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

==== 8.4.1.5 Runoff, Streamflow and Flooding ====

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The AR5 assessed that projected changes in runoff had ''low confidence'' over the period 2016–2035; however, under the RCP8.5 scenario, runoff increases by 2100 are ''likely'' in high northern latitudes. This is consistent with projected regional precipitation increases, based on consistency of changes across different generations of models and different forcing scenarios, and with runoff decreases being ''likely'' in southern Europe, the Middle East and southern Africa. There was considerable uncertainty in the magnitude and direction of change for some regions, largely driven by the uncertainty in projected precipitation changes, particularly across south Asia. For flooding, AR5 assessed with ''medium confidence'' that flooding would increase over parts of South and South East Asia, tropical Africa, north-east Eurasia, and South America, and decrease for parts of Northern and Eastern Europe, Anatolia, Central Asia, Central North America, and southern South America. The SR1.5 assessed with ''medium confidence'' that warming of 2°C would increase the fraction of global area affected by flood hazard relative to warming of 1.5°C. Projected climate-driven changes to runoff, streamflow, and flooding will occur in the context of potential human-caused land-use and land-cover changes, which can have a large influence on surface water ( [[#Sterling--2013|Sterling et al., 2013]] ) but which have considerable uncertainty in projections ( [[#Prestele--2016|Prestele et al., 2016]] ).

Since AR5, studies confirm that global mean annual runoff increases with global surface temperature increase (X. Zhang et al. , 2014, 2018; Lehner et al. , 2019) , but varies regionally (Chen et al. , 2017; H. Yang et al. , 2017; Cook et al. , 2020) . CMIP5 models display a large spread in the ratio of runoff to precipitation for the present-day climate, which applies also to future runoff changes under global warming ( [[#Lehner--2019|Lehner et al., 2019]] ). In studies of CMIP6 projections, runoff increases in most parts of the northern high latitudes and Asia and north and eastern Africa, and decreases in the Mediterranean region, southern Africa, southern Australia and in parts of western Africa, as well as in Central and South America ( [[#Greve--2018|Greve et al., 2018]] ; [[#Cook--2020|Cook et al., 2020]] ). Projected changes in runoff also vary seasonally. In the Northern Hemisphere (NH), runoff increases during winter since more precipitation falls as rain than snow and decreases in the summer as less snow is available to contribute to runoff during the warm season ( [[#Cook--2020|Cook et al., 2020]] ). Global maps of projected changes for December–January–February and June–July–August are shown in Figure 8.18, showing projected changes becoming larger and more consistent in the higher emissions scenarios. Runoff projections for CMIP6 are also shown in Figure 8.16 for tropical and extratropical averages at a range of global mean warming levels and in Table 8.1 for global land in different future scenarios. In the tropics, both the mean and interannual variability of runoff increase with warming. The increase in variability is roughly twice as large as the increase in the mean, and has a large spread across models. In the extratropics, changes are small in the summer but there are large increases in the winter, with the mean increasing much more than the variability, in contrast to the tropics.

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[[File:9188b40f55d99aba09a8dbd80987021e IPCC_AR6_WGI_Figure_8_18.png]]

'''Figure 8.18 |''' '''Projected long-term relative changes in seasonal mean runoff.''' Global maps of projected relative change (%) in runoff seasonal mean for December–January–February (DJF; left panels) and June–July–August (JJA; right panels) averaged across available CMIP6 models (number provided at the top right of each panel) SSP1.2-6 '''(a, b)''' , SSP2-4.5 '''(c, d)''' and SSP5-8.5 '''(e, f)''' scenario respectively. All changes are estimated in 2081–2100 relative to 1995–2014. Uncertainty is represented using the simple approach. No overlay indicates regions with high model agreement, where ≥80% of models agree on sign of change, diagonal lines indicate regions with low model agreement, where &lt;80% of models agree on sign of change. For more information on the simple approach, please refer to the Cross-Chapter Box Atlas.1. Further details on data sources and processing are available in the chapter data table (Table 8.SM.1).

