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

=== 12.4.6 North America ===

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Major changes in North American CIDs were assessed in WGII AR5 Chapter 26 ( [[#Romero-Lankao--2014|Romero-Lankao et al., 2014]] ), with additional detail on connections to warming levels provided by SR1.5 ( [[#IPCC--2018|IPCC, 2018]] ), and climate information related to land degradation and land-use suitability in SRCCL ( [[#IPCC--2019c|IPCC, 2019c]] ), and ocean and coastal hazards in the SROCC ( [[#IPCC--2019b|IPCC, 2019b]] ). Recent national assessments in the USA (USGCRP, 2017, 2018) and Canada (Bush and Lemmen, 2019) enhance the local perspective and assessments across a number of CIDs and their sectoral connections. For the purpose of this assessment, North America is sub-divided into six sub-regions, as defined in Chapter 1: North Central America (NCA), Western North America (WNA), Central North America (CNA), Eastern North America (ENA), North-Eastern North America (NEN), and North-Western North America (NWN). Greenland and Arctic regions of North-Eastern and North-Western North America are further assessed in [[#12.4.9|Section 12.4.9]] , and the Caribbean and Hawaiian Islands are assessed in [[#12.4.7|Section 12.4.7]] .

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==== 12.4.6.1 Heat and Cold ====

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'''Mean air temperature:''' Atlas.9.2 assessed ''very likely'' mean warming in observations across North America, with highest increases at higher latitudes and in the winter season. Atlas.9.4 assessed ''very likely'' mean warming in future decades in all North American regions, with CMIP and CORDEX models showing median increases exceeding 2°C in much of the continental interior under RCP8.5 (2041–2060 compared to 1995–2014) and higher increases towards the north. Mean temperatures at the end of century show strong scenario dependence, rising between 1°C and 2.5°C in RCP2.6 and about 4°C to 8°C in RCP8.5 (Figures Atlas.12, Atlas.26 and Atlas.27). Warming also raises stream temperatures across the continent ( [[#DOE--2015|DOE, 2015]] ; [[#Trtanj--2016|Trtanj et al., 2016]] ; [[#van%20Vliet--2016|van Vliet et al., 2016]] ; [[#Chapra--2017|Chapra et al., 2017]] ), and [[#Hill--2014|Hill et al. (2014)]] projected US stream warming by 0.6°C (±0.3°C) per 1°C increase in local air temperature. Mean warming drives shifts in the seasonal timing of temperature thresholds, including increasing growing degree days ( [[#Mu--2017|Mu et al., 2017]] ), longer growing seasons ( [[#Gowda--2018|Gowda et al., 2018]] ; G. [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|Li et al., 2018]] ; [[#Vincent--2018|L.A. Vincent et al., 2018]] ), reduced chill hours ( [[#Luedeling--2012|Luedeling, 2012]] ; [[#Lee--2015|Lee and Sumner, 2015]] ; [[#Xie--2015|Xie et al., 2015]] ; [[#Parker--2019|Parker and Abatzoglou, 2019]] ), and longer pollen and allergy seasons ( [[#Fann--2016|Fann et al., 2016]] ; [[#Anenberg--2017|Anenberg et al., 2017]] ; [[#Sapkota--2019|Sapkota et al., 2019]] ). Warmer temperatures reduce heating degree days and increase cooling degree days ( '''high confidence''' ) ( [[#Bartos--2016|Bartos et al., 2016]] ; [[#US%20EPA--2016|US EPA, 2016]] ; [[#Craig--2018|Craig et al., 2018]] ; X. [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|Zhang et al., 2019]] ; [[#Coppola--2021b|Coppola et al., 2021b]] )

'''Extreme heat:''' Section 11.9 assessed that extreme temperatures in North America have increased in recent decades ( ''medium evidence'' , ''medium agreement'' ) other than in Central and Eastern North America ( ''low confidence'' ), and extreme heat in all regions is projected to increase with climate change ( ''high confidence'' ). Observed trends in extreme heat are more positive for heat extreme indices that include temperature and humidity given historical expansion of irrigation and intensification of agriculture ( [[#Mueller--2017|Mueller et al., 2017]] ; [[#Grotjahn--2018|Grotjahn and Huynh, 2018]] ; [[#Thiery--2020|Thiery et al., 2020]] ). Several studies noted statistically significant increases in intensity and particularly the frequency, duration, and seasonal length of the physiologically hazardous extreme heat conditions across North America ( [[#Grineski--2015|Grineski et al., 2015]] ; [[#Habeeb--2015|Habeeb et al., 2015]] ; [[#Martínez-Austria--2016|Martínez-Austria et al., 2016]] ; [[#Petitti--2016|Petitti et al., 2016]] ; [[#Vincent--2018|L.A. Vincent et al., 2018]] ; [[#García-Cueto--2019|García-Cueto et al., 2019]] ).

Figure 12.4b shows over a month of additional days at CMIP6 SSP5-8.5 mid-century where temperatures exceed 35°C across much of southern Mexico and regions near the US–Mexico border, and these extreme temperatures occur at least once per year up to southern Canada. [[#Coppola--2021b|Coppola et al. (2021b)]] found similar patterns in CMIP5 and CORDEX-Core. Using locally tailored heat thresholds, [[#Maxwell--2018|Maxwell et al. (2018)]] found that ‘very hot’ days in five US cities will occur a median of three to five times more often by 2036–2065 under RCP8.5 (2 to 3.5 times more often in RCP4.5), [[#Oleson--2018|Oleson et al. (2018)]] projected that annual heatwave duration will exceed one month in Houston in RCP8.5 2061–2080, and [[#Anderson--2018|Anderson et al. (2018)]] projected 7 to 12 times more exceedances of thresholds associated with high-mortality by 2061–2080 under RCP8.5 (6 to 7 times more exceedances in RCP4.5). [[#Schwingshackl--2021|Schwingshackl et al. (2021)]] found that Central and Eastern North America are among the regions with the strongest trend in heat stress indicators. Studies also project increasingly surpassed heat extreme thresholds for North American crops ( [[#Gourdji--2013|Gourdji et al., 2013]] ), airplane weight restrictions ( [[#Coffel--2017|Coffel et al., 2017]] ), and peak load energy systems ( [[#Auffhammer--2017|Auffhammer et al., 2017]] ).

