Editing IPCC:AR6/WGI/TS (section)

=== TS.2.5 The Cryosphere ===

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'''Over recent decades, widespread loss of snow and ice has been observed, and several elements of the cryosphere are now in states unseen in centuries (''high confidence''). Human influence was ''very likely'' the main driver of observed reductions in Arctic sea ice since the late 1970s (with late-summer sea ice loss ''likely'' unprecedented for at least 1000 years) and the widespread retreat of glaciers (unprecedented in at least the last 2,000 years, ''medium confidence''). Furthermore, human influence ''very likely'' contributed to the observed Northern Hemisphere spring snow cover decrease since 1950.''' '''By contrast, Antarctic sea ice area experienced no significant net change since 1979, and there is only ''low confidence'' in its projected changes. The Arctic Ocean is projected to become practically sea ice-free in late summer under high CO <sub>2</sub> emissions scenarios by the end of the 21st century (''high confidence''). It is ''virtually certain'' that further warming will lead to further reductions of Northern Hemisphere snow cover, and there is ''high confidence'' that this is also the case for near-surface permafrost volume.''' '''Glaciers will continue to lose mass at least for several decades even if global temperature is stabilized (''very high confidence''), and mass loss over the 21st century is ''virtually certain'' for the Greenland Ice Sheet and ''likely'' for the Antarctic Ice Sheet. Deep uncertainty persists with respect to the possible evolution of the Antarctic Ice Sheet within the 21st century and beyond, in particular due to the potential instability of the West Antarctic Ice Sheet. Links to chapters 2.3, 3.4, 4.3, 8.3, 9.3–9.6, Box 9.4, 12.4'''

Current Arctic sea ice coverage levels (both annual and late summer) are at their lowest since at least 1850 (''high confidence''), and for late summer for the past 1000 years (''medium confidence''). Since the late 1970s, Arctic sea ice area and thickness have decreased in both summer and winter, with sea ice becoming younger, thinner and more dynamic (''very high confidence''). It is ''very likely'' that anthropogenic forcing, mainly due to greenhouse gas increases, was the main driver of this loss, although new evidence suggests that anthropogenic aerosol forcing has offset part of the greenhouse gas-induced losses since the 1950s (''medium confidence''). The annual Arctic sea ice area minimum will ''likely'' fall below 1 million km <sup>2</sup> at least once before 2050 under all assessed SSP scenarios. This practically sea ice-free state will become the norm for late summer by the end of the 21st century in high CO <sub>2</sub> emissions scenarios (''high confidence''). Arctic summer sea ice varies approximately linearly with global surface temperature, implying that there is no tipping point and observed/projected losses are potentially reversible (''high'' ''confidence''). Links to chapters 2.3.2, 3.4.1, 4.3.2, 9.3.1, 12.4.9

For Antarctic sea ice, there is no significant trend in satellite-observed sea ice area from 1979 to 2020 in both winter and summer, due to regionally opposing trends and large internal variability. Due to mismatches between model simulations and observations, combined with a lack of understanding of reasons for substantial inter-model spread, there is ''low confidence'' in model projections of future Antarctic sea ice changes, particularly at the regional level. Links to chapters 2.3.2, 3.4.1, 9.3.2

In permafrost regions, increases in ground temperatures in the upper 30 m over the past three to four decades have been widespread (''high confidence''). For each additional 1°C of warming (up to 4°C above the 1850–1900 level), the global volume of perennially frozen ground to 3 m below the surface is projected to decrease by about 25% relative to the present volume (''medium confidence''). However, these decreases may be underestimated due to an incomplete representation of relevant physical processes in ESMs (''low confidence''). Seasonal snow cover is treated in Section TS.2.6. Links to chapters 2.3.2, 9.5.2, 12.4.9

There is ''very high confidence'' that, with few exceptions, glaciers have retreated since the second half of the 19th century; this behaviour is unprecedented in at least the last 2000 years (''medium confidence''). Mountain glaciers ''very likely'' contributed 67.2 [41.8 to 92.6] mm to the observed GMSL change between 1901 and 2018. This retreat has occurred at increased rates since the 1990s, with human influence ''very likely'' being the main driver. Under RCP2.6 and RCP8.5, respectively, glaciers are projected to lose 18% ± 13% and 36% ± 20% of their current mass over the 21st century (''medium confidence''). Links to chapters 2.3.2, 3.4.3, 9.5.1, 9.6.1

The Greenland Ice Sheet was smaller than at present during the Last Interglacial period (roughly 125,000 years ago) and the mid-Holocene (roughly 6,000 years ago) (''high confidence''). After reaching a recent maximum ice mass at some point between 1450 and 1850, the ice sheet retreated overall, with some decades ''likely'' close to equilibrium (i.e., mass loss approximately equalling mass gained). It is ''virtually certain'' that the Greenland Ice Sheet has lost mass since the 1990s, with human influence a contributing factor (''medium confidence''). There is ''high confidence'' that annual mass changes have been consistently negative since the early 2000s. Over the period 1992–2020, Greenland ''likely'' lost 4890 ± 460 Gt of ice, contributing 13.5 ± 1.3 mm to GMSL rise. There is ''high confidence'' that Greenland ice mass losses are increasingly dominated by surface melting and runoff, with large interannual variability arising from changes in surface mass balance. Projections of future Greenland ice-mass loss (Box TS.4, Table 1; Figure TS.11e) are dominated by increased surface melt under all emissions scenarios (''high confidence''). Potential irreversible long-term loss of the Greenland Ice Sheet, and of parts of the Antarctic Ice Sheet, is assessed in Box TS.9. Links to chapters 2.3.2, 3.4.3, 9.4.1, 9.4.2, 9.6.3, Atlas.11.2

It is ''likely'' that the Antarctic Ice Sheet has lost 2670 ± 530 Gt, contributing 7.4 ± 1.5 mm to GMSL rise over 1992–2020. The total Antarctic ice mass losses were dominated by the West Antarctic Ice Sheet, with combined West Antarctic and Peninsula annual loss rates increasing since about 2000 (''very high confidence''). Furthermore, it is ''very likely'' that parts of the East Antarctic Ice Sheet have lost mass since 1979. Since the 1970s, snowfall has ''likely'' increased over the western Antarctic Peninsula and eastern West Antarctica, with large spatial and interannual variability over the rest of Antarctica. Mass losses from West Antarctic outlet glaciers, mainly induced by ice shelf basal melt (''high confidence''), outpace mass gain from increased snow accumulation on the continent (''very high confidence''). However, there is only ''limited evidence'' , with ''medium agreement'' , of anthropogenic forcing of the observed Antarctic mass loss since 1992 (with ''low confidence'' in process attribution). Increasing mass loss from ice shelves and inland discharge will ''likely'' continue to outpace increasing snowfall over the 21st century (Figure TS.11f). Deep uncertainty persists with respect to the possible evolution of the Antarctic Ice Sheet along high-end mass-loss storylines within the 21st century and beyond, primarily related to the abrupt and widespread onset of marine ice sheet instability and marine ice cliff instability. (See also Boxes TS.3 and TS.4). Links to chapters 2.3.2, 3.4.3, 9.4.2, 9.6.3, Box 9.4, Atlas.11.1

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