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

=== Atlas.11.2 Arctic ===

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==== Atlas.11.2.1 Key Features of the Regional Climate and Findings From Previous IPCC Assessments ====

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===== Atlas.11.2.1.1 Key Features of the Regional Climate =====

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The Arctic region comprises the Arctic Ocean (ARO), Russian Arctic (RAR), Greenland and Iceland (GIC), and other surrounding land areas in Europe (NEU) and North America (NEN, NWN) (Figure Atlas.29). The region is one of the coldest and driest regions on Earth and plays a key role influencing global and regional climates and the hydrological cycle. A number of physical processes contribute to amplified Arctic temperature variations as compared to the global temperature, in particular thermodynamic changes that include the increase in surface absorption of solar radiation due to surface albedo feedbacks related with sea ice, ice, and snow cover retreat as well as poleward energy transports, water-vapour-radiation and cloud-radiation feedbacks ( [[#Screen--2010|Screen and Simmonds, 2010]] ; [[#Serreze--2011|Serreze and Barry, 2011]] ; [[#Pithan--2014|Pithan and Mauritsen, 2014]] ; [[#Bintanja--2016|Bintanja and Krikken, 2016]] ; [[#Graversen--2016|Graversen and Burtu, 2016]] ; [[#Franzke--2017|Franzke et al., 2017]] ; [[#Stuecker--2018|Stuecker et al., 2018]] ). Precipitation in the Arctic is dominated by snowfall, with rainfall present mostly during the summer period. Arctic climate is influenced by the North Atlantic Oscillation (NAO), the leading mode of atmospheric variability in the North Atlantic basin with a northward extension into the Arctic affecting temperature, precipitation and sea ice over the region, with ENSO and Atlantic Multi-decadal Variability (AMV) also affecting parts of the region (Annex IV). Further, the Greenland Ice Sheet contribution to sea level results from the imbalance between mass gain by net snow accumulation and mass loss by meltwater runoff and ice discharge into the ocean ( [[#IMBIE%20team--2020|IMBIE team, 2020]] ), highlighting that the ice sheet is a major contributor to sea level changes.

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===== Atlas.11.2.1.2 Findings From Previous IPCC Assessments =====

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The following summary from previous IPCC reports is derived from the SROCC ( [[#IPCC--2019a|IPCC, 2019a]] ) unless otherwise stated. Arctic surface air temperatures have increased from the mid-1950s, with feedbacks from loss of sea ice and snow cover contributing to the amplified warming ( ''high confidence'' ) ( [[#IPCC--2018c|IPCC, 2018c]] ), and have ''likely'' increased by more than double the global average over the last two decades ( ''high confidence'' ). Arctic snow cover in June has declined from 1967 to 2018 ( ''high confidence'' ). Arctic glaciers are losing mass ( ''very high confidence'' ) and this along with changes in high-mountain snowmelt have caused changes in hydrology, including river runoff, that are projected to continue in the near term ( ''high confidence'' ). The rate of ice loss from the Greenland Ice Sheet has increased; during 2006–2015 the loss was 278 ± 11 Gt yr <sup>–1</sup> with the rate for 2012–2016 higher than for 2002–2011 and several times higher than during 1992–2001 ( ''high confidence'' ).

The Arctic sea ice area is declining in all months of the year ( ''very high confidence'' ) with the September sea ice minimum ''very likely'' having reduced by 12.8 ± 2.3% per decade during the satellite era (1979–2018) to levels unprecedented for at least 1000 years ( ''medium confidence'' ).

The high latitudes are ''likely'' to experience an increase in annual mean precipitation under RCP8.5 ( [[#IPCC--2013c|IPCC, 2013c]] ). Further, changes in precipitation will not be uniform. Autumn and spring snow cover duration are projected to decrease by a further 5–10% from current conditions in the near term (2031–2050). No further losses are projected under RCP2.6 whereas a further 15–25% reduction in snow cover duration is projected by the end of century under RCP8.5 ( ''high confidence'' ).

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==== 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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==== Atlas.11.2.3 Assessment of Model Performance ====

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Evaluating simulated temperature and precipitation is problematic in the Arctic due to sparse weather station observations. The lack of reliable observed precipitation datasets for the Arctic thus makes it ''very unlikely'' to be able to evaluate objectively the skill of models to reproduce precipitation patterns ( [[#Takhsha--2018|Takhsha et al., 2018]] ).