The number of days crossing dangerous heat thresholds such as HI &gt; 41°C will be very sensitive to the mitigation scenario at the end of the century ( [[#Wuebbles--2014|Wuebbles et al., 2014]] ; [[#Zhao--2015|Zhao et al., 2015]] ; [[#Dahl--2019|Dahl et al., 2019]] ; [[#Schwingshackl--2021|Schwingshackl et al., 2021]] ). At the end of the century under SSP5-8.5, a CMIP6 median increase of exceedances of 75–150 days per year is projected over much of North Central America, Central North America and the south-western USA while this increase is projected to remain limited below 60 days under SSP1-2.6 (Figure 12.4d,f and Figure 12.SM.2). [[#Steinberg--2018|Steinberg et al. (2018)]] also projected more frequent and longer ‘heat-health’ events in California extending into October.

'''Cold spell:''' [[IPCC:Wg1:Chapter:Chapter-11|Chapter 11]] assessed ''high confidence'' in decreasing frequency and intensity of cold spells over North America ( [[IPCC:Wg1:Chapter:Chapter-11#11.9|Section 11.9]] ). The number of days with extreme wind chill hours (humidex &lt;–30) decreased at 76% of examined Canadian stations from 1953 to 2012 ( [[#Mekis--2015|Mekis et al., 2015]] ) and cold days and coldest nights decreased in Mexico from 1980 to 2010 ( [[#García-Cueto--2019|García-Cueto et al., 2019]] ).

Cold spells are projected to decrease over North America under climate change, with the largest decreases most common in the winter season ( ''high confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-11#11.9|Section 11.9]] ). Minimum winter temperatures are projected to rise faster than the mean winter temperature ( [[#Underwood--2017|Underwood et al., 2017]] ) and alter cold-hardiness zones used to determine agricultural suitability ( [[#Parker--2016|Parker and Abatzoglou, 2016]] ). [[#Wuebbles--2014|Wuebbles et al. (2014)]] projections for RCP8.5 end-of-century show that the four-day cold spell that happens on average once every five years is projected to warm by more than 10°C and CMIP5 models do not project current 1-in-20-year annual minimum temperature extremes to recur over much of the continent. Multiple studies have shown that Arctic warming can alter large-scale variability and change the frequency and duration of mid-latitude cold air outbreaks, potentially leading to increasing cold hazards in some regions ( ''low agreement'' ) ( [[#Barcikowska--2019|Barcikowska et al., 2019]] ; [[#Cohen--2020|Cohen et al., 2020]] ; [[#Zhou--2021|Zhou et al., 2021]] ).

'''Frost:''' An expansion of the frost-free season is underway and projections for North America indicate a continuation of this trend in the future ( ''high confidence'' ). Significant decreases in frost days, consecutive frost days, and ice days were identified in 1948–2016 station observations across Canada, along with a resulting lengthening of the frost-free season by more than a month in many regions ( [[#Vincent--2018|L.A. Vincent et al., 2018]] ). Frost days also declined in nearly all Mexican cities from 1980 to 2010 ( [[#García-Cueto--2019|García-Cueto et al., 2019]] ), and a 1917–2016 decline of about three weeks in frigid winter conditions challenges ecosystems in the north-east USA and south-east Canada ( [[#Contosta--2020|Contosta et al., 2020]] ). Studies connect projections of a longer frost-free season in North America to a longer outdoor construction season, shifts in frost variance to orchard damages, and lower weight tolerances for runways ( [[#Daniel--2018|Daniel et al., 2018]] ; [[#DeGaetano--2018|DeGaetano, 2018]] ; [[#Jacobs--2018|Jacobs et al., 2018]] ). Frosts are projected to persist as an episodic hazard in many regions given natural variability and cold air outbreaks even as mean temperature rises ( ''high confidence'' ).

'''Climate change is''' virtually certain '''to shift the balance of temperature towards warming trends and away from cold extremes, with increases in the magnitude, frequency, duration and seasonal and spatial extent of heat extremes driving impacts across North America. The frequency of dangerous heat threshold exceedance (such as HI &gt; 41°C) is particularly sensitive to scenario pathway, with 7''' '''5–1''' '''50 days more under SSP5-8.5 but less than 60 days more under SSP1-2.6 by the end of the century in NCA, CNA and the south-western USA.'''

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==== 12.4.6.2 Wet and Dry ====

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'''Mean precipitation:''' Atlas.9.2 found that trends in annual precipitation over 1960–2015 are generally non-significant, though there are consistent positive trends over parts of ENA and CNA, together with significant decreases in precipitation in parts of the south-western USA and north-western Mexico.

Atlas.9.4 assessed ''very high confidence'' in increases in mean precipitation over most of northern and Eastern North America, with ''medium confidence'' of decrease over Northern Central America and ''low confidence'' elsewhere (see Figure Atlas.26, and Cross-Chapter Box Atlas.1, Figure 1). Changes are most dramatic in the spring and winter, when wet conditions are projected to extend from the northern portions of the continent as far south as the central Great Plains, while Mexico becomes drier; in contrast, summer changes are uncertain across most of the continent other than wetter conditions in northern Canada ( [[#Easterling--2017|Easterling et al., 2017]] ; [[#Bukovsky--2020|Bukovsky and Mearns, 2020]] ; [[#Almazroui--2021|Almazroui et al., 2021]] ; [[#Teichmann--2021|Teichmann et al., 2021]] ).

