinsberg--2017|Schweinsberg et al., 2017]] , 2018; [[#Pendleton--2019|Pendleton et al., 2019]] ).

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[[File:e8a440f7a81bbc3bb2e4a294feacc564 IPCC_AR6_WGI_Figure_2_23.png]]

'''Figure 2.23''' '''|''' '''Mountain glacier advance and annual mass change. (a)''' Number of a finite selection of surveyed glaciers that advanced during the past 2000 years. '''(b)''' Annual and decadal global glacier mass change (Gt yr <sup>–1</sup> ) from 1961 until 2018. In addition, mass change mean estimates are shown. Ranges show the 90% confidence interval. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

New glacier outline ( [[#RGI%20Consortium--2017|RGI Consortium, 2017]] ) and glacier mass compilations ( [[#Zemp--2019|Zemp et al., 2019]] , 2020; [[#Ciracì--2020|Ciracì et al., 2020]] ; [[#Hugonnet--2021|Hugonnet et al., 2021]] ) improve, refine and update the quantification of glacier areal and mass changes based on observations from in situ and remote sensing data. Observations between the 1960s and 2019 indicate that mass loss has increased over recent decades (Figure 2.23b). The overall global glacier mass loss rate has increased from 240 ± 9 Gt yr <sup>–1</sup> over 2000–2009 to 290 ± 10 Gt yr <sup>–1</sup> over 2010–2019 ( [[#Hugonnet--2021|Hugonnet et al., 2021]] ), confirming that the last decade exhibits the most negative glacier mass balance since the beginning of the observational record. Observations are in general consistent with trends revealed by global glacier mass change modelling for almost the entire 20th century (1901–1990) implying an estimated mass loss (without uncharted glaciers ( [[#Parkes--2018|Parkes and Marzeion, 2018]] ) and excluding peripheral glaciers of Greenland and Antarctica) of ''very likely'' 210 ± 90 Gt yr <sup>–1</sup> and ''very likely'' 170 ± 80 Gt yr <sup>–1</sup> for the period 1971–2019 ( [[#Marzeion--2015|Marzeion et al., 2015]] ; Section 9.5.1 and Table 9.5).

In summary, there is ''very high confidence'' that, with few exceptions, glaciers worldwide have retreated since the second half of the 19th century, and continue to retreat. The current global character of glacier mass loss is highly unusual (almost all glaciers simultaneously receding) in the context of at least the last 2 kyr ( ''medium confidence'' ). Glacier mass loss rates have increased since the 1970s ( ''high confidence'' ). Although many surveyed glaciers are currently more extensive than during the MH ( ''high confidence'' ), they generally are in disequilibrium with respect to current climate conditions and hence are committed to further ice loss.

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==== 2.3.2.4 Ice-sheet Mass and Extent ====

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During glacial periods, ice sheets were more extensive and the state of knowledge on their paleo-reconstruction can be found in recent publications (e.g., [[#Stokes--2015|Stokes et al., 2015]] ; [[#Batchelor--2019|Batchelor et al., 2019]] ). This section focuses only on the large-scale aspects of those ice sheets, Greenland and Antarctic, that still exist today.

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===== 2.3.2.4.1 Greenland Ice Sheet (GrIS) =====

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The AR5 concluded the volume of the Greenland Ice Sheet (GrIS) was reduced compared to present during periods of the past few million years that were globally warmer than present ( ''high confidence'' ). It reported that the GrIS had lost ice during the prior two decades ( ''very high confidence'' ), that the ice loss had occurred in several sectors, and that high rates of mass loss had both expanded to higher elevations ( ''high confidence)'' and ''very likely'' accelerated since 1992. The SROCC concluded that it was ''extremely lik'' e ''ly'' that ice loss increased through the early 21st century. The SROCC also found that summer melting rate had increased since the 1990s to a rate unprecedented over the last 350 years ''(very high confidence'' ), being two to five times greater than the pre-industrial rates ( ''medium confidence'' ).