The CMIP5 models reproduce the observed Arctic warming over the past century ( ''medium confidence'' ) ( [[#Chylek--2016|Chylek et al., 2016]] ; [[#Hao--2018|Hao et al., 2018]] ; [[#Huang--2019|Huang et al., 2019]] ). The simulated mean Arctic warming for 1900–2014 averaged over 40 CMIP5 models is 2.7°C compared to the observed values of 2.2°C (NASA GISS data smoothed using a 1200-km radius) or 1.7°C (using a 250-km smoothing radius) ( [[#Chylek--2016|Chylek et al., 2016]] ). However, there are large inter-model differences in the simulated warming which ranges from 1.2°C to 5.0°C. Although the CMIP5 models reproduce the spatially averaged observed warming over the past 50 to 100 years, the pattern is different from that of observations and reanalysis ( [[#Xie--2016|Xie et al., 2016]] ; [[#Franzke--2017|Franzke et al., 2017]] ; [[#Hao--2018|Hao et al., 2018]] ). Zonal mean temperature trends in the CMIP5 models overestimate the warming in the cold season over high latitudes in the Northern Hemisphere ( [[#Xie--2016|Xie et al., 2016]] ). Overall, the amplified Arctic warming in recent decades is overestimated by CMIP5 models ( [[#Huang--2019|Huang et al., 2019]] ). Possible reasons are modelled sea surface temperature biases and an overestimated temperature response to the Arctic sea ice decline. Furthermore, some models, which have a warm or weak bias in their Arctic temperature simulations, closely relate the Arctic warming to changes in the large-scale atmospheric circulation. In other models, which show large cold biases, the albedo feedback effect plays a more important role for the temperature trend magnitude. This implies that the dominant simulated Arctic warming mechanism and trend may be dependent on the bias of the model mean state ( [[#Franzke--2017|Franzke et al., 2017]] ). Compared to CMIP5 models, [[#Davy--2020|Davy and Outten (2020)]] found lower biases in CMIP6 models’ representation of sea ice extent and volume with improved extents linked to a better seasonal cycle in the Barents Sea.

Rapid temperature changes, such as the pronounced increase of 2°C yr <sup>–1</sup> during 2003–2012 over the Kara and Barents seas in March is well captured in Arctic CORDEX simulations ( [[#Kohnemann--2017|Kohnemann et al., 2017]] ). The models show adequate skill in capturing the general temperature patterns ( [[#Koenigk--2015|Koenigk et al., 2015]] ; [[#Matthes--2015|Matthes et al., 2015]] ; [[#Hamman--2016|Hamman et al., 2016]] ; [[#Cassano--2017|Cassano et al., 2017]] ; [[#Brunke--2018|Brunke et al., 2018]] ; [[#Diaconescu--2018|Diaconescu et al., 2018]] ; [[#Takhsha--2018|Takhsha et al., 2018]] ), but tend to show a cold temperature bias which is largest in winter and depends on the reference dataset. [[#Cassano--2017|Cassano et al. (2017)]] showed a large sensitivity of the simulated surface climate to changes in atmospheric model physics. In particular, large changes in radiative flux biases, driven by changes in simulated clouds, lead to large differences in temperature and precipitation biases.

The CMIP5 models perform well in simulating 20th-century snowfall for the Northern Hemisphere, although there is a positive bias in the multi-model ensemble relative to the observed data in many regions ( [[#Krasting--2013|Krasting et al., 2013]] ). Lack of sufficient spatial resolution in the model topography has a serious impact on the simulation of snowfall. The patterns of relative maxima and minima of snowfall, however, are captured reasonably well by the models.

Arctic CORDEX RCMs reproduce the dominant features of regional precipitation patterns and extremes (e.g., [[#Glisan--2014|Glisan and Gutowski, 2014]] ; [[#Hamman--2016|Hamman et al., 2016]] ). Due to their higher spatial resolution, RCMs simulates larger amounts of orographic precipitation compared to reanalyses. Overall, the simulated precipitation is within the reanalysis and global model ensemble spread, but the Arctic river basin precipitation is closer to observations ( [[#Brunke--2018|Brunke et al., 2018]] ). However, [[#Takhsha--2018|Takhsha et al. (2018)]] show that the RCMs’ precipitation bias highly depends on the observational reference dataset used.

The annual mean precipitation pattern of ensemble global atmospheric simulations with a high horizontal resolution agrees well with the observations, with precipitation maxima over the Greenland and Norwegian seas ( [[#Kusunoki--2015|Kusunoki et al., 2015]] ). However, the simulated Arctic average annual precipitation shows a positive bias with excessive precipitation over Alaska and the western Arctic ( [[#Kattsov--2017|Kattsov et al., 2017]] ).