'''River flood:''' There is ''limited evidence'' and ''low agreement'' on observed climate change influences for river floods in North America ( [[IPCC:Wg1:Chapter:Chapter-11#11.5|Section 11.5]] ). Trends in streamflow indices are mixed and difficult to separate from river engineering influences, with large changes but little spatial coherence across the USA, making it difficult to identify trends with confidence ( [[#Peterson--2013|Peterson et al., 2013]] ; [[#Mallakpour--2015|Mallakpour and Villarini, 2015]] ; [[#Archfield--2016|Archfield et al., 2016]] ; [[#Wehner--2017|Wehner et al., 2017]] ; [[#Villarini--2018|Villarini and Slater, 2018]] ; [[#Hodgkins--2019|Hodgkins et al., 2019]] ; [[#Neri--2019|Neri et al., 2019]] ). There is ''high confidence'' in historical shifts in the timing of peak streamflow towards higher winter and earlier spring flows in snowmelt-driven basins in Canada ( [[#Burn--2016|Burn and Whitfield, 2016]] ; [[#Bonsal--2019|Bonsal et al., 2019]] ) and the USA ( [[#Dudley--2017|Dudley et al., 2017]] ; [[#Wehner--2017|Wehner et al., 2017]] , [[#Neri--2020|Neri et al., 2020]] ). Some rivers show ice-jam floods occurring a week earlier, but changes are mixed, given localized positive and negative changes across the continent ( [[#Rokaya--2018|Rokaya et al., 2018]] ). There is ''medium confidence'' that climate change will increase river floods over the USA and Canada but ''low confidence'' for changes in Mexico. [[#Wobus--2017a|Wobus et al. (2017a)]] used a regional hydrologic model for 57,000 streams to project more than a doubling in the frequency of current 1-in-100-year flow events in many portions of the USA for RCP8.5 2050 with additional contributions from earlier snowmelt. CMIP6 projections for SSP5-8.5 2065–2099 show strongest peak USA runoff increases in the east ( [[#Villarini--2020|Villarini and Zhang, 2020]] ); however, several studies applying global hydrological models disagree with regional streamflow projections, indicating a decrease in the magnitude or frequency of floods over a large portion of North America (e.g., [[#Hirabayashi--2013|Hirabayashi et al., 2013]] ; [[#Arnell--2016|Arnell and Gosling, 2016]] ; see Figure 12.10a,c).

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[[File:2789e0f641f8b8383f22e976ab34b8be IPCC_AR6_WGI_Figure_12_10.png]]

'''Figure 12.10''' '''|''' '''Projected changes in selected climatic impact-driver indices for North America. (a)''' Mean change in 1-in-100-year river discharge per unit catchment area (Q100, m <sup>3</sup> s <sup>–1</sup> km <sup>–2</sup> ), and '''(b)''' median change in the number of days with snow water equivalent (SWE) over 100 mm (from November to March), from CORDEX-North America models for 2041–2060 relative to 1995–2014 and RCP8.5. Diagonal lines indicate where less than 80% of models agree on the sign (direction) of change. '''(c)''' Bar plots for Q100 (m <sup>3</sup> s <sup>–1</sup> km <sup>–2</sup> ) averaged over land areas for the AR6 WGI Reference Regions (defined in Chapter 1). The left-hand column within each panel (associated with the left-hand y-axis) shows the ‘recent past’ (1995–2014) Q100 absolute values in grey shades. The other columns (associated with the right-hand y-axis) show the Q100 changes relative to the recent past values for two time periods (‘mid’ 2041–2060 and ‘long’ 2081–2100) and for three global warming levels (defined relative to the pre-industrial period 1850–1900): 1.5°C (purple), 2°C (yellow) and 4°C (brown). The bars show the median (dots) and the 10–90th percentile range of model ensemble values across each model ensemble. CMIP6 is shown by the darkest colours, CMIP5 by medium, and CORDEX by light. SSP5-8.5/RCP8.5 is shown in red and SSP1-2.6/RCP2.6 in blue. '''(d)''' As for (c) but showing absolute values for number of days with SWE &gt; 100 mm, masked to grid cells with at least 14 such days in the recent past. See Technical ( [[IPCC:Wg1:Chapter:Annex-vi|Annex VI]] for details of indices. A Caribbean (CAR) Q100 bar plot is included here but assessed in the Small Islands section ( [[#12.4.7|Section 12.4.7]] ). Further details on data sources and processing are available in the chapter data table (Table 12.SM.1).

'''Heavy precipitation and pluvial flood:''' Section 11.4 assessed ''high confidence'' in observed increases in extreme precipitation events (including hourly totals) in Central and Eastern North America with ''low confidence'' in broad trends elsewhere in the continent despite observational increases in some portions of each region ( [[#Vincent--2018|L.A. Vincent et al., 2018]] ; [[#García-Cueto--2019|García-Cueto et al., 2019]] ; X. [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|Zhang et al., 2019]] ).