Details of the history of the GrIS fluctuations during warm interglacials continue to be elucidated. Oscillations over the past 7.5 Myr, including the Pliocene and through the glacial – interglacial cycles of the Pleistocene are not well-constrained, but most studies indicate that Greenland was at least partially glaciated over this time with extended periods when it was predominantly deglaciated ( [[#Bierman--2016|Bierman et al., 2016]] ; [[#Schaefer--2016|Schaefer et al., 2016]] ). Geological evidence and modelling studies suggest periods of glacial intensification during the Pliocene at 4.9 Myr, 4.0 Myr, 3.6 Myr and 3.3 Myr ( [[#De%20Schepper--2014|De Schepper et al., 2014]] ; [[#Bierman--2016|Bierman et al., 2016]] ; [[#Bachem--2017|Bachem et al., 2017]] ). Retreat of the GrIS occurred during the MPWP and GrIS extent was reduced compared to today with some studies suggesting that the ice sheet was limited to the highest elevations ( [[#De%20Schepper--2014|De Schepper et al., 2014]] ; [[#Koenig--2015|Koenig et al., 2015]] ; [[#Haywood--2016|Haywood et al., 2016]] ; [[#Blake-Mizen--2019|Blake-Mizen et al., 2019]] ). There is apparent glacial intensification following the MPWP, 2.75–2.72 Myr ( [[#Nielsen--2013|Nielsen and Kuijpers, 2013]] ; [[#De%20Schepper--2014|De Schepper et al., 2014]] ; [[#Blake-Mizen--2019|Blake-Mizen et al., 2019]] ; [[#Knutz--2019|Knutz et al., 2019]] ). Several studies agree that during the LIG the total GrIS extent was ''likely'' less than present day (Section 9.4.1, Figure 9.17) with the total mass loss ranging from 0.3–6.2 m sea level equivalent (SLE), although timing and magnitude of this mass loss are not well constrained ( [[#Helsen--2013|Helsen et al., 2013]] ; [[#Stone--2013|Stone et al., 2013]] ; [[#Vasskog--2015|Vasskog et al., 2015]] ; [[#Goelzer--2016|Goelzer et al., 2016]] ; [[#Sinclair--2016|Sinclair et al., 2016]] ; [[#Yau--2016|Yau et al., 2016]] ; [[#Clark--2020|Clark et al., 2020]] ). During the LGM, the GrIS reached a peak ice volume greater than present (2–5 m SLE), as revealed by the limited number of available geological records ( [[#Simpson--2009|Simpson et al., 2009]] ; [[#Lecavalier--2014|Lecavalier et al., 2014]] ; [[#Batchelor--2019|Batchelor et al., 2019]] ).

Recent studies of marine and lake sediments, glacier ice, and geomorphic features show that the GrIS retreated rapidly during the early Holocene but halted periodically, with a complex ice-margin chronology ( [[#Carlson--2014|Carlson et al., 2014]] ; [[#Larsen--2014|Larsen et al., 2014]] , 2015; [[#Young--2015|Young and Briner, 2015]] ; [[#Briner--2016|Briner et al., 2016]] ; [[#Young--2020|Young et al., 2020]] ). It is probable that its total volume during 8–3 ka was smaller than today ( [[#Larsen--2015|Larsen et al., 2015]] ; [[#Young--2015|Young and Briner, 2015]] ; [[#Briner--2016|Briner et al., 2016]] ), but uncertainties exist regarding precisely when the minimum MH extent and volume was reached, due to uncertainties in reconstructions. The GrIS then re-advanced reaching its maximum extent in most places during 1450–1850 CE, although the timing and extent of this maximum differed by sector ( [[#Larsen--2015|Larsen et al., 2015]] ; [[#Briner--2016|Briner et al., 2016]] ).

Greenland-wide estimates of mass change based on direct observations were limited prior to 1992 at the time of AR5 ( [[#Kjeldsen--2015|Kjeldsen et al., 2015]] ). Combined records based on airborne observations, model-based estimates and geodetic approaches indicate an average mass loss of 75 ± 29.4 Gt yr <sup>–1</sup> for 1900–1983 ( [[#Kjeldsen--2015|Kjeldsen et al., 2015]] ). Integration of proxies and modelling indicates that the last time the rate of mass loss of the GrIS was plausibly similar to 20th century rates was during the early Holocene ( [[#Buizert--2018|Buizert et al., 2018]] ; [[#Briner--2020|Briner et al., 2020]] ).

