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

=== Confidence in Projections, Non-linear Responses and the Potential for Abrupt Changes ===

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'''Representation of key physical processes has improved in global climate models, but they are still limited in their ability to simulate all aspects of the present-day water cycle and to agree on future changes''' ( ''high confidence'' ''')''' . '''Climate change studies benefit from sampling the full distribution of model outputs when considering future projections at regional scales.''' Increasing horizontal resolution in global climate models improves the representation of small-scale features and the statistics of daily precipitation ( ''high confidence'' ). 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 ( ''high confidence'' ). There is ''high confidence'' in the potential added value of regional climate models but only ''medium confidence'' that this potential is currently realized. {8.5.1}

'''Natural climate variability will continue to be a major source of uncertainty in near-term (2021–2040) water cycle projections''' ( ''high confidence'' ''').''' 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 ( ''high confidence'' ). {8.5.2, Cross-Chapter Box 4.1}

'''Continued global warming will further amplify''' greenhouse gas '''-induced changes in large-scale atmospheric circulation and precipitation patterns''' ( ''high confidence'' '''), 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 ( ''high confidence'' ). Non-linear responses of regional runoff, groundwater recharge and water scarcity highlight the limitations of simple pattern-scaling techniques ( ''medium confidence'' ). Water resources fed by melting glaciers are particularly exposed to non-linear responses ( ''high confidence'' ). {8.5.3}

'''Abrupt human-caused changes to the water cycle cannot be excluded.''' There is evidence of abrupt change in some high-emissions 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 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 ( ''low confidence'' ). The paleoclimate records show that a collapse in the Atlantic Meridional Overturning Circulation (AMOC) causes abrupt shifts in the water cycle ( ''high confidence'' ), such as a southward shift in the tropical rain belt, weakening of the African and Asian monsoons, strengthening of Southern Hemisphere monsoons, and drying in Europe. 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. {8.6.1, 8.6.2}

'''Solar radiation modification could drive abrupt changes in the water cycle''' ( ''high confidence'' ''').''' It is ''very likely'' that abrupt water cycle changes will occur if solar radiation modification (SRM) techniques are implemented rapidly or terminated abruptly. The impact of SRM is spatially heterogeneous ( ''high confidence'' ), will not fully mitigate the greenhouse gas-forced water cycle changes ( ''medium confidence'' ), and can affect different regions in potentially disruptive ways ( ''low confidence'' ). {8.6.3}

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