( [[IPCC:Wg1:Chapter:Chapter-11#11.4|Section 11.4]] found that high precipitation is projected to increase across North America ( ''high confidence'' ) except for portions of Western North America where projections are mixed ( ''medium confidence'' of increase). [[#Maxwell--2018|Maxwell et al. (2018)]] identified regional ‘heavy precipitation day’ thresholds for five cities across the USA and projected that a tripling (or more) of these events is possible by RCP8.5 mid-century. Projections indicate changes to intensity-duration-frequency (IDF) curves typically used for construction design and automobile hazards, as well as increases in the 10-year recurrence level of 24-hour rainfall intensities that challenge storm water drainage systems ( [[#Hambly--2013|Hambly et al., 2013]] ; [[#Cheng--2015|Cheng and AghaKouchak, 2015]] ; J.E. [[#Neumann--2015|]] [[#Neumann--2015|Neumann et al., 2015]] ; [[#Prein--2017b|Prein et al., 2017b]] ; [[#Hettiarachchi--2018|Hettiarachchi et al., 2018]] ; [[#Ragno--2018|Ragno et al., 2018]] ). Precise levels of regional IDF characteristics may still depend substantially on the method and resolution of downscaling applied ( [[#DeGaetano--2017|DeGaetano and Castellano, 2017]] ; L.M. [[#Cook--2020|]] [[#Cook--2020|Cook et al., 2020]] ).

'''Landslide:''' There is growing yet ''limited evidence'' for unique climate-driven changes in landslide and rockfall hazards in North America, even as theory suggests decreases in slope and rockface stability due to more intense rainfall, rain-on-snow events, mean warming, permafrost thaw, glacier retreat, and coastal erosion ( [[#Cloutier--2017|Cloutier et al., 2017]] ; [[#Coe--2018|Coe et al., 2018]] ; [[#Handwerger--2019|Handwerger et al., 2019]] ; [[#Hock--2019|Hock et al., 2019]] ; [[#Patton--2019|Patton et al., 2019]] ) although dry trends can decelerate mass movements ( [[#Bennett--2016|Bennett et al., 2016]] ). Landslide frequency has increased in British Columbia (Canada; [[#Geertsema--2006|Geertsema et al., 2006]] ) and is expected to increase in North-Western North America given the combination of these factors ( ''medium confidence'' ) ( [[#Gariano--2016|Gariano and Guzzetti, 2016]] ). [[#Cloutier--2017|Cloutier et al. (2017)]] projected an increase in landslides in western Canada due to wetter overall conditions and reduced return period for extreme rainfall. [[#Robinson--2017|Robinson et al. (2017)]] used scenarios based upon projection of 50-year recurrence of 7-day precipitation periods to highlight the potential for increased landslide hazards near Seattle (USA). Broad projections for the USA are more uncertain given increases in evapotranspiration that will counteract precipitation changes over much of the country ( [[#Coe--2016|Coe, 2016]] ) ''.''

'''Aridity:''' [[IPCC:Wg1:Chapter:Chapter-8|Chapter 8]] showed that aridity in North America generally moves opposite to mean precipitation change with an added evaporative demand from warmer temperatures ( ''high confidence'' in aridity increase for Northern Central America; ''medium confidence'' for an increase in Central North America; ''high confidence'' for a decrease in North-Eastern North America; ''medium confidence'' decreases in Eastern and North-Western North America; see also [[IPCC:Wg1:Chapter:Chapter-11#11.9|Section 11.9]] ). Projected soil moisture declines (Figure 12.4j–l) are most widespread across North America during the summer, with the largest declines in Mexico and the southern Great Plains but also extending into Canada ( [[IPCC:Wg1:Chapter:Chapter-8#8.4.1.6|Section 8.4.1.6]] ; [[#Swain--2015|Swain and Hayhoe, 2015]] ; [[#Easterling--2017|Easterling et al., 2017]] ; [[#Bonsal--2019|Bonsal et al., 2019]] ; J. [[#Lu--2019|]] [[#Lu--2019|Lu et al., 2019]] ). [[#Yoon--2018|Yoon et al. (2018)]] found net reduction in southern Great Plains groundwater storage in RCP8.5 mid-century projections despite increases in mean precipitation and both wet and dry extremes. Soil moisture drying could reach unprecedented levels by the CMIP6 RCP8.5 end-of-century, even when evaluating deeper soil columns relevant for crop rooting depth (B.I. [[#Cook--2020|]] [[#Cook--2020|Cook et al., 2020]] ). Projected changes in the aridity index portray a shift the geographic range of temperate drylands northward and eastward in Central and Western North America ( [[#Schlaepfer--2017|Schlaepfer et al., 2017]] ; [[#Seager--2018|Seager et al., 2018]] ) which also diminishes aquifer recharge rates in the southern Great Plains and in some western regions where snowpack is reduced ( [[#Meixner--2016|Meixner et al., 2016]] ).

'''Hydrological drought:''' Section 11.9 asssessed ''low confidence'' of significant observational trends and projected future changes in the characteristics of episodic hydrological drought in North America given ''limited evidence'' and ''low agreement'' in modeled changes. [[#Zhao--2020|]] [[#Zhao--2020|C. Zhao et al. (2020)]] found that increases in hydrological drought frequency (particularly the 100 yr drought) were far more prevalent than for meteorological drought across 5797 watersheds in the USA and Canada, indicating a strong influence of evaporative demand. Reductions in the overall supply of meltwater from a declining snowpack also increase the potential for intermittent hydrological droughts in the western USA ( [[#Mote--2018|Mote et al., 2018]] ; [[#Livneh--2020|Livneh and Badger, 2020]] ).

