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

==== 12.4.6.5 Coastal and Oceanic ====

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'''Relative sea level:''' [[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] found that observations indicate increasing sea levels along most North American coasts ( ''robust evidence'' , ''high agreement'' ), although there is substantial regional variation in relative sea level rise ( ''robust evidence'' , ''high agreement'' ). Around North America, over 1900–2018, a new tide gauge-based reconstruction finds a regional mean RSL change of 1.08 [0.79 to 1.38] mm yr <sup>–1</sup> in the subpolar North Atlantic, 2.49 [1.89 to 3.06] mm yr <sup>–1</sup> in the subtropical North Atlantic, and 1.20 [0.76 to 1.62] in the East Pacific ( [[#Frederikse--2020|Frederikse et al., 2020]] ), compared to a GMSL change of around 1.7 mm yr <sup>–1</sup> [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] and Table 9.5). For the period 1993–2018, these RSLR rates, based on satellite altimetry, increased to 2.17 [1.66 to 2.66] mm yr <sup>–1</sup> , 4.04 [2.77 to 5.24] mm yr <sup>–1</sup> and 2.35 [0.70 to 4.06] mm yr <sup>–1</sup> , respectively ( [[#Frederikse--2020|Frederikse et al., 2020]] ), compared to a GMSL change of 3.25 mm yr <sup>–1</sup> [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] and Table 9.5). Relative sea level (RSL) is falling in portions of southern Alaska ( [[#Sweet--2018|Sweet et al., 2018]] ) and much of the northern part of north-eastern Canada and around Hudson Bay (where land is rising by &gt;10 mm/year; [[#Greenan--2018|Greenan et al., 2018]] ).

Relative sea level rise is ''virtually certain'' to continue in the oceans around North America, except in the northern part of north-eastern Canada and portions of southern Alaska. Regional mean RSLR projections for the oceans around North America range from 0.4–1.0 m under SSP1-2.6 to 0.7–1.4 m under SSP5-8.5 for 2081–2100 relative to 1995–2014 (median values), which means that there are locally large deviations from the projected GMSL change ( [[IPCC:Wg1:Chapter:Chapter-9#9.6.3.3|Section 9.6.3.3]] ), including decreases in RSL in northern north-eastern Canada from land uplift (see also [[#Sweet--2017|Sweet et al., 2017]] ; [[#Greenan--2018|Greenan et al., 2018]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ). The RSLR projections here may however be underestimated due to potential partial representation of land subsidence in their assessment ( [[IPCC:Wg1:Chapter:Chapter-9#9.6.3.2|Section 9.6.3.2]] ).

'''Coastal flood:''' Observations indicate that episodic coastal flooding is increasing along many coastlines in North America ( ''robust evidence'' , ''high agreement'' ), and this episodic coastal flooding will increase in many North American regions under future climate change ( ''high confidence'' ) although land uplift from glacial isostatic adjustment in northern and Hudson Bay portions of North-Eastern North America leads to only ''medium confidence'' of coastal flood increases in that region. [[#Sweet--2018|Sweet et al. (2018)]] found 2000–2015 observed increases of about 125% in high-tide flooding frequencies along the southern Atlantic USA coastline, with 75% increases along the USA Gulf Coast and USA northern Atlantic coastlines. That same study noted that a GMSL of 0.5 m in 2100 would increase high tide (‘nuisance’) flooding from current rates of about once a month for most coastal regions to about once every other day along the USA Atlantic and Gulf coasts and smaller increases in frequency along the Pacific coast, and [[#Dahl--2017a|Dahl et al. (2017a)]] found similar trends on the USA East Coast prior to mid-century. The present day 1-in-50-year ETWL is projected to occur around three times per year by 2100 with an SLR of 1 m all around North America, except in most of Eastern North America where it is expected to have return periods of 1-in-1-year to 1-in-2-years ( [[#Vitousek--2017|Vitousek et al., 2017]] ). [[#Ghanbari--2019|Ghanbari et al. (2019)]] projected corresponding shifts towards higher frequencies of major flooding events for 20 US cities. Figure 12.4r and Figure 12.SM.6 show increases of 70 cm or more in the 100-year return period extreme total water level (ETWL) over much of the USA East Coast, British Columbia, Alaska, and the Hudson Bay under RCP8.5 by 2100 (relative to 1980–2014), with lower increases in northern Mexico, northern Canada, Labrador, and the Pacific and Gulf coasts of the USA ( [[#Vousdoukas--2018|Vousdoukas et al., 2018]] ). Projected increases in coastal flooding generally follow patterns of RSL change, although sea ice loss in the north also increases open water storm surge ( [[#Greenan--2018|Greenan et al., 2018]] ).

