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

== 1.4 Changes in the Ocean and Cryosphere ==

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Earth’s climate, ocean and cryosphere vary across a wide range of time scales. This includes the seasonal growth and melting of sea ice, interannual variation of ocean temperature caused by the El Niño-Southern Oscillation and ice age cycles across tens to hundreds of thousands of years.

Climate variability can arise from internally generated (i.e., unforced) fluctuations in the climate system. Variability can also occur in response to external forcings, including volcanic eruptions, changes in the Earth’s orbit around the Sun, oscillations in solar activity and changing atmospheric greenhouse gas concentrations.

Since the onset of the industrial revolution, human activities have had a strong impact on the climate system, including the ocean and cryosphere. Human activities have altered the external forcings acting on Earth’s climate (Myhre et al., 2013 <sup>[[#fn:r88|88]]</sup> ) by changes in land use (albedo), and changes in atmospheric aerosols (e.g., soot) from the burning of biomass and fossil fuels. Most significantly, human activities have led to an accumulation of greenhouse gases (including CO 2 ) in the atmosphere as a result of the burning of fossil fuels, cement production, agriculture and land use change. In 2016, the global average atmospheric CO 2 concentration crossed 400 parts per million, a level Earth’s atmosphere did not experience for at least the past 800,000 years and possibly much longer (Lüthi et al., 2008 <sup>[[#fn:r89|89]]</sup> ; Fischer et al., 2018 <sup>[[#fn:r90|90]]</sup> ). These anthropogenic forcings have not only warmed the ocean and begun to melt the cryosphere, but have also led to widespread biogeochemical changes driven by the oceanic uptake of anthropogenic CO 2 from the atmosphere (IPCC, 2013 <sup>[[#fn:r91|91]]</sup> ).

It is now nearly three decades since the first assessment report of the IPCC, and over that time evidence and confidence in observed and projected ocean and cryosphere changes have grown ( ''very high confidence'' ; Table SM1.1). Confidence in climate warming and its anthropogenic causes has increased across assessment cycles; robust detection was not yet possible in 1990, but has been characterised as unequivocal since AR4 in 2007. Projections of near-term warming rates in early reports have been realised over the subsequent decades, while projections have tended to err on the side of caution for sea level rise and ocean heat uptake that have developed faster than predicted (Brysse et al., 2013 <sup>[[#fn:r92|92]]</sup> ; Section 4.2, 5.2). Areas of concern in early reports which were expected but not observable are now emerging. The expected acceleration of sea level rise is now observed with ''high confidence'' (Section 4.2). There is emerging evidence in sustained observations and from long-term palaeoclimate reconstructions for the expected slow-down of AMOC ( ''medium confidence'' ), although this remains to be properly attributed (Section 6.7). Significant sea level rise contributions from Antarctic ice sheet mass loss ( ''very high confidence'' ), which earlier reports did not expect to manifest this century, are already being observed (Section 3.3.1). Other newly emergent characteristics of ocean and cryosphere change (e.g., marine heat waves; Section 6.4) are assessed for the first time in SROCC.

AR5 (IPCC, 2013 <sup>[[#fn:r93|93]]</sup> ; IPCC, 2014 <sup>[[#fn:r94|94]]</sup> ) provides ample evidence of profound and pervasive changes in the ocean and cryosphere (Sections 1.4.1, 1.4.2), and along with the recent SR15 report (IPCC, 2018 <sup>[[#fn:r95|95]]</sup> ), is the point of departure for the updated assessments made in SROCC.

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=== 1.4.1 Observed and Projected Changes in the Ocean ===

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Increasing greenhouse gases in the atmosphere cause heat uptake in the Earth system (Section 1.2) and as reported since 1970, there is ''high confidence <sup>[[#fn:3|3]]</sup>'' that the majority (more than 90%) of the extra thermal energy in the Earth’s system is stored in the global ocean (IPCC, 2013 <sup>[[#fn:r96|96]]</sup> ). Mean ocean surface temperature has increased since the 1970s at a rate of 0.11 (0.09 – 0.13)°C per decade ( ''high confidence'' ), and forms part of a long-term warming of the surface ocean since the mid-19th century. The upper ocean (0 – 700 m, ''virtually certain'' ) and intermediate ocean (700 to 2,000 m, ''likely'' ) have warmed since the 1970s. Ocean heat uptake has continued unabated since AR5 (Sections 3.2.1.2.1, 5.2 ), increasing the risk of marine heat waves and other extreme events (Section 6.4). During the 21st century, ocean warming is projected to continue even if anthropogenic greenhouse gas emissions cease (Sections 1.3, 5.2). The global water cycle has been altered, resulting in substantial regional changes in sea surface salinity ( ''high confidence'' ; Rhein et al., 2013 <sup>[[#fn:r97|97]]</sup> ), which is expected to continue in the future (Sections 5.2.2, 6.3, 6.5).

The rate of sea level rise since the mid-19th century has been larger than the mean rate of the previous two millennia ( ''high confidence'' ). Over the period 1901 to 2010, global mean sea level rose by 0.19 (0.17 – 0.21) m ( ''high confidence)'' (Church et al., 2013 <sup>[[#fn:r98|98]]</sup> ; Table SM1.1). Sea level rise continues due to freshwater added to the ocean by melting of glaciers and ice sheets, and as a result of ocean expansion due to continuous ocean warming, with a projected acceleration and century to millennial-scale commitments for ongoing rise (Section 4.2.3). In SROCC, recent developments of ice sheet modelling are assessed (Sections 1.8, 4.3, Cross-Chapter Box 8 in Chapter 3) and the projected sea level rise at the end of 21st century is higher than reported in AR5 but with a larger uncertainty range (Sections 4.2.3.2, 4.2.3.3).

By 2011, the ocean had taken up about 30 ± 7% of the anthropogenic CO 2 that had been released to the atmosphere since the industrial revolution (Ciais et al., 2013 <sup>[[#fn:r99|99]]</sup> ; Section 5.2). In response, ocean pH decreased by 0.1 since the beginning of the industrial era ( ''high confidence'' ), corresponding to an increase in acidity of 26% (Table SM1.1) and leading to both positive and negative biological and ecological impacts ( ''high confidence'' ) (Gattuso et al., 2014 <sup>[[#fn:r100|100]]</sup> ) . Evidence is increasing that the ocean’s oxygen content is declining (Oschlies et al., 2018 <sup>[[#fn:r101|101]]</sup> ). AR5 did not come to a final conclusion with regard to potential long-term changes in ocean productivity due to short observational records and divergent scientific evidence (Boyd et al., 2014 <sup>[[#fn:r102|102]]</sup> ; Section 5.2.2). Ocean acidification and deoxygenation are projected to continue over the next century with ''high confidence'' (Sections 3.2.2.3, 5.2.2).

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=== 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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