'''Agricultural and ecological drought:''' Section 11.9 assessed ''medium confidence'' for an increase in agricultural and ecological drought in Western North America but otherwise found ''limited evidence'' for broadly observed changes in North American agricultural and ecological drought, even as increasing evaporative demand intensified vegetation stress and soil moisture deficits in recent events (Sections 11.6, 11.9). [[IPCC:Wg1:Chapter:Chapter-11#11.9|Section 11.9]] asssessed ''medium confidence'' for more intense agricultural and ecological drought conditions over North Central America, Western North America and Central North America in a 2°C global warming level (about mid-century), with ''medium confidence'' extending to Eastern North America and ''high confidence'' for Northern Central America and Central North America under a 4°C global warming level associated with higher emissions scenarios past 2050. Figure 12.4g–i shows that the frequency of meteorological droughts (which often initiate hydrological, agricultural and ecological droughts) is largely projected to increase in North American areas where total precipitation decreases (and vice versa; see [[IPCC:Wg1:Chapter:Chapter-11#11.9|Section 11.9]] and [[#Coppola--2021b|Coppola et al., 2021b]] ), and higher evaporative demand will extend the regions where more intense ecological and agricultural droughts develop when meteorological droughts occur ( [[#Wehner--2017|Wehner et al., 2017]] ; B.I. [[#Cook--2019|Cook et al., 2019]] , 2020). Studies utilizing a variety of drought indices and soil moisture projections consistently project increased drought extending from Mexico into the southern Canadian Plains during the summer ( [[#Swain--2015|Swain and Hayhoe, 2015]] ; [[#Ahmadalipour--2017|Ahmadalipour et al., 2017]] ; [[#Feng--2017|Feng et al., 2017]] ; [[#Bonsal--2019|Bonsal et al., 2019]] ; [[#Tam--2019|Tam et al., 2019]] ; B.I. [[#Cook--2020|]] [[#Cook--2020|Cook et al., 2020]] ).

'''Fire weather:''' Climatic conditions conducive to wildfire have increased in Mexico, Western and North-Western North America, primarily due to warming ( ''high confidence'' ). [[#Abatzoglou--2016|Abatzoglou and Williams (2016)]] found climate change led to higher values for eight fuel aridity indices over the western USA in recent decades, with 2000–2015 changes exposing 75% more forested area to high fuel aridity and adding nine more high fire-potential days each year, similar to 1979–2013 western USA and Mexico fire season expansion reported in [[#Jolly--2015|Jolly et al. (2015)]] . Increases in lightning-initiated fires have been distinguished from trends in man-made fire in western Canada and the USA ( [[#Balch--2017|Balch et al., 2017]] ; [[#Hanes--2019|Hanes et al., 2019]] ). [[#Jain--2017|Jain et al. (2017)]] identified a 1979–2015 expansion in fire weather season in eastern Canada and the south-western USA (with a smaller reduction in the northern Mountain West) along with regional shifts in the 99th percentile Canadian Fire Weather Index (FWI) and potential fire-spread days. [[#Girardin--2009|Girardin and Wotton (2009)]] noted that 1951–2002 trends in the Monthly Drought Code fire index in eastern Canada could hardly be distinguished from decadal variability.

Climate change drives future increases in North American fire weather, particularly in the south-west ( ''high confidence'' ), although further studies on shifts in exposure and vulnerability are needed to understand overall fire risks (see WGII Chapter 14). A significant increase of FWI is apparent before 2050 under RCP8.5 in much of North America, including the frequency of 95th-percentile FWI days, peak seasonal FWI average, fire weather season length, and maximum fire weather index ( [[#Abatzoglou--2019|Abatzoglou et al., 2019]] ), and fire season across North America expands dramatically beyond 2°C global warming levels (Q. [[#Sun--2019|]] [[#Sun--2019|]] [[#Sun--2019|Sun et al., 2019]] ; [[#Jain--2020|Jain et al., 2020]] ). X. [[#Wang--2017b|Wang et al. (2017b)]] simulated fire-spread days across Canada and found increases across most of the areas studied by 2071–2100, with median changes of –20 to +140% (RCP2.6), –20 to +250% (RCP4.5) and 40 to 360% (RCP8.5) compared to 1976–2015. [[#Prestemon--2016|Prestemon et al. (2016)]] found more conducive conditions for lightning-ignited fires in the south-eastern USA by mid-century, while warming conditions in Alaska increasingly push July temperatures above 13.4°C, a threshold for fire danger across Alaska’s tundra and boreal forest ( [[#Partain--2016|Partain et al., 2016]] ; [[#Young--2017|Young et al., 2017]] ). Longer and more intense fire seasons would also raise particulate matter and black carbon concentrations in the western USA, reducing visibility at many National Parks ( [[#Yue--2013|Yue et al., 2013]] ; [[#Val%20Martin--2015|Val Martin et al., 2015]] ).

'''Changes in North American wet and dry climatic impact-drivers are largely organized by the ‘north-east (more wet) to south-west (more dry)’ pattern of mean precipitation change, although heavy precipitation increases are widespread and increasing evaporative demand expands aridity, agricultural and ecological drought, and fire weather (particularly in summer)''' ( high confidence ''').'''

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==== 12.4.6.3 Wind ====

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'''Mean wind speed:''' Mean wind speeds have declined in North America – as in other Northern Hemisphere areas – over the past four decades ( ''medium confidence'' ) (AR5 WGI) with a reversal in the last decade ( ''low confidence'' ) not fully consistent across studies ( [[#Tian--2019|Tian et al., 2019]] ; [[#Zeng--2019|Zeng et al., 2019]] ; Z. [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|Zhang et al., 2019]] ). [[#Tian--2019|Tian et al. (2019)]] found a corresponding reduction in the wind power potential across the eastern parts of North America.

Mean wind speeds are expected to decline over much of North America (Figure 12.4m–o), but the only broad signal of consistent change across model types is a reduction in wind speed in Western North America ( ''high confidence'' ). These declines reduce wind power endowment by 2050 and as early as the 2020–2040 near-term period in the USA Mountain West, while there is disagreement between global- and regional-model change projections in the upper and lower Great Plains, Ohio River Valley, Mexico and eastern Canada ( [[#Karnauskas--2018a|Karnauskas et al., 2018a]] ; [[#Jung--2019|Jung and Schindler, 2019]] ; [[#Chen--2020|Chen, 2020]] ).