Regarding the Greenland Ice Sheet (region GIC), modelled surface mass balance (SMB) has decreased since the end of the 1990s ( [[#Fettweis--2020|Fettweis et al., 2020]] ). A multi-model intercomparison study ( [[#Fettweis--2020|Fettweis et al., 2020]] ) emphasized a simulated positive mean annual SMB of 338 ± 68 Gt yr <sup>–1</sup> between 1980 and 2012, with a decreasing average rate of 7.3 ± 2.0 Gt yr <sup>–2</sup> , mainly driven by an increase in meltwater runoff. [[#Mouginot--2019|Mouginot et al. (2019)]] stated that SMB played a strong role in the ice-sheet mass loss, where SMB dominated in the last two decades. [[#Mottram--2019|Mottram et al. (2019)]] found that SMB processes dominate the ice-sheet mass budget over most of the interior, highlighting that the ice sheet is a contributor to global mean sea level rise between 1991 and 2015. More specifically, SMB models have improved ( [[#Fettweis--2020|Fettweis et al., 2020]] ; [[#Hanna--2021|Hanna et al., 2021]] ) due to increased availability and quality of remotely sensed ( [[#Koenig--2016|Koenig et al., 2016]] ; [[#Overly--2016|Overly et al., 2016]] ) and in situ observations ( [[#Machguth--2016|Machguth et al., 2016]] ; [[#Fausto--2018|Fausto et al., 2018]] ; [[#Vandecrux--2019|Vandecrux et al., 2019]] , 2020). [[#Fettweis--2020|Fettweis et al. (2020)]] showed that the models’ ensemble mean provides the best estimate of the present-day SMB relative to observations. This is the case for the patterns in all seven regions (regional division after [[#Mouginot--2019|Mouginot et al., 2019]] ) apart from the SE accumulation zone where large discrepancies in modelled snowfall accumulation occurred where the spread can reach 2-m water equivalent per year. [[#Montgomery--2020|Montgomery et al. (2020)]] confirmed this, highlighting that RCMs (MAR and RACMO) are underestimating accumulation in south-east Greenland and that models misrepresent spatial heterogeneity due to an orographically forced bias in snowfall near the coast. Further, for north-east Greenland, [[#Karlsson--2020|Karlsson et al. (2020)]] found RCMs underestimate snow accumulation rates by up to 35%. The regional time series show that SMB has been gradually decreasing in all seven regions (1979–2017), although the trend is less strong in central-eastern and south-eastern regions. In the south-west, north-east and north-west, SMB turns negative or close to zero after 2000 and remains above zero in other regions ( ''medium confidence'' ) (Figure Atlas.3 0).

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==== Atlas.11.2.4 Assessment and Synthesis of Projections ====

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Mean temperature in the Arctic is projected to continue to rise throughthe 21st century significantly higher than the global average (Figure Atlas.29 and the Interactive Atlas). For the regions NWN and NEN, see [[#Atlas.9|Atlas.9]] . The Arctic is projected to reach a 2°C annual mean warming above the 1981–2005 baseline about 25 to 50 years before the globe as a whole under RCP8.5 and RCP4.5 ( [[#Post--2019|Post et al., 2019]] ). The Arctic warming may be as much as 4°C in the annual mean and 7°C in late autumn under 2°C global warming, regardless of which scenario is considered ( ''high confidence'' ) ( [[#Post--2019|Post et al., 2019]] ).

Projections from 40 CMIP5 models of the 2014–2100 Arctic annual warming under RCP4.5 vary from 0.9°C to 6.7°C, with a multi-model mean of 3.7°C ( [[#Chylek--2016|Chylek et al., 2016]] ). All models agree on a projected Arctic amplification (of at least 1.5 times), but they disagree on the magnitude and spatial patterns. Arctic warming trends projected by models that include a full direct and indirect aerosol effect (‘fully aerosol–cloud interactive’) are significantly higher than those projected by models without a full indirect aerosol effect ( [[#Chylek--2016|Chylek et al., 2016]] ).

Projected Arctic warming exhibits a very pronounced seasonal cycle, with exceptionally strong warming in the winter. In projections from 30 CMIP5 models, winter warming over ARO varies regionally from 3°C to 5°C by mid-century and 5°C to 9°C by late-century under RCP4.5 ( ''high confidence'' ) ( [[#AMAP--2017|AMAP, 2017]] ). Averaged over the Arctic and based on 36 CMIP5 models, winter warming is 5.8°C ± 1.5°C by mid-century and 7.1°C ± 2.3°C by 2100 under RCP4.5 ( [[#Overland--2019|Overland et al., 2019]] ), and an exceptionally strong warming of up to 14.1°C ± 2.9°C is projected in December under RCP8.5 ( [[#Bintanja--2016|Bintanja and Krikken, 2016]] ). [[#Bintanja--2013|Bintanja and Van Der Linden (2013)]] estimated the Arctic winter warming over the 21st century to exceed the summer warming by at least a factor of four, irrespective of the magnitude of the climate forcing.