Since AR5, a combination of remote sensing, in situ observations and modelling has provided new insights regarding surface processes and their contribution to recent GrIS mass changes ( [[#AMAP--2017|AMAP, 2017]] ; [[#van%20den%20Broeke--2017|van den Broeke et al., 2017]] ; [[#Bamber--2018|Bamber et al., 2018]] ; [[#Mouginot--2019|Mouginot et al., 2019]] ; [[#IMBIE%20Consortium--2020|IMBIE Consortium, 2020]] ; [[#Khan--2020|Khan et al., 2020]] ). Estimates of total ice loss during the post-1850 period ( [[#Kjeldsen--2015|Kjeldsen et al., 2015]] ) and recent observations show that the rate of loss has increased since the beginning of the 21st century ( [[#IMBIE%20Consortium--2020|IMBIE Consortium, 2020]] ; [[#Sasgen--2020|Sasgen et al., 2020]] ; [[#Velicogna--2020|Velicogna et al., 2020]] ) (Section 9.4.1.1 and Figures 2.24 and 9.17).

The GrIS lost 4890 [4140 to 5640] Gt (SLE 13.5 [11.4 to 15.6] mm) of ice between 1992 and 2020 (Section 9.4.1 and Figure 2.24; [[#IMBIE%20Consortium--2020|IMBIE Consortium, 2020]] ). The ice sheet was close to mass balance in the 1990s, but increases in mass loss have occurred since ( [[#Bamber--2018|Bamber et al., 2018]] ; [[#WCRP%20Global%20Sea%20Level%20Budget%20Group--2018|WCRP Global Sea Level Budget Group, 2018]] ; [[#Mouginot--2019|Mouginot et al., 2019]] ; [[#IMBIE%20Consortium--2020|IMBIE Consortium, 2020]] ). The rate of ice-sheet (including peripheral glaciers) mass loss rose from 120 [70 to 170] Gt yr <sup>–1</sup> (SLE 0.33 [0.18 to 0.47] mm yr <sup>–1</sup> ) in 1901–1990 to 330 [290 to 370] Gt yr <sup>–1</sup> (SLE 0.91 [0.79 to 1.02] mm yr <sup>–1</sup> ) for 2006–2018 (Section 9.4.1, Table 9.5).

In summary, the GrIS was smaller than present during the MPWP ( ''medium confidence'' ), LIG ( ''high confidence'' ) and the MH ( ''high confidence'' ). GrIS mass loss began following a peak volume attained during the 1450–1850 period and the rate of loss has increased substantially since the turn of the 21st century ( ''high confidence'' ).

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===== 2.3.2.4.2 Antarctic Ice Sheet (AIS) =====

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The AR5 reported that there was ''high confidence'' that the Antarctic Ice Sheet (AIS) was losing mass. The average ice mass loss from Antarctica was 97 [58 to 135] Gt yr <sup>–1</sup> (GMSL equivalent of 0.27 [0.16 to 0.37] mm yr <sup>–1</sup> ) over 1993–2010, and 147 [74 to 221] Gt yr <sup>–1</sup> (0.41 [0.20 to 0.61] mm yr <sup>–1</sup> ) over 2005–2010. These assessments included the Antarctic peripheral glaciers. The AR5 reported with ''high confidence'' that the volume of the West Antarctic Ice Sheet (WAIS) was reduced during warm periods of the past few million years. The SROCC concluded that over 2006–2015, the AIS lost mass at an average rate of 155 ± 19 Gt yr <sup>–1</sup> ( ''very high confidence'' ). The SROCC also stated that it is ''virtually certain'' that the Antarctic Peninsula and WAIS combined have cumulatively lost mass since widespread measurements began in 1992, and that the rate of loss has increased since around 2006.

Process understanding and, to some extent, paleoclimate records show that changes in parts of the AIS can occur over multi-century time scales (&lt;2kyr; Sections 9.4.2.3 and 9.6.2; e.g., [[#Dowdeswell--2020|Dowdeswell et al., 2020]] ). Based on physical understanding, paleo evidence and numerical simulations, it is ''very likely'' that the AIS has been smaller than today during at least some past warm climates (such as MCO and LIG), in particular the WAIS (Figure 9.18; [[#Golledge--2014|Golledge et al., 2014]] ; [[#de%20Boer--2015|de Boer et al., 2015]] ; [[#DeConto--2016|DeConto and Pollard, 2016]] ; [[#Levy--2016|Levy et al., 2016]] ). Results from sediment studies suggest a smaller AIS during the MPWP compared with current levels, with main differences in the WAIS (Section 9.6.2; SROCC, [[#IPCC--2019|IPCC, 2019]] ; [[#Bertram--2018|Bertram et al., 2018]] ; [[#Shakun--2018|Shakun et al., 2018]] ). Marine sediments indicate that during the Pleistocene repeated ungrounding and loss of large marine-based parts of the AIS occurred during interglacial periods, with at least seven transitions between floating and grounded ice in the Ross Sea during the last 780 kyr ( [[#McKay--2012|McKay et al., 2012]] ) and at least three reductions in ice volume in the Wilkes Basin during the last 500 kyr ( [[#Wilson--2018|Wilson et al., 2018]] ). Proxies, modelling and process understanding ( [[#Rohling--2019|Rohling et al., 2019]] ; [[#Clark--2020|Clark et al., 2020]] ) indicate that the AIS was smaller during the LIG than present.