'''Severe wind storm:''' There is ''limited evidence'' and ''low agreement'' in observed changes in North American CID indices associated with extratropical cyclones ( [[IPCC:Wg1:Chapter:Chapter-11#11.7|Section 11.7]] ), severe thunderstorms, severe wind bursts ( ''derechos'' ), tornadoes, or lightning strikes ( [[#Vose--2014|Vose et al., 2014]] ; [[#Easterling--2017|Easterling et al., 2017]] ; [[#Kossin--2017|Kossin et al., 2017]] ). Observational studies have indicated a reduction in the number of tornado days in the USA, but increases in outbreaks with 30 or more tornadoes in one day ( [[#Brooks--2014|Brooks et al., 2014]] ), the density of tornado clusters ( [[#Elsner--2015|Elsner et al., 2015]] ), and overall tornado power ( [[#Elsner--2019|Elsner et al., 2019]] ).

There is ''medium confidence'' of a general decrease in the number of extratropical cyclones producing high wind speeds in North America, except over northernmost parts, for a global warming level of 2°C or by the end of the century under RCP4.5 and RCP8.5 ( [[#Kumar--2015|Kumar et al., 2015]] ; [[#Jeong--2018a|Jeong and Sushama, 2018a]] ; [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|C. Li et al., 2018]] ). GCMs cannot directly resolve tornadoes and severe thunderstorms, however projections of favourable environments for severe storms (based on convective available potential energy and wind shear) indicate ''medium confidence'' for more severe storms and a longer convective storm season in the USA, weaker increases extending north and east ( [[#Seeley--2015|Seeley and Romps, 2015]] ; [[#Glazer--2021|Glazer et al., 2021]] ), and a corresponding increase in autumn and winter tornadic storms (H.E. [[#Brooks--2013|]] [[#Brooks--2013|Brooks, 2013]] ; [[#Diffenbaugh--2013|Diffenbaugh et al., 2013]] ; [[#Brooks--2014|Brooks et al., 2014]] ; see also [[IPCC:Wg1:Chapter:Chapter-11#11.7.2|Section 11.7.2]] ). [[#Prein--2017a|Prein et al. (2017a)]] used a convection-permitting model to project a tripling of mesoscale convective systems over the USA for end-of-century RCP8.5.

'''Tropical cyclone''' : [[IPCC:Wg1:Chapter:Chapter-11#11.7.1|Section 11.7.1]] identified recent reductions in tropical cyclone translation speed and higher tropical cyclone rainfall totals over the North Atlantic, as well as substantial natural variability. Projections indicate ''low confidence'' in change in North Atlantic tropical cyclone numbers, but ''medium confidence'' in Mexico and the US Gulf and Atlantic coasts for more intense storms with higher wind, precipitation, and storm surge totals when they do occur ( [[IPCC:Wg1:Chapter:Chapter-11#11.7.1|Section 11.7.1]] ; [[#Diro--2014|Diro et al., 2014]] ; [[#DOE--2015|DOE, 2015]] ; [[#Walsh--2016a|Walsh et al., 2016a]] ; [[#Kossin--2017|Kossin et al., 2017]] ; [[#Marsooli--2019|Marsooli et al., 2019]] ; [[#Ting--2019|Ting et al., 2019]] ; [[#Knutson--2020|Knutson et al., 2020]] ). A more rapid intensification of tropical cyclone winds and destructive power also heightens the tropical cyclone hazard ( [[#Bhatia--2019|Bhatia et al., 2019]] ). Greenhouse gas forcing is projected to shift tropical cyclones poleward ( [[#Kossin--2016|Kossin et al., 2016]] ), while also holding the potential for higher precipitation totals ( [[#Risser--2017|Risser and Wehner, 2017]] ; [[#Knutson--2020|Knutson et al., 2020]] ) particularly given evidence that storms increasingly stall near North American coastlines ( [[#Hall--2019|Hall and Kossin, 2019]] ).

'''Sand and dust storm:''' Land-use change has increased dust emissions in the western USA in the past 200 years ( [[#Neff--2008|Neff et al., 2008]] ). However, there is ''medium confidence'' for observed increases in Western North American sand and dust storm activity since 1980. In their study of Valley Fever spread, [[#Tong--2017|Tong et al. (2017)]] identified a rapid intensification of dust storm activity using PM <sub>10</sub> and PM <sub>2.5</sub> observations from 1980–2011 across 29 monitoring sites in the south-western USA, similar to contiguous USA observations by [[#Brahney--2013|Brahney et al. (2013)]] . [[#Hand--2016|Hand et al. (2016)]] attributed the earlier onset of spring dusts in the south-west in large part to the Pacific Decadal Oscillation, however. The increasing trend in dust since the 1990s in the south-western USA can be explained by precipitation deficit and surface bareness ( [[#Pu--2018|Pu and Ginoux, 2018]] ). Projections of future sand and dust storms over North America are based on aridity as a primary proxy for conducive conditions which lends ''medium confidence'' of an increase over Mexico and the south-western USA. [[#Pu--2017|Pu and Ginoux (2017)]] project about five more dusty days in spring and summer in the southern Great Plains under RCP8.5 at the end of the century, while dusty days decrease in northern regions where mean precipitation tends towards wetter conditions.

'''Tropical cyclones, severe wind and dust storms''' '''in North America are shifting towards more extreme characteristics, with a stronger signal towards heightened intensity than increased frequency, although specific regional patterns are more uncertain''' ( medium confidence '''). Mean wind speed and wind power potential are projected to decrease in Western North America''' ( medium confidence ''') with differences between global and regional models lending''' low confidence '''elsewhere.'''