[[#Overland--2014|Overland et al. (2014)]] highlighted the difference between the near-term ‘adaptation timescale’ and the long-term ‘mitigation timescale’ for the Arctic. Only in the latter half of the century do the projections under RCP4.5 and RCP8.5 noticeably separate. End-of-the-century warming is approximately twice as large under RCP8.5 demonstrating the impact of the lower emissions under RCP4.5 ( ''high confidence'' ) ( [[#AMAP--2017|AMAP, 2017]] ). More specifically under the strong forcing scenario, annual mean surface air temperature in the Arctic is projected to increase by 8.5°C ± 2.1°C over the course of the 21st century ( [[#Bintanja--2017|Bintanja and Andry, 2017]] ), and emerges as a ‘new Arctic’ climate being significantly different from that of the mid-20th century ( [[#Landrum--2020|Landrum and Holland, 2020]] ). The end-of-the-century warming (2080–2099 relative to 1980–1999, RCP8.5) can exceed 15°C in autumn and winter over the Arctic Ocean ( [[#Koenigk--2015|Koenigk et al., 2015]] ). Projections averaged over the four best-performing CMIP5 models show an Arctic annual warming of 4.1°C (RCP2.6), 5.7°C (RCP4.5) and 10.6°C (RCP8.5) by 2100 compared to 1951–1980 ( [[#Hao--2018|Hao et al., 2018]] ). Also, for neighbouring Arctic regions, for example NEU, WSB and ESB, temperature is projected to increase towards the end of the century under both RCP4.5 and RCP8.5 ( [[#Atlas.8|Atlas.8]] and [[#Atlas.5.2|Atlas.5.2]] ).

The ensemble of CMIP6 shows ''likely'' greater warming compared to CMIP5 (Figure Atlas.29). There is weak agreement among the models in projected temperature change over the Arctic North Atlantic under SSPs until the mid-century, but a robust warming signal clearly emerges even there by 2100 (Interactive Atlas). Generally, the largest annual warming is simulated over the Arctic Ocean, particularly over the Barents Sea and the Kara Sea. Future warming in CORDEX RCMs and the CMIP5 models are similar ( [[#Spinoni--2020|Spinoni et al., 2020]] ). The RCM warming over the AO is smaller, while the warming over land is larger in winter and spring but smaller in summer, compared with CMIP5 ( [[#Koenigk--2015|Koenigk et al., 2015]] ).

Mean precipitation in ARO, GIC and RAR is projected to rise in a warming climate (Figure Atlas.29), with different rates for the different seasons and scenarios. For NWN and NEN, see [[#Atlas.9|Atlas.9]] . The CMIP5 multi-model mean projected precipitation increase in the Arctic is ''likely'' of the order of 50% under RCP8.5 by the end of the 21st century, which is among the highest globally ( [[#Bintanja--2014|Bintanja and Selten, 2014]] ). Over 70°N–90°N, the precipitation increase is ''likely'' 62 ± 20% and 56 ± 13% for RCP4.5 and RCP8.5 respectively. For ARO (Svalbard), the increase in annual precipitation by 2100 is estimated to be about 45% for RCP4.5 and 65% for RCP8.5 (CMIP5 ensemble median; [[#Van%20der%20Bilt--2019|Van der Bilt et al., 2019]] ). However, importantly the simulated Arctic precipitation increase varies by a factor of three to four between models ( [[#Bintanja--2014|Bintanja and Selten, 2014]] ). The projected increase is strongest in late autumn and winter ( [[#Vihma--2016|Vihma et al., 2016]] ). The interannual variability of Arctic precipitation will likely increase markedly (up to 40% over the 21st century), especially in summer ( ''medium confidence'' ) ( [[#Bintanja--2020|Bintanja et al., 2020]] ).

The CMIP6 projections confirm precipitation will ''likely'' increase almost everywhere in the Arctic (Interactive Atlas). The largest increase is simulated over the Barents Sea, Kara Sea and East Siberian Sea regions, and over north-east Greenland. A pronounced uncertainty in the projection exists over the Arctic North Atlantic and south Greenland. There, the precipitation signal is not significant even by the end of the 21st century and under high-emissions scenarios (RCP8.5, SSP5-8.5). Consistent with the generally higher warming in CMIP6, compared to CMIP5, the projected precipitation increase is also higher ( ''high confidence'' ) (Figure Atlas.29).