Changes in streamflow vary regionally and increase in magnitude with emissions scenarios, as with runoff (although the two are not equivalent, as runoff includes both surface runoff and streamflow). Streamflow projections additionally require the use of hydrologic models forced by the output from climate models and have not been as widely explored as they are not variables directly included in climate models. On an annual basis, streamflows have been projected to increase in the northern high latitudes and tropical Asia and Africa, and to decrease in the Mediterranean, tropical South America, and South Africa ( [[#Döll--2018|Döll et al., 2018]] ). For a 4°C global warming, half of the global land area is projected to be exposed to increased high flows (average increase 25%), while about 60% may be exposed to decreased low flows (average decrease 50%) ( [[#Asadieh--2017|Asadieh and Krakauer, 2017]] ).

Changes in the seasonality of runoff and streamflow are assessed in Box 8.2. The seasonality of runoff and streamflow (calculated as the annual difference between the wettest and driest months of the year), is expected to increase with global warming in the subtropics, especially in the Mediterranean and southern Africa with ''high confidence'' , and in the Amazon with ''medium confidence'' . For regions where snowmelt is an important contributor to streamflow, there is ''high confidence'' that snowmelt occurring earlier in the year will result in peak flows also occurring earlier in the year, and ''medium confidence'' that reduced snow volume and the weaker solar radiation earlier in the year will reduce the most intense flows (see [[#8.2.3.1|Section 8.2.3.1]] ). In roughly half of 56 large-scale glacierized drainage basins, projected runoff changes show an increase until a maximum is reached, beyond which runoff steadily declines because of limited ice volumes ( [[#Huss--2018|Huss and Hock, 2018]] ).

As future changes in flood events are assessed in Chapters 9, 11 and 12, only a summary is presented here. There are a number of complicating factors for projecting both pluvial (overland) and fluvial (river) flooding that limit confidence in their assessment. In addition to precipitation, flooding also depends on basin and river characteristics such as permeability, antecedent soil moisture, and antecedent flow levels for river flooding, so projections of extreme precipitation and flooding are not always closely linked ( [[#8.2.3.2|Section 8.2.3.2]] ). Possible changes in water resources management and land use add another layer of complexity to future changes. There is ''medium confidence'' in a general increase in pluvial and fluvial flooding, although there are large regional variations, discussed further in Sections 11.5.5, and 12.4. There is ''medium confidence'' in a substantial increase in the frequency of extreme sea level events for coastal regions ( [[IPCC:Wg1:Chapter:Chapter-9#9.6.4.2|Section 9.6.4.2]] ) and the associated coastal flooding is regionally assessed in [[IPCC:Wg1:Chapter:Chapter-12#12.4|Section 12.4]] . The risk of glacier lake outburst floods (GLOFs) is expected to increase with glacier melting in some high mountain regions ( [[IPCC:Wg1:Chapter:Chapter-12#12.4|Section 12.4]] ).

In summary, there is ''medium confidence'' that global runoff will increase with global warming, but with large regional and seasonal variations. There is ''high confidence'' that runoff will increase in the northern high latitudes and decrease in the Mediterranean region and southern Africa. There is ''medium confidence'' that runoff will increase in regions of central and eastern Africa, and decrease in Central America and parts of southern South America, with the magnitude of the change increasing with emissions. There is ''medium confidence'' that the seasonality of runoff and streamflow will increase with global warming in the subtropics. In snow-dominated regions, there is ''high confidence'' that peak flows associated with spring snowmelt will occur earlier in the year and ''medium confidence'' that snowmelt-induced runoff will decrease with reduced snow, except in glacier-fed basins where runoff may increase in the near term. There is ''medium confidence'' that flooding in general will increase, although with considerable variation based on geographic region and flood type. These projected climate-related changes will occur in the context of human-caused land-use and land-cover changes, which may also have a large influence.

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