'''Coastal erosion:''' There is ''limited evidence'' of changes in North American episodic storm erosion caused by waves and storm surges. Observations show increased extreme wave energy on the Pacific coast, but no clear trend on other USA coasts given substantial natural variability ( [[#Bromirski--2013|Bromirski et al., 2013]] ; [[#Vose--2014|Vose et al., 2014]] ). In terms of long-term coastal erosion, shoreline retreat rates of around 1 m yr <sup>–1</sup> have been observed during 1984–2015 along the sandy coasts of NWN and NCA while portions of the US Gulf Coast have seen a retreat rate approaching 2.5 m yr <sup>–1</sup> ( [[#Luijendijk--2018|Luijendijk et al., 2018]] ; [[#Mentaschi--2018|Mentaschi et al., 2018]] ). Sandy shorelines along ENA and WNA have remained more or less stable during 1984–2014, but a shoreline progradation rate of around 0.5 m yr <sup>–1</sup> has been observed in NEN. [[#Mentaschi--2018|Mentaschi et al. (2018)]] report 1984–2015 coastal area land losses of 630 km <sup>2</sup> and 1260 km <sup>2</sup> along the Pacific and Atlantic coasts of the USA, respectively.

Projections indicate that sandy coasts in most of the region will experience shoreline retreat through the 21st century ( ''high confidence'' ). Median shoreline change projections presented by [[#Vousdoukas--2020b|Vousdoukas et al. (2020b)]] show that sandy shorelines in NWN, ENA, and NCA will retreat by between 40 and 80 m by mid-century (relative to 2010) under both RCP4.5 and RCP8.5. Projections for NEN and WNA are lower at 20–30 m under the same RCPs. The highest median mid-century projection in the region is for CNA at around 125 m under both RCPs. RCP4.5 projections for 2100 show shoreline retreats of 100 m or more along the sandy coasts of NWN, CNA, and NCA, while retreats of between 40 and 80 m are projected in other regions. Under RCP8.5, retreats exceeding 100 m are projected in all regions except NEN and WNA (approximately 80 m) by 2100, with particularly high retreats in NWN (160 m) and CNA (330 m). The total length of sandy coasts in North America that are projected to retreat by more than a median of 100 m by 2100 under RCP4.5 and RCP8.5 is about 15,000 km and 25,000 km respectively, an increase of approximately 70%.

'''Marine heatwave:''' There is ''high confidence'' in observed increases in marine heatwave (MHW) frequency and future increases in marine heatwaves are ''very likely'' around North America (Box 9.2). The total number of MHW days per decade increased in the North American coastal zone, albeit somewhat more in the Pacific ( [[#Oliver--2018|Oliver et al., 2018]] ; [[#Smale--2019|Smale et al., 2019]] ). Projected increases in degree heating weeks ( [[#Heron--2016|Heron et al., 2016]] ) and degree heating months ( [[#Frieler--2013|Frieler et al., 2013]] ) indicate increasing bleaching-level and mortality-level heating stress threshold events for reefs in Florida and Mexico.

Mean SST is projected to increase by 1°C (3°C) around North America by 2100, with a hotspot of around 4°C (5°C) off the North American Atlantic coastline under RCP4.5 (RCP8.5) conditions (see Interactive Atlas). [[#Frölicher--2018|Frölicher et al. (2018)]] projected increasing MHW frequency and spatial extent at a 2°C global warming level with the largest increases in the Gulf of Mexico and off the southern USA East Coast (&gt;20×) as well as off the coast of the Pacific Northwest (&gt;15×). Projections for SSP1-2.6 and SSP5-8.5 both show an increase in MHWs around North America by 2081–2100, relative to 1985–2014 (Box 9.2, Figure 1).

'''There is''' high confidence '''that most coastal CIDs in North America will continue to increase in the future with climate change. An observed increase in relative sea level rise is''' virtually certain '''to continue in North America (other than around the Hudson Bay and southern Alaska) contributing to more frequent and severe coastal flooding in low-lying areas''' ( high confidence ''') and shoreline retreat along most sandy coasts''' ( high confidence '''). Marine heatwaves are also expected to increase all around the region over the 21st century''' ( high confidence ''').'''

The assessed direction of change in climatic impact-drivers for North America and associated confidence levels are illustrated in Table 12.8.

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'''Table 12.8''' '''|''' '''Summary of confidence in direction of projected change in climatic impact-drivers in North America, representing their aggregate characteristic changes for mid-century for scenarios RCP4.5, SSP2-4.5, SRES A1B, or above within each AR6 region (defined in Chapter 1), approximately corresponding (for CIDs that are independent of sea level rise) to global warming levels between 2°C and 2.4°C (see [[#12.4|Section 12.4]] for more details of the assessment method).''' The table also includes the assessment of observed or projected time-of-emergence of the CID change signal from the natural interannual variability if found with at least ''medium confidence'' in [[#12.5.2|Section 12.5.2]] .

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