Editing IPCC:AR6/WGII/Chapter-14 (section)

== 14.2 Current and Future Climate in North America ==

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Trends in observed and projected physical climate variables, and changes in extreme weather and climate events, are summarised in this section. Many of the assessments here are adapted from AR6 WGI ( [[#IPCC--2021|IPCC, 2021]] ), especially Chapters 11 ( [[#Seneviratne--2021|Seneviratne et al., 2021]] ) and 12 ( [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ), and the Atlas ( [[#Gutiérrez--2021a|Gutiérrez et al., 2021a]] , b). [[#Ranasinghe--2021|Ranasinghe et al., 2021]] , [[IPCC:Wg2:Chapter:Chapter-12#12.4|Section 12.4.6]] , assesses North American climatic impact drivers without assessing their impacts or associated risks. The WGI assessments are augmented in this section with regionally specific support from recent national climate assessments or original literature.

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=== 14.2.1 Observed Changes in North American Climate ===

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Climate changes directly related to increasing mean and extreme temperature, including reduced snowpack, sea and lake ice and glacier extent, and marine heatwaves (MHWs), can be attributed to human activity and are affecting most of North America ( ''high confidence'' ). Upward trends in annual mean temperature across North America since 1960 are widespread ( [[#Gutiérrez--2021a|Gutiérrez et al., 2021a]] ) but non-uniform (Figure 14.2A). Pronounced polar amplification of warming is observed in high latitudes (Figure 14.2A), particularly in winter ( [[#Gutiérrez--2021a|Gutiérrez et al., 2021a]] ; [[#Vose--2017|Vose et al., 2017]] ; [[#Zhang--2019a|Zhang et al., 2019a]] ). As average temperature rises, extreme high temperature records across North America are being set more frequently than extreme cold records ( [[#Meehl--2016|Meehl et al., 2016]] ) and the probability of cold extreme events is reduced ( WGI Chapter 11 [ [[#Seneviratne--2021|Seneviratne et al., 2021]] ]). Trends in daily maximum and minimum temperature are significant in high latitudes (US-AK, CA-NW, CA-NE). Summer daily maximum temperature is increasing in southwest desert regions (US-SW, MX-NW) ( [[#Martinez-Austria--2016|Martinez-Austria et al., 2016]] ; [[#Martinez-Austria--2017|Martinez-Austria and Bandala, 2017]] ; [[#Navarro-Estupinan--2018|Navarro-Estupinan et al., 2018]] ).

Annual precipitation has increased in recent decades in northern and eastern areas (CA-PR, CA-QU, US-NP, US-SP, US-MW, US-NE, US-AK) ( ''high confidence'' ), and has decreased across the western part of the continent (CA-BC, US-SW, US-NW, MX-NW) ( ''medium confidence'' ), with considerable spatial variability within these regions ( [[#Zhang--2019a|Zhang et al., 2019a]] ; [[#Gutiérrez--2021a|Gutiérrez et al., 2021a]] ). Elsewhere across North America there is ''limited evidence'' and ''low agreement'' on detection of observed trends in total precipitation and river flood hazards. The intensity and frequency of 1-day heavy precipitation events have ''very likely'' [[#footnote-019|2]] increased since the mid-20th Century across most of the USA (US-NP, US-MW, US-NE, but not in US-SE) and in Mexico, but no detectable trend is reported in Canada ( [[#Seneviratne--2021|Seneviratne et al., 2021]] ; [[#Zhang--2019a|Zhang et al., 2019a]] ). Recent flooding events along the mid-latitude Pacific Coast have been attributed to increasingly intense atmospheric river events ( [[#Douville--2021|Douville et al., 2021]] ; [[#Gershunov--2019|Gershunov et al., 2019]] ; [[#Vano--2019|Vano et al., 2019]] ), but there is ''low confidence'' in detecting trends in atmospheric river activity.

