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

==== 12.4.6.3 Wind ====

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'''Mean wind speed:''' Mean wind speeds have declined in North America – as in other Northern Hemisphere areas – over the past four decades ( ''medium confidence'' ) (AR5 WGI) with a reversal in the last decade ( ''low confidence'' ) not fully consistent across studies ( [[#Tian--2019|Tian et al., 2019]] ; [[#Zeng--2019|Zeng et al., 2019]] ; Z. [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|]] [[#Zhang--2019|Zhang et al., 2019]] ). [[#Tian--2019|Tian et al. (2019)]] found a corresponding reduction in the wind power potential across the eastern parts of North America.

Mean wind speeds are expected to decline over much of North America (Figure 12.4m–o), but the only broad signal of consistent change across model types is a reduction in wind speed in Western North America ( ''high confidence'' ). These declines reduce wind power endowment by 2050 and as early as the 2020–2040 near-term period in the USA Mountain West, while there is disagreement between global- and regional-model change projections in the upper and lower Great Plains, Ohio River Valley, Mexico and eastern Canada ( [[#Karnauskas--2018a|Karnauskas et al., 2018a]] ; [[#Jung--2019|Jung and Schindler, 2019]] ; [[#Chen--2020|Chen, 2020]] ).

'''Severe wind storm:''' There is ''limited evidence'' and ''low agreement'' in observed changes in North American CID indices associated with extratropical cyclones ( [[IPCC:Wg1:Chapter:Chapter-11#11.7|Section 11.7]] ), severe thunderstorms, severe wind bursts ( ''derechos'' ), tornadoes, or lightning strikes ( [[#Vose--2014|Vose et al., 2014]] ; [[#Easterling--2017|Easterling et al., 2017]] ; [[#Kossin--2017|Kossin et al., 2017]] ). Observational studies have indicated a reduction in the number of tornado days in the USA, but increases in outbreaks with 30 or more tornadoes in one day ( [[#Brooks--2014|Brooks et al., 2014]] ), the density of tornado clusters ( [[#Elsner--2015|Elsner et al., 2015]] ), and overall tornado power ( [[#Elsner--2019|Elsner et al., 2019]] ).

There is ''medium confidence'' of a general decrease in the number of extratropical cyclones producing high wind speeds in North America, except over northernmost parts, for a global warming level of 2°C or by the end of the century under RCP4.5 and RCP8.5 ( [[#Kumar--2015|Kumar et al., 2015]] ; [[#Jeong--2018a|Jeong and Sushama, 2018a]] ; [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|C. Li et al., 2018]] ). GCMs cannot directly resolve tornadoes and severe thunderstorms, however projections of favourable environments for severe storms (based on convective available potential energy and wind shear) indicate ''medium confidence'' for more severe storms and a longer convective storm season in the USA, weaker increases extending north and east ( [[#Seeley--2015|Seeley and Romps, 2015]] ; [[#Glazer--2021|Glazer et al., 2021]] ), and a corresponding increase in autumn and winter tornadic storms (H.E. [[#Brooks--2013|]] [[#Brooks--2013|Brooks, 2013]] ; [[#Diffenbaugh--2013|Diffenbaugh et al., 2013]] ; [[#Brooks--2014|Brooks et al., 2014]] ; see also [[IPCC:Wg1:Chapter:Chapter-11#11.7.2|Section 11.7.2]] ). [[#Prein--2017a|Prein et al. (2017a)]] used a convection-permitting model to project a tripling of mesoscale convective systems over the USA for end-of-century RCP8.5.

'''Tropical cyclone''' : [[IPCC:Wg1:Chapter:Chapter-11#11.7.1|Section 11.7.1]] identified recent reductions in tropical cyclone translation speed and higher tropical cyclone rainfall totals over the North Atlantic, as well as substantial natural variability. Projections indicate ''low confidence'' in change in North Atlantic tropical cyclone numbers, but ''medium confidence'' in Mexico and the US Gulf and Atlantic coasts for more intense storms with higher wind, precipitation, and storm surge totals when they do occur ( [[IPCC:Wg1:Chapter:Chapter-11#11.7.1|Section 11.7.1]] ; [[#Diro--2014|Diro et al., 2014]] ; [[#DOE--2015|DOE, 2015]] ; [[#Walsh--2016a|Walsh et al., 2016a]] ; [[#Kossin--2017|Kossin et al., 2017]] ; [[#Marsooli--2019|Marsooli et al., 2019]] ; [[#Ting--2019|Ting et al., 2019]] ; [[#Knutson--2020|Knutson et al., 2020]] ). A more rapid intensification of tropical cyclone winds and destructive power also heightens the tropical cyclone hazard ( [[#Bhatia--2019|Bhatia et al., 2019]] ). Greenhouse gas forcing is projected to shift tropical cyclones poleward ( [[#Kossin--2016|Kossin et al., 2016]] ), while also holding the potential for higher precipitation totals ( [[#Risser--2017|Risser and Wehner, 2017]] ; [[#Knutson--2020|Knutson et al., 2020]] ) particularly given evidence that storms increasingly stall near North American coastlines ( [[#Hall--2019|Hall and Kossin, 2019]] ).

'''Sand and dust storm:''' Land-use change has increased dust emissions in the western USA in the past 200 years ( [[#Neff--2008|Neff et al., 2008]] ). However, there is ''medium confidence'' for observed increases in Western North American sand and dust storm activity since 1980. In their study of Valley Fever spread, [[#Tong--2017|Tong et al. (2017)]] identified a rapid intensification of dust storm activity using PM <sub>10</sub> and PM <sub>2.5</sub> observations from 1980–2011 across 29 monitoring sites in the south-western USA, similar to contiguous USA observations by [[#Brahney--2013|Brahney et al. (2013)]] . [[#Hand--2016|Hand et al. (2016)]] attributed the earlier onset of spring dusts in the south-west in large part to the Pacific Decadal Oscillation, however. The increasing trend in dust since the 1990s in the south-western USA can be explained by precipitation deficit and surface bareness ( [[#Pu--2018|Pu and Ginoux, 2018]] ). Projections of future sand and dust storms over North America are based on aridity as a primary proxy for conducive conditions which lends ''medium confidence'' of an increase over Mexico and the south-western USA. [[#Pu--2017|Pu and Ginoux (2017)]] project about five more dusty days in spring and summer in the southern Great Plains under RCP8.5 at the end of the century, while dusty days decrease in northern regions where mean precipitation tends towards wetter conditions.

'''Tropical cyclones, severe wind and dust storms''' '''in North America are shifting towards more extreme characteristics, with a stronger signal towards heightened intensity than increased frequency, although specific regional patterns are more uncertain''' ( medium confidence '''). Mean wind speed and wind power potential are projected to decrease in Western North America''' ( medium confidence ''') with differences between global and regional models lending''' low confidence '''elsewhere.'''

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