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

==== Atlas.11.1.1 Key Features of the Regional Climate and Findings From Previous IPCC Assessments ====

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===== Atlas.11.1.1.1 Key Features of the Regional Climate =====

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The Antarctic region, covered by an ice sheet and surrounded by the Southern Ocean, is characterized by polar climate. It is the coldest, windiest and driest continent on Earth and plays a pivotal role in regulating the global climate and hydrological cycle. Antarctica has a mean temperature of –35°C ( [[#Lenaerts--2016|Lenaerts et al., 2016]] ) and receives 171 mm yr <sup>–1</sup> water equivalent of snowfall (north of 82°S, estimate based on satellite measurements during 2006–2011; [[#Palerme--2014|Palerme et al., 2014]] ). Precipitation in Antarctica occurs mostly in the form of snowfall and diamond dust, with sporadic coastal rainfall during the summer over the Antarctic Peninsula and sub-Antarctic islands. Drizzle events sometimes occur during warm air intrusions ( [[#Nicolas--2017|Nicolas et al., 2017]] ) at relatively low temperatures ( [[#Silber--2019|Silber et al., 2019]] ). Precipitation constitutes the largest component of the surface mass balance (SMB) '','' which also includes sublimation (from the surface or drifting snow), meltwater runoff and redistribution by wind ( [[#Lenaerts--2019|Lenaerts et al., 2019]] ). SMB can be considered as a proxy of precipitation if averaged over an annual cycle ( [[#Gorodetskaya--2015|Gorodetskaya et al., 2015]] ; [[#Bracegirdle--2019|Bracegirdle et al., 2019]] ). Precipitation and SMB exhibit spatial and temporal variability controlled by atmospheric large-scale low-pressure systems and moisture advection from lower latitudes. SMB is an important component of the total ice-sheet mass balance ( [[IPCC:Wg1:Chapter:Chapter-9#9.4.2.1|Section 9.4.2.1]] ). The Antarctic contribution to sea level results from the imbalance between net snow accumulation and ice discharge into the ocean (Box 9.1). Ice shelves buttress the ice sheet and are influenced by oceanic and atmospheric drivers (Box 9.1).

Antarctic climate variability is influenced by the Southern Annular Mode (SAM) and regionally by other modes, including ENSO, Pacific–South American pattern, Pacific Decadal Variability (PDV), Indian Ocean Dipole and Zonal Wave 3 (Annex IV). Climate change in Antarctica and the Southern Ocean is influenced by interactions between the ice sheet, ocean, sea ice and atmosphere (Sections 9.2.3.2, 9.3.2 and 9.4.2; [[#Meredith--2019|Meredith et al., 2019]] ). In addition to Chapter 9, Antarctica is discussed across the report: global climate links (Chapters 2 and 10), attribution (Chapter 3), global water cycle (Chapter 8), extremes (Chapter 11), and climatic impact-drivers (Chapter 12).

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===== Atlas.11.1.1.2 Findings From previous IPCC Assessments =====

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The AR5 ( [[#Vaughan--2013|Vaughan et al., 2013]] ) reported warming over Antarctica since the 1950s, mostly over the AP and WAN, attributed to the positive trend in the SAM. These trends in the Antarctic temperature were given ''low confidence'' due to substantial multi-annual to multi-decadal variability, as well as uncertainties in magnitude and spatial trend structure. The AR5 reported ''low confidence'' that anthropogenic forcing has contributed to the temperature change in Antarctica. The AR5 highlighted a large interannual variability in snow accumulation with no significant trend since 1979 around Antarctica, and ''high confidence'' in the overall mass loss from Antarctica, accelerated since the 1990s.

In this and the following paragraphs, findings are from SROCC ( [[#Meredith--2019|Meredith et al., 2019]] ) unless otherwise stated. Warming trends were reported over parts of WAN with record surface warmth over WAN during the 1990s compared to the past 200 years, and AP surface melting intensifying since the mid-20th century. No significant temperature trends were reported over EAN and there was ''low confidence'' in both WAN and EAN trend estimates due to sparse in situ records and large interannual to inter-decadal variability. In the AP, concomitant increase in temperature and foehn winds due to positive SAM caused increased surface melting over the Larsen ice shelves ( ''medium confidence'' ). Strong warming between the mid-1950s and the late 1990s led to the collapse of the Larsen B ice shelf in 2002, which had been intact for 11,000 years ( ''medium confidence'' ).

Snowfall increased over the Antarctic Ice Sheet over AP and WAN, offsetting some of the 20th-century sea level rise ( ''medium confidence'' ). Longer records suggest either a decrease in snowfall over the Antarctic Ice Sheet over the last 1000 years or a statistically negligible change over the last 800 years ( ''low confidence'' ).

Recent warming in the AP and consequent ice-shelf collapse are ''likely'' linked to anthropogenic ozone and greenhouse gas forcing via the SAM and anthropogenically driven Atlantic sea surface. Also, there is ''high confidence'' in the influence of tropical sea surface temperature on the Antarctic temperature and Southern Hemisphere mid-latitude circulation, as well as the SAM. There is ''medium agreement'' but ''limited evidence'' of an anthropogenic forcing effect on Antarctic ice-sheet mass balance ( ''low confidence'' ) and partitioning between natural and human drivers of atmospheric and ocean circulation changes remains very uncertain.

In AR5, [[#Church--2013|Church et al. (2013)]] gave ''medium confidence'' in model projections of a future Antarctic SMB increase, implying a negative contribution to global mean sea level rise, consistent with a projection of significant Antarctic warming. [[#Church--2013|Church et al. (2013)]] also gave ''high confidence'' to the relationship between future temperature and precipitation increases in Antarctica on physical grounds and from ice-core evidence. In [[#Meredith--2019|Meredith et al. (2019)]] , the total mass balance projections derived from ice-sheet models were reported without separating the SMB, though projections were reported of increased precipitation and continued strengthening of the westerly winds in the Southern Ocean.

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