Physical Basis for Water Cycle Changes
Modifications of Earth’s energy budget by anthropogenic radiative forcings drive substantial and widespread changes in the global water cycle. There is high confidence that global mean precipitation and evaporation increase with global warming, but the estimated rate is model-dependent (very likely range of 1–3% per 1°C). The global increase in precipitation is determined by a robust response to global mean surface air temperature (very likely 2–3% per 1°C) that is partly offset by fast atmospheric adjustments to atmospheric heating by GHGs and aerosols. The overall effect of anthropogenic aerosols is to reduce global precipitation and alter large-scale atmospheric circulation patterns through their well-understood surface radiative cooling effect. Land-use and land-cover changes also drive regional water cycle changes through their influence on surface water and energy budgets.
A warmer climate increases moisture transport into weather systems, which, on average, makes wet seasons and events wetter. An increase in near-surface atmospheric water holding capacity of about 7% per 1°C of warming explains a similar magnitude of intensification of heavy precipitation events (from sub-daily up to seasonal time scales) that increases the severity of flood hazards when these extremes occur. The severity of very wet and very dry events increases in a warming climate, but changes in atmospheric circulation patterns alter where and how often these extremes occur with substantial regional differences and seasonal contrasts. The slowdown of tropical circulation with global warming partly offsets the warming-induced strengthening of precipitation in monsoon regions.
Warming over land drives an increase in atmospheric evaporative demand and the severity of droughts. Greater warming over land than over the ocean alters atmospheric circulation patterns and, on average, reduces continental near-surface relative humidity, which contributes to regional drying. Increasing atmospheric CO2 concentrations increase plant growth and water-use efficiency, but there is low confidence in how these factors drive regional water cycle changes.
Causes of Observed Changes
Human-caused climate change has driven detectable changes in the global water cycle since the mid 20th century. Global warming has contributed to an overall increase in atmospheric moisture and precipitation intensity, increased terrestrial evapotranspiration, influenced global patterns in aridity (very likely), and enhanced contrasts in surface salinity and precipitation minus evaporation patterns over the oceans.
Greenhouse gas forcing has driven increased contrasts in precipitation amounts between wet and dry seasons and weather regimes over tropical land areas, with a detectable precipitation increase in the northern high latitudes. GHG forcing has also contributed to drying in dry summer climates, including the Mediterranean, southwestern Australia, southwestern South America, South Africa, and western North America. Earlier onset of spring snowmelt and increased melting of glaciers have already contributed to seasonal changes in streamflow in high-latitude and low-elevation mountain catchments.
Anthropogenic aerosols have driven detectable large-scale water cycle changes since at least the mid 20th century. Shifts in the tropical rain belt are associated with the inter-hemispheric temperature response to the time-evolving radiative influence of anthropogenic aerosols and the ongoing warming influence of GHGs. Cooling in the Northern Hemisphere by sulphate aerosols explained a southward shift in the tropical rain belt and contributed to the Sahel drought from the 1970s to the 1980s, subsequent recovery from which has been linked with GHG warming. Observed changes in regional monsoon precipitation, especially over South Asia, East Asia and West Africa, have been limited over much of the 20th century due to increases driven by warming from GHGs being counteracted by decreases due to cooling from anthropogenic aerosols.
Land-use change and water extraction for irrigation have influenced local and regional responses in the water cycle. Large-scale deforestation has likely decreased evapotranspiration and precipitation and increased runoff over the deforested regions. Urbanization has increased local precipitation and runoff intensity. Increased precipitation intensities have enhanced groundwater recharge, most notably in tropical regions. There is high confidence that groundwater depletion has occurred since at least the start of the 21st century as a consequence of groundwater withdrawals for irrigation in agricultural areas in drylands (e.g., the United States southern High Plains, California Central Valley, North China Plain, and northwest India).
Southern Hemisphere storm tracks and associated precipitation have shifted polewards since the 1970s, especially in the austral summer and autumn. It is very likely that these changes are associated with a positive trend in the Southern Annular Mode, related to both stratospheric ozone depletion and GHG increases. There is medium confidence that the recent observed expansion of the Hadley Circulation was caused by GHG forcing, especially in the Southern Hemisphere, but there is only low confidence in how it influences the drying of subtropical land areas.
Future Water Cycle Changes
Without large-scale reduction in greenhouse gas emissions, global warming is projected to cause substantial changes in the water cycle at both global and regional scales. Global annual precipitation over land is projected to increase on average by 2.4 [–0.2 to 4.7] % (very likely range) in the SSP1-1.9 low-emission scenario and by 8.3 [0.9 to 12.9] % in the SSP5-8.5 high-emission scenario by 2081–2100, relative to 1995–2014. It is virtually certain that evaporation will increase over the oceans and very likely that evapotranspiration will increase over land with regional exceptions in drying areas. There is low confidence in the sign and magnitude of projected changes in global land runoff in all Shared socioeconomic Pathway scenarios. Projected increases in precipitation amount and intensity will be associated with increased runoff in the northern high latitudes. There is high confidence that mountain glaciers will diminish in all regions and that seasonal snow cover duration will generally decrease. Runoff from small glaciers will typically decrease through loss of ice mass, while runoff from large glaciers is likely to increase with increasing global warming until glacier mass becomes depleted.
