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

==== 12.4.5.1 Heat and Cold ====

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'''Mean air temperature:''' Since AR5, studies have confirmed that the mean warming trend in Europe is increasing (Atlas.8.2). The observed warming trend patterns are largely consistent with those simulated by global and regional climate models and it is ''very likely'' that such trends are, in large part, due to human influence on climate ( [[IPCC:Wg1:Chapter:Chapter-3#3.3.1|Section 3.3.1]] ).

All temperature trends are ''very likely'' to continue for a global warming of 1.5°C or 2°C and 3°C (Atlas.8.4). Future warming leads to the exceedance of different temperature thresholds relevant for vector-borne diseases ( ''medium confidence'' ) ( [[#Caminade--2012|Caminade et al., 2012]] ; [[#Medlock--2013|Medlock et al., 2013]] ), invasive allergens ( ''medium confidence'' ) ( [[#Storkey--2014|Storkey et al., 2014]] ; [[#Hamaoui-Laguel--2015|Hamaoui-Laguel et al., 2015]] ), SST thresholds in the Mediterranean ( ''likely'' to exceed 20°C), or relevant for the ''Vibrio'' bacteria development ( [[#Vezzulli--2015|Vezzulli et al., 2015]] ). Future warming is also projected to lead to the exceedance of cooling degree day index (&gt;22°C) thresholds, characterizing a potential increase in energy demand for cooling in southern Europe with increases ''likely'' exceeding 40% in some areas ( [[#Spinoni--2015|Spinoni et al., 2015]] ) by 2050 under RCP8.5 ( ''high confidence'' ) ( [[#12.3|Section 12.3]] and Atlas.8; [[#Coppola--2021a|Coppola et al., 2021a]] ).

'''Extreme heat:''' The frequency of heatwaves observed in Europe has ''very likely'' increased in recent decades due to human-induced change in atmospheric composition ( [[IPCC:Wg1:Chapter:Chapter-11#11.3|Section 11.3]] ) and a detectable anthropogenic increase in a summer heat stress index over all regions of Europe has been identified based on WBGT index trends for 1973–2012 ( ''medium confidence'' , ''limited evidence'' ) ( [[#Knutson--2016|Knutson and Ploshay, 2016]] ).

It is ''very likely'' that the frequency of heatwaves will increase during the 21st century regardless of the emissions scenario in each European region, and for 1.5°C and 2°C GWLs ( [[IPCC:Wg1:Chapter:Chapter-11#11.3.5|Section 11.3.5]] ). Heat stress due to both high temperature and humidity, affecting morbidity, mortality and labour capacity ( [[#12.3|Section 12.3]] ) is projected to increase under all emissions scenarios and GWLs by the middle of the century (Figure 12.4a–f). Under RCP8.5, the expected number of days with WBGT higher than 31°C is about 25, 30 and 40 days per year, as projected by EURO-CORDEX, CMIP5 and CMIP6 respectively on average over the Mediterranean region, and around 30, 40 and 60 days per year in low coastal plain areas such as the Po Valley, the Italian, Greek and Spanish coasts, and the Mediterranean islands ( [[#Coppola--2021a|Coppola et al., 2021a]] ). An average increase of a few days per year of maximum daily temperature exceeding 35°C, a typical critical threshold for crop productivity, is expected by the mid-century in central Europe, and an increase of 10–20 days is expected for the Mediterranean areas (Figure 12.4b; [[#Coppola--2021a|Coppola et al., 2021a]] ). By contrast, under SSP1-2.6, the increase in this number of days remains limited to less than about 10 days, and confined to the Mediterranean regions. Mitigation is expected to have a strong effect, with the dangerous heat threshold of HI &gt; 41°C projected to be crossed 5–10 days more per year in the Mediterranean regions and a few days per year more in WCE and EEU under SSP5-8.5, while such increases would be virtually absent under SSP1-2.6 (Figure 12.4d–f).

'''Cold spell and frost:''' Temperature observations for winter cold spells in Europe show a long-term decreasing frequency ( [[#Brunner--2018|Brunner et al., 2018]] ), with their probability of occurrence projected to decrease in the future ( ''high confidence'' ) and virtually disappear by the end of the century ( [[IPCC:Wg1:Chapter:Chapter-11#11.3|Section 11.3]] ). The frequency of frost days will ''very likely'' decrease for all scenarios and all time horizons ( [[#Lindner--2014|Lindner et al., 2014]] ; [[#Coppola--2021a|Coppola et al., 2021a]] ) with consequences for agriculture and forests. A simple heating degree day index, characterizing heating demand, shows a large observed decreasing trend for winter heating energy demand in Europe ( [[#Spinoni--2015|Spinoni et al., 2015]] ). This trend is ''very likely'' to continue through the 21st century, with decreases in the range of 20–30% for Northern Europe, about 20% for central Europe and 35% for southern Europe, by mid-century under RCP8.5 ( [[#Spinoni--2018b|Spinoni et al., 2018b]] ; [[#Coppola--2021a|Coppola et al., 2021a]] ; Interactive Atlas).

'''In summary, irrespective of the scenario, it is''' virtually certain '''that warming will continue in Europe, and there is''' high confidence '''that the observed increase in heat extremes is due to human activities. It is''' very likely '''that the frequency of heat extremes will increase over the 21st century with an increasing gradient toward the southern regions. Extreme heat will exceed critical thresholds for health, agriculture and other sectors more frequently''' ( high confidence '''), with strong differences between mitigation scenarios. It is''' very likely '''that the frequency of cold spells and frost days will keep decreasing over the course of this century and it is''' likely '''that cold spells will virtually disappear towards the end of the century.'''

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