Editing IPCC:AR6/WGI/Chapter-12 (section)

==== 12.4.9.5 Coastal and Oceanic ====

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'''Relative sea level:''' Satellite altimetry and tide data show that relative sea levels (with glacial isostatic adjustment) are rising in Arctic Europe and Arctic North-Western North America, declining in portions of southern Alaska and Arctic North-Eastern North America and no clear trend in Greenland and the Russian Arctic ( [[#Sweet--2018|Sweet et al., 2018]] ; [[#Rose--2019|Rose et al., 2019]] ), which is broadly consistent with findings in [[#Oppenheimer--2019|Oppenheimer et al. (2019)]] . Areas with low or negative change have substantial land uplift counteracting the global mean sea level trend ( [[#Greenan--2018|Greenan et al., 2018]] ; [[#Sweet--2018|Sweet et al., 2018]] ; [[#Madsen--2019|Madsen et al., 2019]] ). SROCC projections indicate ''high confidence'' in future rises in relative sea level for all Arctic regions other than areas of substantial land uplift in north-eastern Canada, the west coast of Greenland, and narrow portions of West Antarctica ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ).

'''Coastal flooding and erosion:''' Higher sea levels and reduced coastal sea ice protection will increase future extreme sea levels in the Arctic ( ''high confidence'' for Arctic Northern Europe, the Russian Arctic, and Arctic North-Western North America ( ''medium confidence'' '')'' for Greenland and Iceland and Arctic North-Eastern North America given glacial isostatic adjustment). [[#Vousdoukas--2018|Vousdoukas et al. (2018)]] project that the current 1-in-100-year extreme total water level would have median return periods of 1-in-20-years to 1-in-50-years by 2050, increasing to 1-in-5-years to 1-in-20-years by 2100 under RCP4.5 along nearly the entire Arctic coastline by 2100 (excluding GIC for which projections are not available). Projections for RCP8.5 indicate that the present-day 1-in-100-year ETWL would have median return periods of 1-in-10-years to 1-in-50-years by 2050 and would occur once every five years (or more frequently) by 2100. Arctic coastal erosion is also expected to increase with climate change ( ''medium confidence'' ; ''high agreement'' but ''limited evidence'' of projections), accelerated in some regions by subsurface permafrost thaw and increased wave energy ( [[#Gibbs--2015|Gibbs and Richmond, 2015]] ; [[#Fritz--2017|Fritz et al., 2017]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ; [[#Casas-Prat--2020|Casas-Prat and Wang, 2020]] ). A longer ice-free season for the RCP8.5 2080s is projected to help drive more than 100 m of shoreline retreat in North-Western North America Arctic coastal communities ( [[#Melvin--2017|Melvin et al., 2017]] ; [[#Greenan--2018|Greenan et al., 2018]] ; [[#Magnan--2019|Magnan et al., 2019]] ). Assessment of coastal flooding and erosion changes in Antarctica are limited by a lack of studies.

'''Marine heatwave:''' Recent years have seen marine heatwaves (MHWs) and increasing extreme coastal SSTs in Arctic systems ( [[#Lima--2012|Lima and Wethey, 2012]] ; [[#Collins--2019|Collins et al., 2019]] ; [[#Frölicher--2019|Frölicher, 2019]] ). Projections show increases in MHW intensity, frequency and duration will be larger over the Arctic Ocean than mid-latitude oceans due in part to low interannual variability under current sea ice ( ''high confidence'' ). [[#Frölicher--2018|Frölicher et al. (2018)]] used 12 CMIP5 models to project median MHW days increasing about 25-fold and 50-fold at the 2°C and 3.5°C GWLs, respectively, in response to mean ocean warming and sea ice loss, and the smallest global changes still leading to increases in the Southern Ocean around Antarctica (see also Cross-chapter Box 9.1).

'''Climate change has caused and will continue to induce an enhanced warming trend, increasing heat-related extremes and decreasing cold spells and frosts in the Arctic''' ( high confidence '''), with similar changes in Antarctica but''' medium confidence '''for extreme heat increases and West Antarctic frost change decreases and''' low confidence '''for cold spell changes and East Antarctica frost. The water cycle is projected to intensify in polar regions, leading to more rainfall, higher river flood potential and more intense precipitation''' ( high confidence '''). Projections indicate reductions in glaciers at both poles, with sea ice loss, enhanced permafrost warming, decreasing permafrost extent, and decreasing seasonal duration and extent of snow cover in the Arctic''' ( high confidence ''') even as some of the coldest regions will see higher total snowfall given increased precipitation''' ( medium confidence '''). Projections indicate relative sea level rises in polar regions''' ( high confidence ''')''' , '''with the exception of regions with substantial land uplift including North-Eastern North America''' ( high confidence '''), western Greenland, the northern Baltic Sea, and portions of West Antarctica. Higher sea levels also contribute to''' high confidence '''for projected increases of Arctic coastal flooding and higher coastal erosion (aided by sea ice loss)''' ( medium confidence ''') with lower confidence for those CIDs in regions with substantial land uplift.'''

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