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==== 12.4.6.4 Snow and Ice ====

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'''Snow:''' The seasonal extent of snow cover has reduced over North America in recent decades ( ''robust evidence'' , ''high agreement'' ) (see also Sections 2.3.2.2 and 9.5.3, and Figure Atlas.25). The average snow-cover extent in North America decreased at a rate of about 8500 km <sup>2</sup> yr <sup>–1</sup> over the 1972–2015 period, reducing the average snow cover season by two weeks, primarily due to earlier spring melt ( [[#US%20EPA--2016|US EPA, 2016]] ). Observations indicate earlier spring snowpack melting ( [[#Dudley--2017|Dudley et al., 2017]] ) and a reduction in end-of-season snowpack metrics important to water resources over the Rocky Mountains (particularly since 1980) and Pacific Northwest ( [[#Pederson--2013|Pederson et al., 2013]] ; [[#Kormos--2016|Kormos et al., 2016]] ; [[#Kunkel--2016|Kunkel et al., 2016]] ; [[#Fyfe--2017|Fyfe et al., 2017]] ; [[#Mote--2018|Mote et al., 2018]] ). In situ measurements in Canada show more heterogenous trends in snow amount and density ( [[#Brown--2019|Brown et al., 2019]] ).

Climate change is expected to reduce the total snow amount and the length of the snow cover season over most of North America, with a corresponding decrease in the proportion of total precipitation falling as snow and a reduction in end-of-season snowpack ( ''high confidence'' ) (see Atlas.9.5). Changes include a reduction in the number of days with snowfall in across all of North America, with the exception of northern Canada ( [[#Danco--2016|Danco et al., 2016]] ; [[#McCrary--2019|McCrary and Mearns, 2019]] ), a delay of about a week in first snowfall in the western USA by 2050 under RCP8.5 ( [[#Pierce--2013|Pierce and Cayan, 2013]] ), and more prominent reductions in Canadian snow cover in the October –December period ( [[#Mudryk--2018|Mudryk et al., 2018]] ). Reduced total snowpack and earlier snowmelt lower dry season streamflow ( [[#Kormos--2016|Kormos et al., 2016]] ; [[#Rhoades--2018|Rhoades et al., 2018]] ). Figure 12.10b shows a reduction in days suitable for skiing (SWE &gt; 10 cm; [[#Wobus--2017b|Wobus et al., 2017b]] ) across the USA and southern Canada, although some portions of northern central Canada see an increase.

'''Glacier:''' Section 9.5.1 assessed that glaciers in Alaska, western Canada and the western USA are expected to continue to lose mass and areal extent ( ''high confidence'' ). Compared to their 2015 state, glaciers in the western Canada and the USA region will lose 62 ± 30%, 75 ± 29% and 85 ± 23%, of their mass by the end of the century for RCP2.6, RCP4.5 and RCP8.5 scenarios, respectively ( [[#Marzeion--2020|Marzeion et al., 2020]] ). Meanwhile glaciers in Alaska will lose 26 ± 21%, 31 ± 24% and 44 ± 27%, of their 2015 mass under the same scenarios. The overall loss of glacial mass can act as a meltwater supply for freshwater resources, although this is expected to peak in the middle of the century and then fade as glaciers disappear ( [[#Fyfe--2017|Fyfe et al., 2017]] ; [[#Derksen--2018|Derksen et al., 2018]] ). Continued shrinkage of glaciers is projected to create further glacial lakes ( ''medium confidence'' ) similar to those that have led to outburst floods in Alaska and Canada ( [[#Carrivick--2016|Carrivick and Tweed, 2016]] ; [[#Harrison--2018|Harrison et al., 2018]] ).

'''Permafrost:''' Warmer ground temperatures are expected to extend the geographical extent and depth of permafrost thaw across northern North America ( ''very'' ''high confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.2|Section 9.5.2]] ). Observations across Canada show that permafrost temperature is increasing and the active layer is getting thicker ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.2.5|Section 2.3.2.5]] ; [[#Derksen--2018|Derksen et al., 2018]] ; [[#Biskaborn--2019|Biskaborn et al., 2019]] ; [[#Romanovsky--2020|Romanovsky et al., 2020]] ). [[#Slater--2013|Slater and Lawrence (2013)]] note that the RCP8.5 end-of-century period in North America only has shallow permafrost as the most probable condition in the Canadian Archipelago. [[#Melvin--2017|Melvin et al. (2017)]] noted the loss of shallow permafrost in five RCP8.5 CMIP5 models across a wide swathe of southern Alaska by 2050, along with increases of active layer thickness. There is ''high confidence'' in continued reductions in mountain near-surface permafrost area with high spatial variability given local snow and temperature changes ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.2|Section 9.5.2]] ; [[#Peng--2018|Peng et al., 2018]] ; [[#Hock--2019|Hock et al., 2019]] ).