Snowpack and snow extent across much of Canada and the western USA have declined as temperatures have increased ( ''very high confidence'' ) ( [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ; Gutierrez et al., 2021a; [[#Kunkel--2016|Kunkel et al., 2016]] ; [[#Mote--2018|Mote et al., 2018]] ; [[#Mudryk--2018|Mudryk et al., 2018]] ; [[#Derksen--2019|Derksen et al., 2019]] ). Warm ‘snow droughts’ describing a deficit of snowpack available for runoff, even in the absence of a winter precipitation deficit ( [[#Cooper--2016|Cooper et al., 2016]] ; [[#Harpold--2017|Harpold et al., 2017]] ), have become more common in North American mountains ( [[#Sproles--2016|Sproles et al., 2016]] ; [[#Nicholls--2018|Nicholls et al., 2018]] ; [[#Pershing--2018|Pershing et al., 2018]] ). Glaciers have retreated over the past half-century at high elevation across North America ( [[#Frans--2018|Frans et al., 2018]] ; [[#Zemp--2019|Zemp et al., 2019]] ) and in the Arctic ( [[#Burgess--2017|Burgess, 2017]] ; [[#Box--2019|Box et al., 2019]] ; [[#Derksen--2019|Derksen et al., 2019]] ). Lake ice in Canada, south of the Arctic region delineated in Figure 14.1, has declined ( [[#Alexeev--2016|Alexeev et al., 2016]] ; [[#Derksen--2019|Derksen et al., 2019]] ).

There is limited evidence of trends in meteorological or hydrological droughts over the historical record (see [[#Douville--2021|Douville et al. (2021)]] and [[#Seneviratne--2021|Seneviratne et al. (2021)]] for multiple perspectives on drought; [[#Wehner--2017|Wehner et al., 2017]] ), but there is ''medium confidence'' in increasing atmospheric evaporative demand acting to intensify surface aridity during recent droughts (e.g., US-SW) ( [[#Seneviratne--2021|Seneviratne et al., 2021]] ; [[#Williams--2020|Williams et al., 2020]] ). The ongoing multi-decadal dry period in the Colorado River basin is as extreme as any drought in the past 1000 years ( [[#Murphy--2019|Murphy and Ellis, 2019]] ; [[#Williams--2020|Williams et al., 2020]] ).

The proportion of hurricanes in stronger categories has ''likely'' increased globally over the past 40 years, with ''medium confidence'' that the onshore propagation speed of hurricanes making landfall in the USA has slowed detectably since 1900 ( [[#Seneviratne--2021|Seneviratne et al., 2021]] ; [[#Kossin--2018|Kossin, 2018]] ), contributing to detectable increases in local rainfall and coastal flooding associated with these storms. There is ''high confidence'' ( [[#Seneviratne--2021|Seneviratne et al., 2021]] ) that anthropogenic climate change has contributed to extreme precipitation associated with recent intense hurricanes, such as Harvey in 2017.

North American sea ice extent and volume (thickness) have declined up to 10% per decade since 1981 ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Ding--2017|Ding et al., 2017]] ; [[#Mudryk--2018|Mudryk et al., 2018]] ; [[#Derksen--2019|Derksen et al., 2019]] ; [[#IPCC--2019b|IPCC, 2019b]] ), with changes accelerating during this time ( ''robust evidence, high agreement'' ) ( [[#Schweiger--2019|Schweiger et al., 2019]] ), resulting in longer and larger periods of open water ( [[#Wang--2018a|Wang et al., 2018a]] ). Recent (2018) sea ice extent in the Bering Sea was the lowest in a 5500-year record and appears to lag atmospheric CO 2 by about two decades (Jones et al. 2021). High Arctic sea ice retreat since 1971 and increases in open-water duration in the most recent decade are unprecedented ( [[#Box--2019|Box et al., 2019]] ) and most pronounced in the Chukchi, Bering and Beaufort seas (US-AK, CA-NW) ( ''high confidence'' ) ( [[#Wang--2015|Wang and Overland, 2015]] ; [[#Jones--2020|Jones et al., 2020]] ).