Geological evidence has been used to reconstruct Holocene glacial fluctuations of the ice sheet margin and lowerings of its surface, which occurred at different times in different places, as recently reviewed by [[#Noble--2020|Noble et al. (2020)]] . In West Antarctica, marine sediments below the ice sheet ( [[#Kingslake--2018|Kingslake et al., 2018]] ) corroborate a previous glacial isostatic adjustment modelling study ( [[#Bradley--2015|Bradley et al., 2015]] ), which suggests that ice had retreated behind the present grounding line prior to about 10 ka, and then readvanced. Geophysical imaging indicates a readvance in this area around 6 ± 2 ka ( [[#Wearing--2019|Wearing and Kingslake, 2019]] ). Other studies from the region conclude that ice-sheet retreat and thinning was fastest from 9 to 8 ka ( [[#Johnson--2014|Johnson et al., 2014]] ; [[#McKay--2016|McKay et al., 2016]] ; [[#Spector--2017|Spector et al., 2017]] ), or millennia later, during the MH ( [[#Hein--2016|Hein et al., 2016]] ; [[#Johnson--2019|Johnson et al., 2019]] ), with indications of a subsequent readvance ( [[#Venturelli--2020|Venturelli et al., 2020]] ). In East Antarctica, rapid ice-sheet thinning occurred between around 9 and 5 ka ( [[#Jones--2015|Jones et al., 2015]] ), consistent with previous work indicating that the ice sheet in many regions was at or close to its current position by 5 ka ( [[#Bentley--2014|Bentley et al., 2014]] ). Overall, during the MH, the AIS was retreating, but remained more extensive than present, while some parts of the ice sheet might have been smaller than now ( ''low confidence'' ).

Improved estimates of surface mass balance (SMB) in Antarctica from 67 ice core records do not show any substantial changes in accumulation rates over most of Antarctica since 1200 CE ( [[#Frezzotti--2013|Frezzotti et al., 2013]] ). The SMB growth rate in Antarctica is estimated to be 7.0 ± 0.1 Gt per decade between 1800 and 2010 and 14.0 ± 1.8 Gt per decade since 1900 ( [[#Thomas--2017|Thomas et al., 2017]] ). For the period 1979–2000, an insignificant Antarctic-wide negative SMB trend has been estimated ( [[#Medley--2019|Medley and Thomas, 2019]] ). The Antarctic Ice Sheet lost 2670 [1800 to 3540] Gt (SLE 7.4 [5.0 to 9.8] mm) of ice between 1992 and 2020. The rate of ice-sheet (including peripheral glaciers) mass loss rose from 0 [–36 to +40] Gt yr <sup>–1</sup> (SLE 0.0 [–0.10 to 0.11] mm yr <sup>–1</sup> ) in 1901–1990 to 192 [145 to 239] Gt yr <sup>–1</sup> (SLE 0.54 [0.47 to 0.61] mm yr <sup>–1</sup> ) for 2006–2018 (Section 9.4.2, Figure 2.24, and Table 9.5). Within quantified uncertainties, this estimate agrees with other recent estimates ( [[#Rignot--2019|Rignot et al., 2019]] ; [[#Smith--2020|]] [[#Smith--2020|B. Smith et al., 2020]] ; [[#Velicogna--2020|Velicogna et al., 2020]] ). There is therefore ''very high confidence'' that the AIS has been losing mass over 1992–2020 (Section 9.4.2.1 and Figure 2.24). Major contributions to recent AIS changes arise from West Antarctica and Wilkes Land in East Antarctica ( [[#Rignot--2019|Rignot et al., 2019]] ). For the East Antarctic most studies suggest that the mass balance is not significantly different from zero ( [[#Bamber--2018|Bamber et al., 2018]] ; [[#IMBIE%20Consortium--2018|IMBIE Consortium, 2018]] ; [[#Mohajerani--2018|Mohajerani et al., 2018]] ; [[#Rignot--2019|Rignot et al., 2019]] ).