Increased evapotranspiration due to growing atmospheric water demand will decrease soil moisture over the Mediterranean, southwestern North America, south Africa, southwestern South America, and southwestern Australia. The total land area subject to increasing drought frequency and severity will expand, and in the Mediterranean, southwestern South America, and western North America, future aridification will far exceed the magnitude of change seen in the last millennium. Some tropical regions are also projected to experience increased aridity, including the Amazon basin and Central America.
Water cycle variability and extremes are projected to increase faster than average changes in most regions of the world and under all emission scenarios. In the tropics and in the extratropics of both hemispheres during summer/warm season, interannual variability of precipitation and runoff over land is projected to increase at a faster rate than changes in seasonal mean precipitation amount It is very likely that rainfall variability related to the El Niño–Southern Oscillation will be amplified by the end of the 21st century. Sub-seasonal precipitation variability is also projected to increase, with fewer rainy days but increased daily mean precipitation intensity over many land regions. Precipitation extremes will increase in almost all regions, even where seasonal mean precipitation is projected to decrease. There is high confidence that heavy precipitation events associated with both tropical and extratropical cyclones will intensify.
There are contrasting projections in monsoon precipitation, with increases in more regions than decreases. Summer monsoon precipitation is projected to increase for the South, Southeast and East Asian monsoon domains, while North American monsoon precipitation is projected to decrease. West African monsoon precipitation is projected to increase over the Central Sahel and decrease over the far western Sahel. There is low confidence in projected precipitation changes in the South American and Australian monsoons (for both magnitude and sign). There is high confidence that the monsoon season will be delayed in North and South America and medium confidence that it will be delayed in the Sahel.
Precipitation associated with extratropical storms and atmospheric rivers will increase in the future in most regions. A continued poleward shift of storm tracks in the Southern Hemisphere (likely) and the North Pacific will lead to similar shifts in annual or seasonal precipitation. There is low confidence in projections of blocking and stationary waves and therefore their influence on precipitation for almost all regions.
The seasonality of precipitation, water availability and streamflow will increase with global warming over the Amazon and in the subtropics, especially in the Mediterranean and southern Africa. The annual contrast between the wettest and driest month of the year is likely to increase by 3–5% per 1°C in most monsoon regions in terms of precipitation, precipitation minus evaporation, and runoff. There is high confidence in an earlier onset in spring snowmelt, with higher peak flows at the expense of summer flows in snow-dominated regions globally, but medium confidence that reduced snow volume in lower-latitude regions will reduce runoff from snowmelt.
Confidence in Projections, Non-Linear Responses and the Potential for Abrupt Changes
Representation of key physical processes has improved in global climate models (GCMs), but they are still limited in their ability to simulate all aspects of the present-day water cycle and to agree on future changes. Climate change studies benefit from sampling the full distribution of model outputs when considering future projections at regional scales. Increasing horizontal resolution in GCMs improves the representation of small-scale features and the statistics of daily precipitation. High-resolution climate and hydrological models provide a better representation of land surfaces, including topography, vegetation and land-use change, which improve the accuracy of simulations of regional changes in the water cycle. There is high confidence in the potential added value of regional climate models but only medium confidence that this potential is currently realized.
Natural climate variability will continue to be a major source of uncertainty in near-term (2021–2040) water cycle projections. Decadal predictions of water cycle changes should be considered with low confidence in most land areas because the internal variability of precipitation is difficult to predict and can offset or amplify the forced water cycle response. Water cycle changes that have already emerged from natural variability will become more pronounced in the near term, but the occurrence of volcanic eruptions (either single large events or clustered smaller ones) can alter the water cycle for several years, decreasing global mean land precipitation and altering monsoon circulation.
Continued global warming will further amplify GHG-induced changes in large-scale atmospheric circulation and precipitation patterns, but in some cases regional water cycle changes are not linearly related to global warming. Non-linear water cycle responses are explained by the interaction of multiple drivers, feedbacks and time scales. Nonlinear responses of regional runoff, groundwater recharge and water scarcity highlight the limitations of simple pattern-scaling techniques. Water resources fed by melting glaciers are particularly exposed to nonlinear responses.
Abrupt human-caused changes to the water cycle cannot be excluded. There is evidence of abrupt change in some high-emission scenarios, but there is no overall consistency regarding the magnitude and timing of such changes. Positive land-surface feedbacks, including vegetation and dust, can contribute to abrupt changes in aridity, but there is only low confidence that such changes will occur during the 21st 15 century. Continued Amazon deforestation, combined with a warming climate, raises the probability that this ecosystem will cross a tipping point into a dry state during the 21st century. The paleoclimate record shows that a collapse in the Atlantic Meridional Overturning Circulation (AMOC) causes abrupt shifts in the water cycle, such as a southward shift in theh tropical rain belt, weakening of the African and Asian monsoons and strengthening of Southern Hemisphere monsoons. There is medium confidence that AMOC will not collapse before 2100, but should it collapse, it is very likely that there would be abrupt changes in the water cycle.
Solar radiation modification (SRM) could drive abrupt changes in the water cycle. It is very likely that abrupt water cycle changes will occur if SRM techniques are implemented rapidly or terminated abruptly. The impact of SRM is spatially heterogeneous, will not fully mitigate the GHG-forced water cycle changes, and can affect different regions in potentially disruptive ways.