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

==== 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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