Editing IPCC:AR6/WGI/Chapter-8 (section)

==== 8.1.2.2 Key Findings of AR6 Special Reports ====

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The SR1.5 assessed the impacts of global warming of 1.5°C above pre-industrial levels. The dominant human influence on observed global warming and related water cycle changes was confirmed. Further evidence that anthropogenic global warming has caused an increase in the frequency, intensity and/or amount of heavy precipitation events at the global scale ( ''medium confidence'' ), as well as in drought occurrence in the Mediterranean region ( ''medium confidence'' ) was also reported. [[IPCC:Wg1:Chapter:Chapter-3|Chapter 3]] of SR1.5 ( [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ) highlights that each half degree of additional global warming influences the climate response. Heavy precipitation shows a global tendency to increase more at 2°C compared to 1.5°C, though there is ''low confidence'' in projected regional differences in heavy precipitation at 1.5°C compared to 2°C global warming, except at high latitudes or at high altitude where there is ''medium confidence'' . A key finding is that ‘limiting global warming to 1.5°C compared to 2°C would approximately halve the proportion of the world population expected to suffer water scarcity, although there is considerable variability between regions ( ''medium confidence'' )’ (SR1.5). This is consistent with greater adverse impacts found at 2°C compared to 1.5°C for a number of dryness or drought indices (Schleussner et al., 2016; [[#Lehner--2017|Lehner et al., 2017]] ; [[#Greve--2018|Greve et al., 2018]] ). There is also ''medium confidence'' that land areas with increased runoff and exposure to flood hazards will increase more at 2°C compared to 1.5°C of global warming.

The Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) provides a comprehensive assessment of recent and projected changes, specifically in snow and ice-covered areas that form a key component of the water cycle in high-elevation and high-latitude areas. High mountain regions have experienced significant warming since the early 20th century, resulting in reduced snowpack on average (Marty et al., 2017), with glaciers retreating globally since the mid-20th century (Marzeion et al., 2018; [[#Zemp--2019|Zemp et al., 2019]] ). Glacier shrinkage and snow cover changes have led to changes (both increases and decreases) in streamflow in many mountain regions in recent decades (Milner et al., 2017). Permafrost regions have undergone degradation and ground-ice loss due to recent warming (Lu et al., 2017). Glacier mass loss is projected to continue through the 21st century under all scenarios. In high mountain areas, low-elevation snow cover is also projected to decrease, regardless of emissions scenario. Widespread permafrost thaw is projected to continue through this century and beyond. River runoff in snow- or glacier-fed basins is projected to increase in winter and to decrease in summer (and in the annual mean) by 2100. In the oceans, the Atlantic Meridional Overturning Circulation (AMOC) will ''very likely'' weaken over the 21st century under all emissions scenarios (SROCC), with potential effects on atmospheric circulation and the water cycle at the regional scale (see also [[#8.6|Section 8.6]] ).

The Special Report on climate change, desertification, land degradation, sustainable management, food security, and greenhouse gas (GHG) fluxes in terrestrial ecosystems (SRCCL) has clear connections with the water cycle. This Report indicates that since 1850–1900, land surface temperature has risen nearly twice as much as global surface temperature ( ''high confidence'' ), with an increase in dry climates ( ''high confidence'' ). Land surface processes modulate the likelihood, intensity and duration of many extreme events including droughts ( ''medium confidence'' ) and heavy precipitation ( ''medium confidence'' ). The direction and magnitude of hydrological changes induced by land use change and land surface feedbacks vary with location and season ( ''high confidence'' ). Desertification exacerbates climate change through feedbacks involving vegetation cover, greenhouse gases and mineral dust aerosol ( ''high confidence'' ). Urbanization increases extreme rainfall events over or downwind of cities ( ''medium confidence'' ). Intensification of rainy events increase their consequences on land degradation.

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