Warming of North American offshore waters is significant and attributable to human activities, particularly along the Atlantic coast, contributing to sea level rise (SLR) through thermal expansion ( ''very high confidence'' ) ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#IPCC--2019b|IPCC, 2019b]] ). Rates of SLR have accelerated along most North American coasts during the past three decades, excepting coastlines in southern Alaska (US-AK) and northeastern Canada (CA-QC, CA-NE) where land is rising ( [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ; [[#Greenan--2018|Greenan et al., 2018]] ). Tidal flooding frequency has increased in the North Pacific from once every 1–3 years to every 6–12 months ( [[#Sweet--2014|Sweet et al., 2014]] ).

Acidification of North American coastal waters has occurred in conjunction with increased atmospheric CO 2 concentration ( [[#Mathis--2015|Mathis et al., 2015]] ; [[#Jewett--2017|Jewett and Romanou, 2017]] ; [[#Claret--2018|Claret et al., 2018]] ) combined with other local acidifying inputs such as nitrogen and sulphur deposition ( [[#Doney--2007|Doney et al., 2007]] ) and freshwater nutrient input ( ''very high confidence'' ) ( [[#Strong--2014|Strong et al., 2014]] ; [[#IPCC--2019b|IPCC, 2019b]] ). Oxygen minimum zones, particularly in the North Pacific south of US-AK, have expanded in volume and O 2 has declined since 1970 ( [[#IPCC--2019b|IPCC, 2019b]] ).

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=== 14.2.2 Projected Changes in North American Climate ===

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Climate changes related to warming temperature, including more intense heatwaves over land and in the ocean, diminished snowpack, sea ice reduction and SLR, are projected with ''high confidence'' and are strongly sensitive to future GHG concentrations (Figure 14.2). Climatic hazards affected by hydrological change, including humidity-inclusive heat stress, extreme precipitation and more intense storms, are projected to intensify.

Pronounced amplification of warming across the Arctic and continental intensification of warming (Figure 14.2B,C) is projected with ''high confidence'' ( [[#Doney--2007|Doney et al., 2007]] ; [[#Vose--2017|Vose et al., 2017]] ). Extreme heatwaves are projected to intensify, particularly in MX-NW, MX-N, MX-NE, US-SW, US-NP and US-SP (Figure 14.2F,G) and become more frequent and longer in duration as average temperature rises across North America ( [[#Seneviratne--2021|Seneviratne et al., 2021]] ). Extreme cold events are projected to decrease in severity ( [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ; [[#Wuebbles--2014|Wuebbles et al., 2014]] ).

Total precipitation is projected to increase across the northern half of North America ( ''very high confidence'' ) and decrease in southwest North America (MX-SW, MX-NW, US-SW) ( ''medium confidence'' ) (Figure 14.2D,E; [[#Gutiérrez--2021b|Gutiérrez et al., 2021b]] ). Further increases in the intensity of locally heavy precipitation are ''very likely'' across the continent, as a greater fraction of precipitation falls in intense events ( [[#Easterling--2017|Easterling et al., 2017]] ; [[#Prein--2017a|Prein et al., 2017a]] ; [[#Zhang--2019a|Zhang et al., 2019a]] ).

High-humidity hazards are projected to increase ( ''medium confidence'' ) in regions around the Gulf of Mexico and southeast North America (US-SE, US-SP, MX-NE, MX-SE) ( [[#Zhao--2015|Zhao et al., 2015]] ). In subtropical regions that are less influenced by moisture from the Gulf of Mexico (including US-SW, US-SP, MX-NW and MX-N), the combination of higher temperature and less total precipitation leads to projections of increased aridity: drier surface conditions, higher evaporative demand by plants and more intense droughts ( [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ; [[#Jones--2016|Jones and Gutzler, 2016]] ; [[#Easterling--2017|Easterling et al., 2017]] ; [[#Escalante-Sandoval--2017|Escalante-Sandoval and Nuñez-Garcia, 2017]] ).