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[[File:8246f75a6d42767749dc6960bd8237e0 IPCC_AR6_WGI_Figure_2_24.png]]

'''Figure 2.''' '''24 |''' '''Cumulative Antarctic Ice Sheet (AIS) and Greenland Ice Sheet (GrIS) mass changes.''' Values shown are in gigatons and come from satellite-based measurements ( [[#IMBIE%20Consortium--2018|IMBIE Consortium, 2018]] , 2020) for the period 1992–2020. The estimated uncertainties, ''very likely'' range, for the respective cumulative changes are shaded. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

In summary, the AIS has lost mass between 1992 and 2020 ( ''very high confidence'' ), and there is ''medium confidence'' that this mass loss has increased. During the MPWP and LIG, the ice sheet was smaller than present ( ''medium confidence'' ). There is ''low confidence'' as to whether the total mass of the ice sheet was larger or smaller around 6 ka compared to now.

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==== 2.3.2.5 Terrestrial Permafrost ====

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The AR5 concluded that in most regions and at most monitoring sites permafrost temperatures since the 1980s had increased ( ''high confidence'' ). Negligible change was observed at a few sites, mainly where permafrost temperatures were close to 0°C, with slight cooling at a limited number of sites. The AR5 also noted positive trends in active layer thickness (ALT; the seasonally thawed layer above the permafrost) since the 1990s for many high latitude sites ( ''medium confidence'' ). The SROCC concluded permafrost temperatures have increased to record high levels since the 1980s ( ''very high confidence'' ) with a recent increase by 0.29°C ± 0.12°C from 2007 to 2016 averaged across polar and high mountain regions globally.

Permafrost occurrence during the Pliocene has been inferred from pollen in lake sediments in NE Arctic Russia and permafrost-vegetation relationships which indicate that permafrost was absent during the MPWP in this region ( [[#Brigham-Grette--2013|Brigham-Grette et al., 2013]] ; [[#Herzschuh--2016|Herzschuh et al., 2016]] ). Analysis of speleothem records in Siberian caves, indicates that permafrost was absent in the current continuous permafrost zone at 60°N at the start of the 1.5 Ma record, with aggradation occurring around 0.4 Ma ( [[#Vaks--2020|Vaks et al., 2020]] ). There are indications of extensive permafrost thaw during subsequent interglacials especially further south in the current permafrost zone ( [[#Vaks--2013|Vaks et al., 2013]] ). Reconstruction of permafrost distribution during the LGM indicates that permafrost was more extensive in exposed areas ( [[#Vandenberghe--2014|Vandenberghe et al., 2014]] ). In non-glaciated areas of the North American Arctic there is permafrost that survived the LIG ( [[#French--2014|French and Millar, 2014]] ). Trends and timing of permafrost aggradation and thaw over the last 6 kyr in peatlands of the NH were recently summarized ( [[#Hiemstra--2018|Hiemstra, 2018]] ; [[#Treat--2018|Treat and Jones, 2018]] ). Three multi-century periods (ending 1000 Before the Common Era (BCE), 500 CE and 1850 CE) of permafrost aggradation, associated with neoglaciation periods are inferred resulting in more extensive permafrost in peatlands of the present-day discontinuous permafrost zone, which reached a peak approximately 250 years ago, with thawing occurring concurrently with post 1850 warming ( [[#Treat--2018|Treat and Jones, 2018]] ). Although permafrost persists in peatlands at the southern extent of the permafrost zone where it was absent prior to 3 ka, there has been thawing since the 1960s ( [[#James--2013|James et al., 2013]] ; B.M. [[#Jones--2016|]] [[#Jones--2016|Jones et al., 2016]] ; [[#Holloway--2020|Holloway and Lewkowicz, 2020]] ).

Records of permafrost temperature measured in several boreholes located throughout the northern polar regions indicate general warming of permafrost over the last 3–4 decades (Figure 2.25), with marked regional variations ( [[#Romanovsky--2017a|Romanovsky et al., 2017a]] , b, 2020; [[#Biskaborn--2019|Biskaborn et al., 2019]] ). Recent (2018–2019) permafrost temperatures in the upper 20–30 m layer (at depths where seasonal variation is minimal) were the highest ever directly observed at most sites ( [[#Romanovsky--2020|Romanovsky et al., 2020]] ), with temperatures in colder permafrost of northern North America being more than 1°C higher than they were in 1978. Increases in temperature of colder Arctic permafrost are larger (average 0.4°C–0.6°C per decade) than for warmer (temperature &gt;–2°C) permafrost (average 0.17°C per decade) of sub-Arctic regions (Figures 2.25, 9.22).