The Arctic mean annual precipitation sensitivity has been estimated at a 4.5% increase per degree Celsius of temperature rise, compared to a global average of 1.6–1.9% per degree Celsius of temperature rise ( [[#Bintanja--2014|Bintanja and Selten, 2014]] ) based on a set of 37 CMIP5 GCMs. [[#Koenigk--2015|Koenigk et al. (2015)]] stress the different precipitation sensitivity in winter (0.8 mm per month per degree Celsius of temperature rise) and summer (2 mm per month per degree Celsius of temperature rise). The pattern and amplitude of precipitation changes agree in CORDEX simulations with their driving CMIP5 models ( ''high confidence'' ) ( [[#Koenigk--2015|Koenigk et al., 2015]] ; [[#Spinoni--2020|Spinoni et al., 2020]] ). However, more small-scale variations over land and coastlines, and significantly larger precipitation changes in summer are obvious in the downscaling.

Rain is projected to become the dominant form of precipitation in the Arctic region by the end of the 21st century ( [[#Bintanja--2018|Bintanja, 2018]] ). The CMIP5 models show a decrease in annual Arctic snowfall under both RCP4.5 and RCP8.5 ( ''high confidence'' ) ( [[#Krasting--2013|Krasting et al., 2013]] ; [[#Bintanja--2017|Bintanja and Andry, 2017]] ). In the central Arctic, the snowfall fraction barely remains larger than 50%, with only Greenland still having snowfall fractions larger than 80% ( [[#Bintanja--2017|Bintanja and Andry, 2017]] ). The most dramatic reductions in snowfall fraction are projected to occur over the North Atlantic and, especially, the Barents Sea.

With ongoing warming and polar amplification in the Arctic, the Greenland Ice Sheet SMB will inevitably continue to change ( ''high confidence'' ) ( [[#Lenaerts--2019|Lenaerts et al., 2019]] ). For the ice sheet, despite large differences between model scenarios, future projections and regions agree that increasing temperatures will increase runoff which will in turn dominate the future decrease of SMB ( [[#Rae--2012|Rae et al., 2012]] ; [[#van%20Angelen--2014|van Angelen et al., 2014]] ; [[#Mottram--2017|Mottram et al., 2017]] ; [[#Hofer--2020|Hofer et al., 2020]] ), confirming the high sensitivity of the SMB to atmospheric warming. Changes in SMB will continue to dominate future mass loss from the ice sheet, and likely even more when marine-terminating glaciers retreat onto land, and solid ice discharge is reduced ( [[#Vizcaino--2014|Vizcaino, 2014]] ; [[#Lenaerts--2019|Lenaerts et al., 2019]] ).

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==== Atlas.11.2.5 Summary ====

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It is ''very likely'' that the Arctic has warmed at more than twice the global rate over the past 50 years and ''likely'' that annual precipitation has increased with the highest increases during the cold season. This is based on various Arctic amplification processes, in particular, a combination of several feedback-related processes such as sea ice and snow-cover albedo, poleward energy transports, and water-vapour-cloud-radiation feedbacks. The frequency of rainfall increased over the Arctic by 2.7–5.4% over the 2000–2016 period with more frequent rainfall events being reported for northern Europe and Svalbard ( ''medium confidence'' ).

The CMIP5 models reproduce the observed Arctic warming over the past century but overestimate the amplified Arctic warming in the recent decades ( ''medium confidence'' ). Arctic CORDEX simulations show adequate skill in capturing regional temperature and precipitation patterns and precipitation extremes ( ''high confidence'' ). SMB models have improved due to increased availability and quality of remotely sensed and in situ observations, and an ensemble mean of SMB model simulations provides the best estimate of the present-day SMB ( ''medium confidence'' ).

It is ''very likely'' that the Arctic annual mean temperature and precipitation will continue to increase, reaching around 11.5°C ± 3.4°C and 49 ± 19% over the 2081–2100 period (with respect to a 1995–2014 baseline) under the SSP5-8.5 scenario or 4.0°C ± 2.5°C and 17 ± 11% under the SSP1-2.6 scenario. These CMIP6 results show ''likely'' higher Arctic annual mean temperatures compared to CMIP5 for a given time-period and emissions scenario, though the projections are consistent for global warming levels.

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