As temperatures rise, snow extent, duration of snow cover and accumulated snowpack are ''virtually certain'' to decline in subarctic regions of North America (Gutierrez et al., 2021a; [[#McCrary--2019|McCrary and Mearns, 2019]] ; [[#Mudryk--2021|Mudryk et al., 2021]] ), with corresponding effects on snow-related hydrological changes ( ''high confidence'' ). These changes include declines in snowmelt runoff ( [[#Li--2017|Li et al., 2017]] ), increased evaporative losses during snow ablation ( [[#Foster--2016|Foster et al., 2016]] ; [[#Milly--2020|Milly and Dunne, 2020]] ), as well as increases in the frequency of rain-on-snow events ( [[#Jeong--2018a|Jeong and Sushama, 2018a]] ) and consecutive snow drought years in western North America ( [[#Marshall--2019a|Marshall et al., 2019a]] ).

Climate change is projected to magnify the impact of tropical cyclones in US-NE, MX-NE, US-SP, and US-SE by increasing rainfall ( [[#Patricola--2018|Patricola and Wehner, 2018]] ) and extreme wind speed ( ''high confidence'' ) and slowing the speed of land-falling storms ( ''limited evidence, low confidence'' ) ( [[#Seneviratne--2021|Seneviratne et al., 2021]] ; [[#Kossin--2018|Kossin, 2018]] ) ''.'' The coastal region at severe risk from tropical storms is projected to expand northward within US-NE ( ''medium confidence'' ) ( [[#Kossin--2017|Kossin et al., 2017]] ).

Additional reduction in polar sea ice is ''virtually certain'' ( [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ; [[#Mudryk--2021|Mudryk et al., 2021]] ), with the North American Arctic projected to be seasonally ice free at least once per decade under 2°C of global warming ( ''high confidence'' ) ( [[#IPCC--2019b|IPCC, 2019b]] ; [[#Mioduszewski--2019|Mioduszewski et al., 2019]] ; [[#Mudryk--2018|Mudryk et al., 2018]] ). Duration of freshwater lake ice across the northern USA and southern Canada is projected to diminish ( ''high confidence'' ) ( [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ; [[#Dibike--2012|Dibike et al., 2012]] ; [[#Mudryk--2018|Mudryk et al., 2018]] ; [[#Sharma--2019|Sharma et al., 2019]] ).

Ocean surface temperature is ''very likely'' to increase in future decades in waters around North America ( [[#Jewett--2017|Jewett and Romanou, 2017]] ; [[#Greenan--2018|Greenan et al., 2018]] ), but at a slower rate than air temperature over the continent. Rates of change are projected to be relatively higher in northern latitudes, with most rapid warming in summer in the Arctic and Bering Sea (US-AK, CA-NW) ( [[#Wang--2015|Wang and Overland, 2015]] ; [[#Wang--2018a|Wang et al., 2018a]] ; [[#Hermann--2019|Hermann et al., 2019]] ).

Sea level rise is ''virtually certain'' to continue along North American coastlines except for parts of US-AK and around Hudson Bay (HB) with geographically variable rates of rise ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ; see Box 14.4). Relatively greater SLR is projected along the US-SE and MX-SW coastlines and relatively less along CA-BC and US-NW ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ; [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ; see Box 14.4) ( [[#Fasullo--2018|Fasullo and Nerem, 2018]] ; [[#Greenan--2018|Greenan et al., 2018]] [[#IPCC--2019b|IPCC, 2019b]] ).

Ocean acidification (OA) along North American coastlines is projected to increase ( ''very high confidence'' ) ( [[#Jewett--2017|Jewett and Romanou, 2017]] ). The frequency and extent of oxygen minimum and hypoxic zones are projected to increase, with less confidence, exacerbated by climate-driven eutrophication and increasing stratification ( [[#Altieri--2015|Altieri and Gedan, 2015]] ; [[#IPCC--2019b|IPCC, 2019b]] ).

