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

==== 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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