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

=== Causes of Observed Changes ===

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'''Human-caused climate change has driven detectable changes in the global water cycle since the mid-20th century''' ( ''high confidence'' ''').''' Global warming has contributed to an overall increase in atmospheric moisture and precipitation intensity ( ''high confidence'' ), increased terrestrial evapotranspiration ( ''medium confidence'' ), influenced global patterns in aridity ( ''very likely'' ) , and enhanced contrasts in surface salinity and precipitation minus evaporation patterns over the oceans ( ''high confidence'' ). {3.4.2, 3.4.3, 3.5.2, 8.3.1, 9.2.2}

'''Greenhouse gas forcing has driven increased contrasts in precipitation amounts between wet and dry seasons and weather regimes over tropical land areas''' ( ''medium confidence'' ''') and a detectable precipitation increase in the northern high latitudes''' ( ''high confidence'' ''')''' . Greenhouse gas forcing has also contributed to drying in dry summer climates, including the Mediterranean, south-western Australia, south-western South America, South Africa, and western North America ( ''medium to high confidence'' ). 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 ( ''high confidence'' ). {Box 8.2, 8.2.2.1, 8.3.1, 3.3.2, 3.3.3, 3.5.2}

'''Anthropogenic aerosols have driven detectable large-scale water cycle changes since at least the mid-20th century''' ( ''high confidence'' ''').''' 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 greenhouse gases ( ''high confidence'' ) ''.'' 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 ( ''high confidence'' ), subsequent recovery from which has been linked with greenhouse gas warming ( ''medium confidence'' ). 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 greenhouse gases being counteracted by decreases due to cooling from anthropogenic aerosols ( ''high confidence'' ). {8.3.1.3, 8.3.2.4, Box 8.1}

'''Land-use change and water extraction for irrigation have influenced local and regional responses in the water cycle''' ( ''high confidence'' ''')''' . Large-scale deforestation has ''likely'' decreased evapotranspiration and precipitation and increased runoff over the deforested regions. Urbanization has increased local precipitation ( ''medium confidence'' ) and resulting runoff intensity ( ''high confidence'' ). Increased precipitation intensities have enhanced groundwater recharge, most notably in tropical regions ( ''medium confidence'' ). 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 southern High Plains and California Central Valley of the USA, North China Plain, and north-west India). {8.2.3.4, 8.3.1.7, Box 10.3, FAQ 8.1}

'''Southern Hemisphere storm tracks and associated precipitation have shifted polewards since the 1970s, especially in the austral summer and autumn''' ( ''high confidence'' ''')''' . 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 greenhouse gas increases. There is ''medium confidence'' that the recent observed expansion of the Hadley circulation was caused by greenhouse gas forcing, especially in the Southern Hemisphere, but there is only ''low confidence'' in how it influences the drying of subtropical land areas. {8.2.2, 8.3.2, 3.3.3}

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