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

=== 8.1.2 Summary of Water Cycle Changes From AR5 and Special Reports ===

<div id="h2-6-siblings" class="h2-siblings"></div>

This Report is the first IPCC assessment to include a chapter specifically dedicated to providing an integrated assessment of the global water cycle changes, by building on many chapters from previous reports. This section summarizes observed and projected water cycle changes reported in AR5( [[#IPCC--2013|IPCC, 2013]] ) and in the recent IPCC Special Reports on Global Warming of 1.5°C (SR1.5), the Ocean and Cryosphere in a Changing Climate (SROCC), and Climate Change and Land (SRCCL).

<div id="8.1.2.1" class="h3-container"></div>

<span id="summary-of-observed-and-projected-water-cycle-changes-from-ar5"></span>
==== 8.1.2.1 Summary of Observed and Projected Water Cycle Changes from AR5 ====

<div id="h3-3-siblings" class="h3-siblings"></div>

Based on long-term observational evidence ( [[#Hartmann--2013|Hartmann et al., 2013]] ), AR5 concluded it was ''likely'' that anthropogenic influence has affected the water cycle since the 1960s ( [[#IPCC--2018|IPCC, 2018]] ). Detectable human influence on changes to the water cycle were found in atmospheric moisture content ( ''medium confidence'' ), global-scale changes of precipitation over land ( ''medium confidence'' ), intensification of heavy precipitation events over land regions where sufficient data networks exist ( ''medium confidence'' ), and ''very likely'' changes to ocean salinity through its connection with evaporation minus precipitation change patterns (Sections 2.5, 2.6, 3.3, 7.6, 10.3 and 10.4; [[#Stocker--2013|Stocker et al., 2013]] ). The AR5 also reported that it is ''very likely'' that global surface air specific humidity increased since the 1970s. There was ''low confidence'' in the observations of global-scale cloud variability and trends, ''medium confidence'' in reductions of pan-evaporation, and ''medium confidence'' in the non-monotonic changes of global evapotranspiration since the 1980s. In terms of streamflow and runoff, AR5 identified that there is ''low confidence'' in the observed increasing trends of global river discharge during the 20th century. Similarly, AR5 concluded that there is ''low confidence'' in any global-scale observed trend in drought or dryness (lack of rainfall) since the mid-20th century. Yet, the frequency and intensity of drought ''likely'' increased in the Mediterranean and West Africa, while they ''likely'' decreased in central North America and north-western Australia since 1950.

Water cycle projections in AR5 ( [[#Collins--2013|Collins et al., 2013]] ) were considered primarily in terms of water vapour, precipitation, surface evaporation, runoff, and snowpack. Globally-averaged precipitation was projected to increase with global warming with ''virtual certainty'' ( [[IPCC:Wg1:Chapter:Chapter-12|Chapter 12]] Executive Summary and [[IPCC:Wg1:Chapter:Chapter-12#12.4.1.1|Section 12.4.1.1]] ). Regionally, precipitation in some areas of the tropics and polar regions could increase by more than 50% by the end of the 21st century under the RCP8.5 emissions scenario, while precipitation in large areas of the subtropics could decrease by 30% or more (AR5 FAQ 12.2, Figure 12.22). Overall, the contrast of annual mean precipitation between dry and wet regions and between dry and wet seasons (‘wet get wetter, dry get drier’) was projected to increase over most of the globe with ''high confidence'' ( [[IPCC:Wg1:Chapter:Chapter-12|Chapter 12]] Executive Summary and [[IPCC:Wg1:Chapter:Chapter-12#12.4.5.2|Section 12.4.5.2]] ). Globally, the frequency of intense precipitation events was projected to increase while the frequency of all precipitation events was projected to decrease, leading to the contradictory-seeming projection of a simultaneous increase in both droughts and floods (12.2 and [[IPCC:Wg1:Chapter:Chapter-12#12.4.5.5|Section 12.4.5.5]] in AR5 WGI). Surface evaporation change was projected to be positive over most of the ocean and to generally follow the pattern of precipitation change over land [[IPCC:Wg1:Chapter:Chapter-12|Chapter 12]] Executive Summary, and [[IPCC:Wg1:Chapter:Chapter-12#12.4.5.4|Section 12.4.5.4]] ). Near-surface relative humidity reductions over many land areas were projected to be ''likely'' , with ''medium confidence'' ( [[IPCC:Wg1:Chapter:Chapter-12#12.4.5.1|Section 12.4.5.1]] ). General decreases in soil moisture in present-day dry regions were considered ''likely ,'' and projected with ''medium confidence'' under the RCP8.5 scenario ( [[IPCC:Wg1:Chapter:Chapter-12#12.4.5.3|Section 12.4.5.3]] ). Soil moisture drying in the Mediterranean, south-west USA and southern African regions was considered ''likely'' , with ''high confidence'' by the end of this century under the RCP8.5 scenario ( [[IPCC:Wg1:Chapter:Chapter-12#12.4.5.3|Section 12.4.5.3]] ). Projections for annual runoff included both decreases and increases. Decreases in Northern Hemisphere snow cover were assessed as ''very likely'' with continued global warming ( [[IPCC:Wg1:Chapter:Chapter-12#12.4.6.2|Section 12.4.6.2]] ). As temperatures increase, snow accumulation was projected to begin later in the year and melting to start earlier, with related changes in snowmelt-driven river flows (FAQ 12.2 and [[IPCC:Wg1:Chapter:Chapter-12#12.4.6.2|Section 12.4.6.2]] in AR5 WGI). In terms of the potential for abrupt change in components of the water cycle, long-term droughts and monsoonal circulation were identified as potentially undergoing rapid changes, but the assessment was reported with ''low confidence'' (Sections 12.5.5.8.1 and 12.5.5.8.2, and Table 12.4).

<div id="8.1.2.2" class="h3-container"></div>

<span id="key-findings-of-ar6-special-reports"></span>
==== 8.1.2.2 Key Findings of AR6 Special Reports ====

<div id="h3-4-siblings" class="h3-siblings"></div>

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.

<div id="8.1.3" class="h2-container"></div>

<span id="chapter-motivations-framing-and-preview"></span>