'''Lake, river and sea ice:''' Anthropogenic warming reduces the seasonal extent of lake and river ice over many North American freshwater systems, with ice-free winter conditions pushing further north with rising temperatures ( ''high confidence'' ). Observations in Central and Eastern North America show reduced average seasonal lake-ice cover duration ( [[#Benson--2012|Benson et al., 2012]] ; [[#Mason--2016|Mason et al., 2016]] ; [[#US%20EPA--2016|US EPA, 2016]] ). Satellite observations show declines in lake ice ( [[#Du--2017|Du et al., 2017]] ) and loss of more than 20% of winter river-ice length in much of Alaska (2008–2018 compared to 1984–1994; [[#Yang--2020a|Yang et al., 2020a]] ). Spring lake and river ice in Canada is projected to break up 10–25 days earlier while autumn freeze-up occurs 5–15 days later by mid-century, with larger declines in lake-ice season closer to the coasts ( [[#Dibike--2012|Dibike et al., 2012]] ) and for rivers in the Rocky Mountains and north-eastern USA ( [[#Yang--2020a|Yang et al., 2020a]] ), although global models have difficulty with frozen freshwater system dynamics ( [[#Derksen--2018|Derksen et al., 2018]] ). Substantial ice loss is projected over the Laurentian Great Lakes ( [[#Hewer--2019|Hewer and Gough, 2019]] ; [[#Matsumoto--2019|Matsumoto et al., 2019]] ). The southern extent of lakes experiencing intermittent winter ice cover moves northward with rising temperature, pushing nearly out of the continental USA at low elevations under a 4.5°C GWL ( [[#Sharma--2019|Sharma et al., 2019]] ). Higher spring flows and the potential for winter thaws are also projected to heighten the threat of ice jams ( [[#Rokaya--2018|Rokaya et al., 2018]] ; [[#Bonsal--2019|Bonsal et al., 2019]] ) while reducing the seasonal viability of ice roads and recreational use ( [[#Pendakur--2016|Pendakur, 2016]] ; [[#Mullan--2017|Mullan et al., 2017]] ; [[#Knoll--2019|Knoll et al., 2019]] ).

Seasonal sea ice coverage along the majority of Canadian and Alaskan coastlines is declining ( ''robust evidence'' , ''high agreement'' ) and there is ''high confidence'' that sea ice loss continues under climate change, as further assessed in [[#12.4.9|Section 12.4.9]] .

'''Heavy snowfall:''' There is ''low agreement'' ( ''limited evidence'' ) for observed changes in heavy snowfall in North America. [[#Kluver--2015|Kluver and Leathers (2015)]] noted a 1930–2008 frequency increase for all snow intensities in the northern Great Plains but declines in heavier snow events in the Pacific Northwest and declines in the south-eastern USA. [[#Changnon--2018|Changnon (2018)]] found that most extreme 30-day high-snowfall periods in the 1900–2016 record over the eastern USA occurred in the 1959–1987 period, which lies between the 1930s Dust Bowl and recent warming. There is ''low agreement'' and ''medium evidence'' for broad projected changes to heavy snowfall over North America given increased heavy precipitation and warmer winter temperatures. Several recent regional studies have projected that low-intensity events decrease more rapidly than heavy snowfall events, resulting in an increase in the snowfall proportion from heavy snowfall events even as the number of such events decreases ( [[#O’Gorman--2014|O’Gorman, 2014]] ; [[#Lute--2015|Lute et al., 2015]] ; [[#Zarzycki--2016|Zarzycki, 2016]] ; [[#Janoski--2018|Janoski et al., 2018]] ; [[#Ashley--2020|Ashley et al., 2020]] ).

'''Ice storm:''' There is ''limited evidence'' in the literature of unique changes in ice storms observed or projected over North America. [[#Groisman--2016|Groisman et al. (2016)]] examined 40 years of observations and found weak decreases in freezing rain events over the south-eastern USA in the most recent decade. [[#Ning--2015|Ning and Bradley (2015)]] project that the average snow–rain transition line, which is associated with mixed precipitation, moves 2° latitude northward over Eastern North America by the end of the 21st century under RCP4.5 (4° under RCP8.5; see also [[#Klima--2015|Klima and Morgan, 2015]] ).

'''Hail:''' There is ''limited evidence'' and ''low agreement'' for observed changes in the frequency or intensity of North American hail storms. J.T. [[#Allen--2015|]] [[#Allen--2015|Allen et al. (2015)]] and [[#Allen--2018|Allen (2018)]] found that temporal inconsistencies in the US and Canadian hail records made long-term climate analysis difficult, although B.H. [[#Tang--2019|]] [[#Tang--2019|Tang et al. (2019)]] identified an increasing frequency of environmental conditions conducive for large hail (diameter ≥ 5 cm) over the central and eastern USA. There is ''limited evidence'' and ''medium agreement'' in projections of increased hail damage potential over North America. Some regional and convective-permitting model projections indicate a longer hail season with fewer events and larger hail sizes that result in higher hail damage potential ( [[#Brimelow--2017|Brimelow et al., 2017]] ; [[#Trapp--2019|Trapp et al., 2019]] ).

'''Snow avalanche:''' There is ''limited evidence'' of directional changes in snow avalanches over North America. [[#Mock--2000|Mock and Birkeland (2000)]] identified a 1969–1995 decrease in snow avalanches over the western United States, although they note the heavy influence of natural variability. A similar decline was observed over western Canada ( [[#Bellaire--2016|Bellaire et al., 2016]] ; [[#Sinickas--2016|Sinickas et al., 2016]] ), but clear trends are difficult to discern given sparse observations and shifts in avalanche management. We concur with the SROCC assessment of ''medium confidence'' and ''high agreement'' that snow avalanche hazards generally decrease at low elevations given lower snowpack, even as high elevations are increasingly susceptible to wet-snow avalanches ( [[#Hock--2019|Hock et al., 2019]] ; see also [[#Lazar--2008|Lazar and Williams, 2008]] ).

'''Observations and projections agree that snow and ice CIDs over North America are characterized by reduction in glaciers and the seasonality of snow and ice formation, loss of shallow permafrost, and shifts in the rain/snow transition line that alters the seasonal and geographic range of snow and ice conditions in the coming decades''' ( very high confidence ''').'''

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==== 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]] .

[[File:04271e9d5dd8d10bb6f8bffafb98a88f IPCC_AR6_WGI_Chapter12_Table_12_8.jpg]]

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