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[[File:cd36a54a9f35ce7efec6035c4a10cf2d IPCC_AR6_WGI_Figure_2_25.png]]

'''Figure''' '''2.25 |''' '''Changes in permafrost temperature.''' Average departures of permafrost temperature (measured in the upper 20–30 m) from a baseline established during International Polar Year (2007–2009) for Arctic regions. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

Increases in permafrost temperature over the last 10–30 years of up to 0.3°C per decade have been documented at depths of about 20 m in high elevation regions in the NH (European Alps, the Tibetan Plateau and some other high elevation areas in Asia; G. [[#Liu--2017|]] [[#Liu--2017|]] [[#Liu--2017|Liu et al., 2017]] ; [[#Cao--2018|Cao et al., 2018]] ; [[#Biskaborn--2019|Biskaborn et al., 2019]] ; [[#Noetzli--2020|Noetzli et al., 2020]] ; [[#Zhao--2020|Zhao et al., 2020]] ). In Antarctica, where records are limited and short (most &lt;10 years) trends are less evident ( [[#Noetzli--2019|Noetzli et al., 2019]] ).

Assessment of trends in ALT is complicated by considerable ALT interannual variability. For example, in north-western North America during the extreme warm year of 1998, ALT was greater than in prior years. Although ALT decreased over the following few years, it has generally increased again since the late 2000s ( [[#Duchesne--2015|Duchesne et al., 2015]] ; [[#Romanovsky--2017b|Romanovsky et al., 2017b]] , 2020). However, at some sites there has been little change in ALT due to ground subsidence that accompanies thaw of ice-rich permafrost ( [[#Streletskiy--2017|Streletskiy et al., 2017]] ; [[#O’Neill--2019|O’Neill et al., 2019]] ). In the European and Russian Arctic there has been a broad-scale increase in ALT during the 21st century ( [[#Streletskiy--2015|Streletskiy et al., 2015]] ; [[#Romanovsky--2020|Romanovsky et al., 2020]] ). In high elevation areas in Europe and Asia, increases in ALT have occurred since the mid-1990s (Y. [[#Liu--2017|]] [[#Liu--2017|]] [[#Liu--2017|Liu et al., 2017]] ; [[#Cao--2018|Cao et al., 2018]] ; [[#Noetzli--2019|Noetzli et al., 2019]] , 2020; [[#Zhao--2020|Zhao et al., 2020]] ). Limited and shorter records for Antarctica show marked interannual variability and no apparent trend with ALT being relatively stable or decreasing at some sites since 2006 ( [[#Hrbáček--2018|Hrbáček et al., 2018]] ).

Observations of ground subsidence and other landscape change (e.g., thermokarst, slope instability) since the middle of the 20th century in the Arctic associated with ground ice melting have been documented in several studies and provide additional indications of thawing permafrost ( [[#Séjourné--2015|Séjourné et al., 2015]] ; [[#Liljedahl--2016|Liljedahl et al., 2016]] ; [[#Borge--2017|Borge et al., 2017]] ; [[#Kokelj--2017|Kokelj et al., 2017]] ; [[#Nitze--2017|Nitze et al., 2017]] ; [[#Streletskiy--2017|Streletskiy et al., 2017]] ; [[#Derksen--2019|Derksen et al., 2019]] ; [[#Farquharson--2019|Farquharson et al., 2019]] ; [[#Lewkowicz--2019|Lewkowicz and Way, 2019]] ; [[#O’Neill--2019|O’Neill et al., 2019]] ; see Section 9.5.2.1). In mountain areas, destabilization and acceleration of rock glacier complexes that may be associated with warming permafrost have also been observed ( [[#Eriksen--2018|Eriksen et al., 2018]] ; [[#Marcer--2019|Marcer et al., 2019]] ).

In summary, increases in permafrost temperatures in the upper 30 m have been observed since the start of observational programs over the past three to four decades throughout the permafrost regions ( ''high confidence'' ). ''Limited evidence'' suggests that permafrost was less extensive during the MPWP ( ''low confidence'' ). Permafrost that formed after 3ka still persists in areas of the NH, but there are indications of thaw after the mid-1800s ( ''medium confidence'' ).

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