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

==== Atlas.11.2.2 Assessment and Synthesis of Observations, Trends and Attribution ====

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The Arctic has warmedat more than twice the global rate over the past 50 years with the greatest warming during the cold season ( ''high confidence'' ) ( [[#Davy--2018|Davy et al., 2018]] ; [[#Box--2019|Box et al., 2019]] ; [[#Przybylak--2020|Przybylak and Wyszyński, 2020]] ; [[#Xiao--2020|Xiao et al., 2020]] ). This is based on various Arctic amplification processes, in particular the combined effect of several related feedback processes, including between various radiation components and (a) the albedo of sea ice and snow, (b) water vapour, and (c) clouds, as well as poleward energy transports. The annual average Arctic surface air temperature increased by 2.7°C from 1971 to 2017, with a 3.1°C increase in the cold season (October–May) and a 1.8°C increase in the warm season (June–September) ( [[#AMAP--2019|AMAP, 2019]] ). Satellite-based data estimate the rate of annual warming for 1981–2012 over sea ice covered regions to be 0.47°C per decade, whereas the trend was significantly higher at 0.77°C per decade over Greenland and amplified in the northern Barents and Kara seas ( [[#Comiso--2014|Comiso and Hall, 2014]] ). The largest Arctic warming in 2003–2017 was reported over the Barents and Kara seas with trends larger than 2.5°C per decade ( [[#Susskind--2019|Susskind et al., 2019]] ), and Arctic temperatures from 2014 to 2018 have exceeded all previous records since 1900 ( [[#Blunden--2019|Blunden and Arndt, 2019]] ).

Over the ARO, long-term temperature records are available from Spitsbergen (Svalbard Airport). For the period 1898–2018, the annual mean warming was 0.32°C per decade, about 3.5 times the global mean temperature for the same period and since 1991, it was 1.7°C per decade or about seven times the global average for the same period ( [[#Nordli--2020|Nordli et al., 2020]] ). There is a positive trend in the annual temperature for all stations across Svalbard ( [[#Gjelten--2016|Gjelten et al., 2016]] ; [[#Hanssen-Bauer--2019|Hanssen-Bauer et al., 2019]] ; [[#Dahlke--2020|Dahlke et al., 2020]] ) of 0.64°C–1.01°C per decade for 1971–2017 ( [[#Hanssen-Bauer--2019|Hanssen-Bauer et al., 2019]] ), co-varying with regional changes in sea ice conditions ( [[#Dahlke--2020|Dahlke et al., 2020]] ). The largest temperature trends ''very likely'' occur in winter, with Svalbard Airport warming at 0.43°C per decade during 1898–2018 and 3.19°C per decade during 1991–2018 ( [[#Nordli--2020|Nordli et al., 2020]] ), and [[#Isaksen--2016|Isaksen et al. (2016)]] reporting on substantial warming in western Spitsbergen, particularly in winter, while the summer warming is moderate.

A multi-dataset analysis for NEN shows a consistent warming ( [[#Rapaić--2015|Rapaić et al., 2015]] ), with the largest annual temperature trend greater than 0.3°C per decade during 1981–2010 over eastern NEN and also significant warming over northern Quebec and most of the Canadian Arctic north of the treeline. For the longer 1950–2010 period, a consistent warming is found over central and western NEN, but no trend or no consensus is found over the Labrador coast. The latter is related with cooling of the North Atlantic region during the 1970s. For western Greenland, however, summer temperatures increased (2.2°C in June, 1.1°C in July) from 1994 to 2015 ( [[#Saros--2019|Saros et al., 2019]] ). For neighbouring Arctic regions of NEU, WSE and ESB, datasets show a consistent warming of annual mean temperature since the mid-1970s and 1980 ( [[#Atlas.8|Atlas.8]] and [[#Atlas.5.2|Atlas.5.2]] ).

Along with the amplified warming, the Arctic has become moister ( [[#Rinke--2019|Rinke et al., 2019]] ; [[#Nygård--2020|Nygård et al., 2020]] ). AMAP reported Arctic precipitation increases of 1.5–2.0% per decade, with the strongest increase in the cold season (October–May) ( ''medium confidence'' ) ( [[#AMAP--2019|AMAP, 2019]] ). Also, for neighbouring Arctic regions for example NEU, EEU and North Asia, mean annual precipitation has increased since the early 20th century ( [[#Atlas.8|Atlas.8]] and [[#Atlas.5.2|Atlas.5.2]] ). Estimated trends for precipitation and snowfall fraction are mixed for the Arctic, with increases and decreases for different regions and seasons ( [[#Vihma--2016|Vihma et al., 2016]] ). However, annual precipitation trends derived from different reanalyses do not agree, differ in sign and have low significance ( [[#Lindsay--2014|Lindsay et al., 2014]] ; [[#Boisvert--2018|Boisvert et al., 2018]] ). Direct precipitation measurements are difficult and include uncertainties (among others measuring frozen precipitation), therefore precipitation estimates in the Arctic rely on climate models and reanalyses.

An average of five reanalyses for 2000–2010 suggests around 40% of Arctic Ocean precipitation falls as snow, though there is large uncertainty in this estimate ( [[#Boisvert--2018|Boisvert et al., 2018]] ). Rainfall frequency is estimated to have increased over the Arctic by 2.7–5.4% over 2000–2016 ( [[#Boisvert--2018|Boisvert et al., 2018]] ) with more frequent rainfall events reported for NEU and ARO (Svalbard; [[#Maturilli--2015|Maturilli et al., 2015]] ; [[#AMAP--2019|AMAP, 2019]] ), and winter rain totals and frequency have increased in Svalbard since 2000 ( ''medium confidence'' ) ( [[#Łupikasza--2019|Łupikasza et al., 2019]] ). Rain-free winters have rarely occurred since 1998 ( [[#Peeters--2019|Peeters et al., 2019]] ).

Observational records (1966–2010) for the RAR region show changing precipitation characteristics ( [[#Ye--2016|Ye et al., 2016]] ), with higher precipitation intensity but lower frequency and little change in annual precipitation total. Precipitation intensity is reported to have increased in all seasons, strongest in winter and spring, weakest in summer, and at a rate of about 1–3% per degree Celsius of air temperature increase.

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