Editing IPCC:AR6/WGII/Chapter-4 (section)

=== 4.4.3 Projected Changes in Streamflow ===

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AR5 ( [[#Jiménez%20Cisneros--2014|Jiménez Cisneros et al., 2014]] ) concluded that increases in the mean annual runoff are projected in high latitudes and the wet tropics and decreases in dry tropical regions, but with very considerable uncertainty. Both the patterns of change and uncertainties were found to be primarily driven by projected changes in precipitation. SR1.5 ( [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ) concluded with ''medium confidence'' that areas with either positive or negative changes in mean annual runoff/streamflow are projected to be smaller for 1.5°C than for 2°C of global warming. AR6 WGI ( [[#Douville--2021|Douville et al., 2021]] ) conclude with ''medium confidence'' that global runoff will increase with global warming but with significant regional and seasonal variations. WGI further concluded with ''high confidence'' that runoff will increase in the high northern latitudes and decrease in the Mediterranean and southern Africa. However, there was ''medium confidence'' that runoff will increase in central and eastern African regions and decrease in Central America and parts of southern South America. The magnitude of the change is projected to increase 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.

Changes in runoff and streamflow are projected over most of the ice-free land surface with all recent climate and hydrological model ensembles (Figure 4.16). Changes in streamflow could increase the number of people facing water scarcity or insecurity ( ''high confidence'' ) ( [[#Schewe--2014|Schewe et al., 2014]] ; [[#Gosling--2016|Gosling and Arnell, 2016]] ; [[#McMillan--2016|McMillan et al., 2016]] ). Projections of future runoff at basin scales show considerable uncertainty in many regions, including differences in signs in many regions (Figure 4.16). This uncertainty is driven by uncertainties in regional precipitation patterns and hydrological models ( [[#Koirala--2014|Koirala et al., 2014]] ; [[#Asadieh--2016|Asadieh et al., 2016]] ), including vegetation responses to CO 2 and its effects on ET ( [[#Betts--2015|Betts et al., 2015]] ). This uncertainty in future water availability contributes to the policy challenges for adaptation, for example, for managing risks of water scarcity ( [[#Greve--2018|Greve et al., 2018]] ; Box 4.1). In many regions, some models project large changes in runoff/streamflow but with low consistency between models on the sign of the change (Figure 4.16). In streamflow projections driven by 11 CMIP5 models with the RCP8.5 scenario, strong model consistency (agreement by at least 10 models) is only seen over 21% of global land ( [[#Koirala--2014|Koirala et al., 2014]] ). Consensus on the sign of projected change is smaller with the RCP4.5 scenario.

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'''Figure 4.16 |''' '''Projected changes in the annual mean runoff in selected river basins at global warming levels (GWLs) of 1''' '''.''' '''5°C, 2°C and 4°C in a combined ensemble.''' For each named basin, the sinaplot dots show individual model outcomes for percentage increased flows (blue) and decreased flows (red) at each GWL. Black circles show the ensemble median, and black bars show the 95% confidence range in the median. See inset with the Rio Grande sinaplot for additional guidance on interpretation. In the map, the colours in the basins show the percentage model agreement on the sign of the projected change in streamflow at the 4°C GWL. The combined ensemble is comprised of four multi-model ensembles: the CMIP5 multi-model ensemble of GCMs driven with RCP8.5; the CMIP6 multi-model ensemble of GCMs driven with SSP5-8.5; varying combinations of hydrological models with five GCMs in the Inter-Sectoral Impacts Model Intercomparison Project (ISIMIP); and the JULES land ecosystems and hydrology model driven by GCMs from the HELIX study ( [[#Betts--2018|Betts et al., 2018]] ; [[#Koutroulis--2019|Koutroulis et al., 2019]] ). In CMIP5 and CMIP6, the projected runoff changes are directly from the GCM land surface schemes without bias correction. In ISIMIP and HELIX, bias-corrected climate model outputs were used to drive the hydrology models. A comparison of the projected changes at the 4°C GWL for the four individual ensembles is shown in Figure Cross-Chapter Box CLIMATE.1 in Chapter 1.

Considering a wider set of projections, the consensus on increased flows becomes stronger at higher GWLs in (for example) the Yukon, Mackenzie, Kemijoki, Amur, Hwang Ho, Yangtze, Mekong, Ganges-Brahmaputra, Nile, Zaire and Parana basins (Figure 4.16). The consensus on decreased flows becomes stronger for higher GWLs in (for example) the Colorado, Tagus, Helmand, Tigris-Euphrates and Amazon. However, in both cases, some models have projected changes of the opposite sign to the consensus. Moreover, the distribution of projected outcomes becomes notably broader at higher GWLs in (for example) the Mississippi, Yangtze and Amazon. Therefore, even with a strong global climate change signal, uncertainties in changes in mean runoff/streamflow can remain large or even increase. Nevertheless, since projected changes typically increase with global warming, limiting warming to 1.5°C or 2°C substantially reduces the potential for either large increases or decreases in mean streamflow compared to 3°C or 4°C ( [[#Warszawski--2014|Warszawski et al., 2014]] ; [[#Falkner--2016|Falkner, 2016]] ; [[#Gosling--2017|Gosling et al., 2017]] ; Figure 4.16) ( ''high confidence'' ).

