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

==== 2.3.2.5 Terrestrial Permafrost ====

<div id="h3-20-siblings" class="h3-siblings"></div>

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).

<div id="_idContainer064" class="Basic-Text-Frame"></div>

[[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'' ).

<div id="2.3.3" class="h2-container"></div>

<span id="ocean"></span>