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

==== 12.3.1.4 Frost ====

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Frost (T <sub>min</sub> &lt;0°C) is a natural and fundamental aspect of many ecosystems, with more extreme conditions defined as ice (or icing) days (T <sub>max</sub> &lt;0°C) ( [[#Vincent--2018|L.A. Vincent et al., 2018]] ). Agricultural systems planning (e.g., planting calendars, seed selection or the opportunity to double-crop) requires information about the start and end of the frost-free season ( [[#Wypych--2017|Wypych et al., 2017]] ; [[#Wolfe--2018|Wolfe et al., 2018]] ). Crops and wild plants can be directly damaged by frost, but hard or killing frosts (at a threshold several degrees below freezing) can kill crops or lower harvest quality depending on duration (which relates to soil temperature penetration) and plant developmental stage ( [[#Crimp--2016a|Crimp et al., 2016a]] ; [[#Cradock-Henry--2017|Cradock-Henry, 2017]] ; G. [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|Li et al., 2018]] ; [[#Mäkinen--2018|Mäkinen et al., 2018]] ; [[#Grotjahn--2021|Grotjahn, 2021]] ). Earlier disappearance of snow cover reduces natural insulation that protects plants and burrowing animals from hard frost damages ( [[#Trnka--2014|Trnka et al., 2014]] ; [[#Mäkinen--2018|Mäkinen et al., 2018]] ). In some cases an early season warm spell may reduce plant hardiness or induce fruit tree flowering that exposes plants to devastating subsequent frost impacts ( [[#Hufkens--2012|Hufkens et al., 2012]] ; [[#Hatfield--2014|Hatfield et al., 2014]] ; [[#Tripathi--2016|Tripathi et al., 2016]] ; [[#Brunner--2018|Brunner et al., 2018]] ; [[#DeGaetano--2018|DeGaetano, 2018]] ; [[#Unterberger--2018|Unterberger et al., 2018]] ; [[#Wolfe--2018|Wolfe et al., 2018]] ). Shifts in the seasonality of frozen soils also affect groundwater recharge and surface streamflow for water resource applications, particularly when peak precipitation is shifted to a season that no longer has frozen soils ( [[#Jyrkama--2007|Jyrkama and Sykes, 2007]] ).

Regional information about the spring and autumn seasonal periods in which freeze-thaw cycles are common (such as the dates of first spring thaw and last spring frost, or the number of days where T <sub>max</sub> &gt;0°C and T <sub>min</sub> &lt;0°C) are particularly useful in estimating the rate of potential road and building damage or determining seasonal truck weight restrictions ( [[#Kvande--2009|Kvande and Lisø, 2009]] ; [[#Chinowsky--2012|Chinowsky and Arndt, 2012]] ; [[#Palko--2017|Palko, 2017]] ; [[#Daniel--2018|Daniel et al., 2018]] ). The altitude of the freezing level also identifies portions of mountain slopes where freeze/thaw transitions or changes in snowpack condition can influence landslide and snow avalanche hazards ( [[#Coe--2018|Coe et al., 2018]] ). The geographical distribution of frost is also a determining factor in the range of vectors for human diseases such as malaria (X. [[#Zhao--2016|]] [[#Zhao--2016|Zhao et al., 2016]] ; [[#Smith--2020|Smith et al., 2020]] ).

Figure 12.3 illustrates how successive heat and cold hazards can potentially affect important natural and human systems, with climatic pressures reaching new sectoral assets or becoming increasingly severe as conditions become more extreme. While the precise value of any CID threshold may depend strongly on local environmental and system characteristics, there are common patterns and interdependencies in the types of thresholds encountered. Changes in the regional profile of CIDs can thus substantially alter threshold exceedance likelihoods.

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[[File:1fbbe02ba84f8df0d3124b2557ff7990 IPCC_AR6_WGI_Figure_12_3.png]]

'''Figure 12.3''' '''|''' '''Conceptual illustration of representative climatic impact-driver thresholds showing how graduating thresholds affect successive sectoral assets and lead to potentially more acute hazards as conditions become more extreme (exact values are not shown as these must be tailored to reflect diverse vulnerabilities of regional assets).''' Representative threshold definitions (T = instantaneous temperature; T '''''' = mean temperature): '''Cities and Infrastructures:''' T <sub>trans</sub> = temperature at which energy transmission lines efficiency reduced; T <sub>aircraft</sub> = temperature at which aircraft become weight-restricted for takeoff; T <sub>hotroads</sub> = temperature above which roads begin to warp; T <sub>stream</sub> = temperature at which streams are not capable of adequately cooling thermal plants; CDD <sub>min</sub> = minimum temperature for calculating cooling degree days; HDD <sub>max</sub> = maximum temperature for calculating heating degree days; T <sub>ice</sub> = temperature at which ice threatens transportation; T '''''' <sub>permafrost</sub> = mean seasonal temperature above which permafrost thaws at critical depths; T <sub>coldroads</sub> = temperature below which road asphalt performance suffers. '''Health:''' T <sub>deadly</sub> = temperature above which prolonged exposure may be deadly (often combined with humidity for heat indices); T <sub>severe</sub> = temperature above which prolonged exposure may cause elevated morbidity; T '''''' <sub>blooms</sub> = mean temperature for harmful algal or cyanobacteria blooms; T <sub>danger</sub> = level of dangerous cold temperatures (often combined with wind for chill indices); T <sub>overwinter</sub> = temperature below which disease vector species cannot survive winter. '''Ecosystems''' (CID indices for air and ocean temperature): T <sub>hotlim</sub> and T <sub>coldlim</sub> = limiting hot and cold temperatures for a given species range; T <sub>frost</sub> = frost threshold; T '''''' <sub>max</sub> and T '''''' <sub>min</sub> = maximum and minimum suitable annual mean temperatures for a given species; T <sub>crit</sub> = critical temperature above which a given species is stressed. '''Agriculture:''' T <sub>hotlim</sub> = temperature above which a crop or livestock species dies; T <sub>hotpest</sub> = maximum (or ‘lethal’) temperature above which an agricultural pest/disease/weed cannot survive; T <sub>crit</sub> = temperature at which productivity for a given crop is depressed; T '''''' <sub>opt</sub> = optimal mean temperature for a given plant’s productivity; GDD <sub>min</sub> = threshold temperature for growing degree days determining plant development; T <sub>chill</sub> = temperature below which chilling units are accumulated; T <sub>frost</sub> = temperature below which frost occurs; T <sub>hfrost</sub> = temperature below which a hard frost threatens crops or livestock; T <sub>coldpest</sub> = minimum winter temperature below which a given agricultural pest cannot survive; T <sub>coldlim</sub> = minimum temperature below which a given crop cannot survive.

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