Editing IPCC:AR6/WGI/Chapter-11 (section)

=== 11.5.5 Future Projections ===

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The SREX (Chapter 3, [[#Seneviratne--2012|Seneviratne et al., 2012]] ) stressed the low availability of studies on flood projections under different emissions scenarios, and concluded that there was ''low confidence'' in projections of flood events given the complexity of the mechanisms driving floods at the regional scale. The AR5 WGII report (Chapter 3, [[#Jimenez%20Cisneros--2014|Jimenez Cisneros et al., 2014]] ) assessed with ''medium confidence'' the pattern of future flood changes, including flood hazards increasing over about half of the globe (parts of southern and South East Asia, tropical Africa, north-east Eurasia, and South America) and flood hazards decreasing in other parts of the world, despite uncertainties in GCMs and their coupling to hydrological models. The SR1.5 (Chapter 3, [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ) assessed with ''medium confidence'' that global warming of 2°C would lead to an expansion of the fraction of global area affected by flood hazards, compared to conditions at 1.5°C of global warming, as a consequence of changes in heavy precipitation.

The majority of new studies that produce future flood projections based on hydrological models do not typically consider aspects that are also important to actual flood severity or damages, such as flood prevention measures ( [[#Neumann--2015|Neumann et al., 2015]] ; [[#Şen--2018|Şen, 2018]] ), flood control policies ( [[#Barraqué--2017|Barraqué, 2017]] ), and future changes in land cover (see also [[IPCC:Wg1:Chapter:Chapter-8#8.4.1.5|Section 8.4.1.5]] ). At the global scale, [[#Alfieri--2017|Alfieri et al. (2017)]] used downscaled projections from seven GCMs as input to drive a hydrodynamic model. They found successive increases in the frequency of high floods in all continents except Europe, associated with increasing levels of global warming (1.5°C, 2°C, 4°C). These results are supported by [[#Paltan--2018|Paltan et al. (2018)]] , who applied a simplified runoff aggregation model forced by outputs from four GCMs. S. [[#Huang--2018|]] [[#Huang--2018|Huang et al. (2018)]] used three hydrological models forced with bias-adjusted outputs from four GCMs to produce projections for four river basins including the Rhine, Upper Mississippi, Upper Yellow, and Upper Niger under 1.5°C, 2°C, and 3°C global warming. This study found diverse projections for different basins, including a shift towards earlier flooding for the Rhine and the Upper Mississippi, a substantial increase in flood frequency in the Rhine only under the 1.5°C and 2°C scenarios, and a decrease in flood frequency in the Upper Mississippi under all scenarios.

At the continental and regional scales, the projected changes in floods are uneven in different parts of the world, but there is a larger fraction of regions with an increase than with a decrease over the 21st century ( [[#Hirabayashi--2013|Hirabayashi et al., 2013]] ; [[#Dankers--2014|Dankers et al., 2014]] ; [[#Arnell--2016|Arnell and Gosling, 2016]] ; [[#Döll--2018|Döll et al., 2018]] ). These results suggest ''medium confidence'' in flood trends at the global scale, but ''low confidence'' in projected regional changes. Increases in flood frequency or magnitude are identified for south-eastern and northern Asia and India ( ''high agreement'' across studies), eastern and tropical Africa, and the high latitudes of North America ( ''medium agreement'' ), while decreasing frequency or magnitude is found for central and eastern Europe and the Mediterranean ( ''high confidence'' ), and parts of South America, southern and central North America, and south-west Africa ( ''low confidence'' ) ( [[#Hirabayashi--2013|Hirabayashi et al., 2013]] ; [[#Dankers--2014|Dankers et al., 2014]] ; [[#Arnell--2016|Arnell and Gosling, 2016]] ; [[#Döll--2018|Döll et al., 2018]] ). Over South America, most studies based on global and regional hydrological models show an increase in the magnitude and frequency of high flows in the western Amazon ( [[#Guimberteau--2013|Guimberteau et al., 2013]] ; [[#Langerwisch--2013|Langerwisch et al., 2013]] ; [[#Sorribas--2016|Sorribas et al., 2016]] ; [[#Zulkafli--2016|Zulkafli et al., 2016]] ) and the Andes ( [[#Hirabayashi--2013|Hirabayashi et al., 2013]] ; [[#Bozkurt--2018|Bozkurt et al., 2018]] ). [[IPCC:Wg1:Chapter:Chapter-12#12.4|Section 12.4]] provides a detailed assessment of regional flood projections.

In summary, global hydrological models project a larger fraction of land areas to be affected by an increase in river floods than by a decrease in river floods ( ''medium confidence'' ). There is ''medium confidence'' that river floods will increase in the western Amazon, the Andes, and south-eastern and northern Asia. Regional changes in river floods are more uncertain than changes in pluvial floods because complex hydrological processes and forcings are involved, including land cover change and human water management.

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