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

=== 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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