Editing IPCC:AR6/SROCC/Chapter-1 (section)

=== 1.4.2 Observed and Projected Changes in the Cryosphere ===

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Changes in the cryosphere documented in AR5 included the widespread retreat of glaciers ( ''high confidence'' ), mass loss from the Greenland and Antarctic ice sheets ( ''high confidence'' ) and declining extents of Arctic sea ice ( ''very high confidence'' ) and Northern Hemisphere spring snow cover ( ''very high confidence'' ; IPCC, 2013 <sup>[[#fn:r103|103]]</sup> ; Vaughan et al., 2013 <sup>[[#fn:r104|104]]</sup> ).

A particularly rapid change in Earth’s cryosphere has been the decrease in Arctic sea ice extent in all seasons (Section 3.2.1.1). AR5 assessed that there was ''medium confidence'' that a nearly ice-free summer Arctic Ocean is ''likely'' to occur before mid-century under a high emissions future (IPCC, 2013 <sup>[[#fn:r105|105]]</sup> ), and SR15 assessed that ice-free summers are projected to occur at least once per century at 1.5 o C of warming, and at least once per decade at 2 o C of warming above pre-industrial levels (IPCC, 2018 <sup>[[#fn:r106|106]]</sup> ). Sea ice thickness is decreasing further in the Northern Hemisphere and older ice that has survived multiple summers is rapidly disappearing; most sea ice in the Arctic is now ‘first year’ ice that grows in the autumn and winter but melts during the spring and summer (AMAP, 2017 <sup>[[#fn:r107|107]]</sup> ).

AR5 assessed that the annual mean loss from the Greenland ice sheet ''very likely'' substantially increased from 34 (-6 – 74) Gt yr –1 (billion tonnes yr -1 ) over the period 1992 – 2001, to 215 (157 – 274) Gt yr –1 over the period 2002 – 2011 (IPCC, 2013). The average rate of ice loss from the Antarctic ice sheet also ''likely'' increased from 30 (-37 – 97) Gt yr –1 over the period 1992–2001, to 147 (72 – 221) Gt yr –1 over the period 2002 – 2011 (IPCC, 2013 <sup>[[#fn:r108|108]]</sup> ). The average rate of ice loss from glaciers around the world (excluding glaciers on the periphery of the ice sheets), was ''very likely'' 226 (91 – 361) Gt yr -1 over the period 1971 – 2009, and 275 (140 – 410) Gt yr -1 over the period 1993 – 2009 (IPCC, 2013). The Greenland and Antarctic ice sheets are continuing to lose mass at an accelerating rate (Section 3.3) and glaciers are continuing to lose mass worldwide (Section 2.2.3, Cross-Chapter Box 6 in Chapter 2). Confidence in the quantification of glacier and ice sheet mass loss has increased across successive IPCC reports (Table SM1.1) due to the development of remote sensing observational methods (Section 1.8.1).

Changes in seasonal snow are best documented for the Northern Hemisphere. AR5 reported that the extent of snow cover has decreased since the mid-20th century ( ''very high confidence'' ). Negative trends in both snow depth and duration are also detected with station observations ( ''medium confidence'' ), although results depend on elevation and observational period (Section 2.2.2). AR5 assessed that permafrost temperatures have increased in most regions since the early 1980s ( ''high confidence'' ), and the rate of increase has varied regionally (IPCC, 2013 <sup>[[#fn:r109|109]]</sup> ). Methane and carbon dioxide release from soil organic carbon is projected to continue in high mountain and polar regions (Box 2.2), and SROCC has used multiple lines of evidence to reduce uncertainty in permafrost change assessments (Cross-Chapter Box 5 in Chapter 1, Section 3.4.3.1.1).

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