Editing IPCC:AR6/WGI/Chapter-11 (section)

==== 11.6.5.4 Hydrological Deficits ====

<div id="h3-27-siblings" class="h3-siblings"></div>

Some studies support wetting tendencies as a response to a warmer climate when considering globally averaged changes in runoff over land ( [[#Roderick--2015|Roderick et al., 2015]] ; [[#Greve--2017|Greve et al., 2017]] ; Y. [[#Yang--2018|]] [[#Yang--2018|]] [[#Yang--2018|]] [[#Yang--2018|]] [[#Yang--2018|Yang et al., 2018]] ), and streamflow projections respond to enhanced CO <sub>2</sub> concentrations in CMIP5 models ( [[#Yang--2019|Yang et al., 2019]] ). Nevertheless, when focusing regionally on low-runoff periods, model projections also show an increase of hydrological droughts in large world regions ( [[#Wanders--2015|Wanders and Van Lanen, 2015]] ; [[#Dai--2018|Dai et al., 2018]] ; [[#Vicente-Serrano--2020c|Vicente-Serrano et al., 2020c]] ). In general, the frequency of hydrological deficits is projected to increase over most of the continents, although with regionally and seasonally differentiated effects ( [[#11.9|Section 11.9]] ), with ''medium confidence'' of increase in the following AR6 regions: WCE, MED, SAU, WCA, WNA, SCA, NSA, SAM, SWS, SSA, WSAF, ESAF and MDG ( [[#11.9|Section 11.9]] ; [[#Forzieri--2014|Forzieri et al., 2014]] ; [[#Prudhomme--2014|Prudhomme et al., 2014]] ; [[#Giuntoli--2015|Giuntoli et al., 2015]] ; [[#Wanders--2015|Wanders and Van Lanen, 2015]] ; [[#Roudier--2016|Roudier et al., 2016]] ; [[#Marx--2018|Marx et al., 2018]] ; [[#Cook--2019|Cook et al., 2019]] ; [[#Zhao--2020|Zhao et al., 2020]] ). However, there are large uncertainties related to the hydrological/impact model used ( [[#Prudhomme--2014|Prudhomme et al., 2014]] ; [[#Schewe--2014|Schewe et al., 2014]] ; [[#Gosling--2017|Gosling et al., 2017]] ), limited signal-to-noise ratio (due to model spread) in several regions ( [[#Giuntoli--2015|Giuntoli et al., 2015]] ), and also uncertainties in the projection of future human activities, including water demand and land cover changes, which may represent more than 50% of the projected changes in hydrological droughts in some regions ( [[#Wanders--2015|Wanders and Wada, 2015]] ).

Regions dependent on mountainous snowpack as a temporary reservoir may be affected by severe hydrological droughts in a warmer world. In the southern European Alps, both winter and summer low flows are projected to be more severe, with a 25% decrease in the 2050s ( [[#Vidal--2016|Vidal et al., 2016]] ). In western USA, a 22% reduction in winter snow water equivalent is projected at around 2°C of global warming, with a further decrease of a 70% reduction at 4°C global warming ( [[#Rhoades--2018|Rhoades et al., 2018]] ). This decline would cause less predictable hydrological droughts in snowmelt-dominated areas of North America ( [[#Livneh--2020|Livneh and Badger, 2020]] ). The exact magnitude of the influence of higher temperatures on snow-related droughts is, however, difficult to estimate ( [[#Mote--2016|Mote et al., 2016]] ), since the streamflow changes could affect the timing of peak streamflows but not necessarily their magnitude. In addition, projected changes in hydrological droughts downstream of declining glaciers can be very complex to assess (Chapter 9, see also SROCC).

<div id="11.6.5.5" class="h3-container"></div>

<span id="atmospheric-based-drought-indices-4"></span>