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=== FAQ 14.1 | How has climate change contributed to recent extreme events in North America and their impacts? ===

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''Multiple lines of evidence indicate that climate change is already contributing to more intense and more frequent extreme events across North America. The impacts resulting from extreme events represent a huge challenge for adapting to future climate change.''

Extreme events are a fundamental part of how we experience weather and climate. Exceptionally hot days, torrential rainfall and other extreme weather events have a direct impact on people, communities and ecosystems. Extreme weather can lead to other impactful events such as droughts, floods or wildfires. In a changing climate, people frequently ask whether extreme events are generally becoming more severe or more frequent, and whether an actual extreme event was caused by climate change.

Because really extreme events occur rarely (by definition), it can be very difficult to assess whether the overall severity or frequency of such events has been affected by changing climate. Nevertheless, careful statistical analysis shows that record-setting hot temperatures in North America are occurring more often than record-setting cold temperatures as the overall climate has gotten warmer in recent decades. The area burned by large wildfires in the western USA has increased in recent decades. Observed trends in extreme precipitation events are more difficult to detect with confidence, because the natural variability of precipitation is so large and the observational database is limited.

Our understanding of how individual extreme weather events have been influenced by climate change has improved greatly in recent years. Climate scientists have developed a formal technique (‘event attribution’, described in WGI FAQ 11.3) for assessing how climate change affects the severity or frequency of a particular extreme event, such as a record-breaking rainfall event or a marine heatwave. This is a challenging task, because any particular event can be caused by a combination of natural variability and climate change. Event attribution is typically carried out using models to compare the probability of a specific event occurring in today’s climatic environment relative to the probability that the same event might have occurred in a modelled climate in which atmospheric GHGs have not risen due to human activities. Using this strategy, multiple studies have estimated that the historically extreme rainfall amount that fell across the Houston area from Hurricane Harvey (2017) was three to ten times more ''likely'' as the result of climate change.

The ''impacts'' from extreme events depend not just on physical climate system hazards (temperature, precipitation, wind, etc.), but also on the exposure and vulnerability of humans or ecosystems to these events. For example, damage from land-falling hurricanes along the coast of the Gulf of Mexico is expected to increase as very strong hurricanes become more frequent and intense due to climate change. But damage would also increase with additional construction along the shoreline, because coastal development increases ''exposure'' to hurricanes. And if some structures are constructed to poor building standards, as was the case when hurricane Andrew made landfall in Florida in 1992, then ''vulnerability'' to hurricane-caused impacts is increased.

Climate change also contributes to impacts from extreme events by making some building codes and zoning restrictions inadequate or obsolete. Many North American communities limit development in areas known to be flood-prone, to minimise exposure to flooding. But as climate change expands the areas at risk of exposure to flooding beyond historical floodplains, the impacts of potential flooding are increased, as Hurricane Harvey demonstrated. Adapting to climate change may require retrofits for existing structures and revised zoning for new construction. Some structures and neighbourhoods may need to be abandoned altogether to accommodate expanded flooding risk.

Climate change can be an ''added stress'' that increases impacts from extreme events, combined with other non-climatic stressors. For example, climate change in western North America has contributed to more extreme fire weather. The devastating impacts of recent wildfire outbreaks, such as occurred across western Canada in 2016 and 2017, the western United States in 2018 and 2020, and both countries in 2021, are to some extent associated with expanded development and forest management practices (such as policies to suppress low-intensity fires, allowing fuel to accumulate). The effects of development and forest management have dramatically increased the exposure and vulnerability of communities to intense wildfires. Climate change has added to these stressors: warming temperature leads to more extreme weather conditions that are conducive to increasingly severe wildfires.

Biodiversity is affected by climate change in this way too. For example, numerous bird populations across North America are estimated to have declined by up to 30% over the past half-century. Multiple human-related factors, including habitat loss and agricultural intensification, contribute to these declines, with climate change as an added stressor. Increasingly extreme events, such as severe storms and wildfires, can decimate local populations of birds, adding to existing ecological threats.

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