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

=== Physical Basis for Water Cycle Changes ===

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'''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 greenhouse gases 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 ( ''high confidence'' ). Land-use and land-cover changes also drive regional water cycle changes through their influence on surface water and energy budgets ( ''high confidence'' ). {8.2.1, 8.2.3.4, 8.2.2.2, Box 8.1}

'''A warmer climate increases moisture transport into weather systems, which, on average, makes wet seasons and events wetter''' ( ''high confidence'' ''')''' . 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 ( ''high confidence'' ). The severity of very wet and very dry events increases in a warming climate ( ''high confidence'' ), but changes in atmospheric circulation patterns alter where and how often these extremes occur, with substantial regional differences and seasonal contrasts. A slowdown of tropical circulation with global warming partly offsets the warming-induced strengthening of precipitation in monsoon regions ( ''high confidence'' ). {8.2.2, 8.2.3, 8.3.1.7, 8.4.1, 8.5.1}

'''Warming over land drives an increase in atmospheric evaporative demand and the severity of droughts''' ( ''high confidence'' ''').''' 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 ( ''high confidence'' ). Increasing atmospheric CO <sub>2</sub> concentrations increase plant growth and water-use efficiency, but there is ''low confidence'' in how these factors drive regional water cycle changes. {8.2.2, 8.2.3}

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