In CMIP5, strong model consistency on changes in high and low streamflows is seen with similar global patterns to the mean flows, but over smaller areas ( [[#Koirala--2014|Koirala et al., 2014]] ). By the end of the 21st century, with RCP8.5, increases in mean, high and low flows are projected for the Lena, and mean and low flows for the MacKenzie ( [[#Gelfan--2017|Gelfan et al., 2017]] ; [[#Pechlivanidis--2017|Pechlivanidis et al., 2017]] ; [[#Döll--2018|Döll et al., 2018]] ) ''.'' Increased mean and high flows are projected in the Ganges, high flow in the Rhine and Mississippi, while decreasing mean and low flows are projected in the Rhine ( [[#Krysanova--2017|Krysanova et al., 2017]] ; [[#Pechlivanidis--2017|Pechlivanidis et al., 2017]] ; [[#Vetter--2017|Vetter et al., 2017]] ). Decreases in mean, high and low flows are projected for the Tagus (Krysanova et al. 2017; Vetter et al. 2017). Low flows are projected to decrease in the Mediterranean region and increase in the Alps and northern Europe ( [[#Marx--2018|Marx et al., 2018]] ). A general shift in the runoff distribution towards more extreme low runoff is projected in Mexico, western USA, western Europe, southeastern China and the West Siberian Plain, and more extreme high runoff is projected in Alaska, northern Canada and large parts of Asia ( [[#Zhai--2020|Zhai et al., 2020]] ).

While projected changes in high and low flows are similar to those in mean flows in many regions, this is not the case everywhere. When a single hydrological model and a sample of climate models are selected to explore uncertainties systematically, approximately 56% of the global population is projected to be affected by increased extreme high flows at 1.5°C warming, rising to 61% at 2°C warming ( [[#Zhai--2020|Zhai et al., 2020]] ). Those affected by extreme low flows decrease is projected to remain close to 45% at both 1.5°C and 2°C warming. However, these results are based on the median of the ensemble projections, so they are subject to high uncertainty. At 1.5°C global warming, 15% of the population is projected to be affected concurrently by decreased extreme low flows and increased extreme high flows, increasing to 20% at 2°C warming. In 25 combinations of five CMIP5 climate models and five global hydrological models under the RCP8.5 scenario reaching approximately 4°C GWL at the end of the century, 10% of the global land area is projected to face simultaneously increasing high extreme streamflow and decreasing low extreme streamflow. These regions include the British Isles and the shores of the North Sea, large parts of the Tibetan Plateau, South Asia and western Oceania, and smaller regions of Africa and North and South America, affecting over 2.1 billion people with 2015 population distributions ( [[#Asadieh--2017|Asadieh and Krakauer, 2017]] ). With 11 CMIP5 models driving a single hydrological model, simultaneous increases in high flows and decreases in low flows are projected over 7% of global land ( [[#Koirala--2014|Koirala et al., 2014]] ).

By the end of the 21st century, global changes in streamflow extremes are projected to be approximately twice as large with RCP8.5 (over 4°C GWL) than with RCP2.6 (approximately 2°C GWL) ( [[#Asadieh--2017|Asadieh and Krakauer, 2017]] ).

Glacier retreat and associated runoff changes represent a major global sustainability concern ( [[#4.4.2|Section 4.4.2]] ). By 2100, using an ensemble of 14 CMIP5 climate models driven by the RCP4.5 scenario, one third of the 56 large-scale glacierised catchments are projected to experience a mean annual runoff decline by over 10%, with the most significant reductions in central Asia and the Andes ( [[#Huss--2018|Huss and Hock, 2018]] ). Thus, communities dependent on glacier runoff are particularly vulnerable ( [[#Jiménez%20Cisneros--2014|Jiménez Cisneros et al., 2014]] ).

Societal impacts of change in runoff spread throughout several socioeconomic sectors, such as agriculture, health and energy production, affecting overall water security ( [[#Wang--2021a|Wang et al., 2021a]] ). Decreases in runoff may lead to water scarcity and result in increased multi-sectoral effects in sub-Saharan Africa ( [[#Serdeczny--2017|Serdeczny et al., 2017]] ), western Africa, the Middle East, Mexico, Northeastern Brazil, central Argentina, Mediterranean Africa and Europe ( [[#Gosling--2016|Gosling and Arnell, 2016]] ; [[#Greve--2018|Greve et al., 2018]] ), and southeastern Australia ( [[#Barnett--2015|Barnett et al., 2015]] ).

In summary, mean and extreme streamflow changes are projected over most of the ice-free land surface ( ''high confidence'' ). The magnitude of streamflow change is projected to increase with global warming in most regions ( ''high confidence'' ), but there is often high uncertainty on the sign of change. There is ''high confidence'' that mean streamflows will increase in the northern high latitudes and decrease in the Mediterranean and southern Africa. Annual mean runoff in one third of assessed glacierised catchments is projected to decline by at least 10% by 2100 under RCP4.5, with the most significant reductions in central Asia and the Andes ( ''medium confidence'' ). Elsewhere, projections include both increased and decreased flows. Substantial fractions of ensemble projections disagree with the multi-model mean ( ''high confidence'' ), with implications for long-term planning for water management. With 1.5 and 2°C global warming, approximately 15 and 20% of the current global population, respectively, would experience both an increase in high streamflows and a decrease in low streamflows ( ''medium confidence'' ). At 4°C global at the end of the century, 10% of the global land area is projected to simultaneously experience an increase in high extreme streamflow and decrease in low extreme streamflow.

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