Editing IPCC:AR6/WGII/Chapter-10 (section)

==== 10.4.4.4 Projections ====

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Asian and global water demands for irrigation, despite geographic variation in terms of water availability, are ''very likely'' to surpass supply by 2050 ( [[#Chartres--2014|Chartres, 2014]] ). A regional quantitative assessment ( [[#Lutz--2019|Lutz et al., 2019]] ) of the impacts of 1.5°C versus 2°C global warming for a major global climate-change hotspot–the Indus, Ganges and Brahmaputra river basins (IGB) in South Asia–shows adverse impacts of climate change on agricultural production, hydropower production and human health. A global temperature increase of 1.5°C with respect to pre-industrial levels would imply a ≈ 2.1°C temperature increase for IGB, whereas under a 2.0°C global temperature increase scenario, these river basins would warm by ≈ 2.7°C. Future warming is expected to further increase rain-on-snow events that can cause snowmelt flood during winter ( [[#Ohba--2020|Ohba and Kawase, 2020]] ), affecting hydropower and resulting in river flooding, avalanches and landslides.

In the Mekong River Delta (in Vietnam), with an area of 40,500 km 2 and home to 17.8 million people in 2018, climate change is projected to increase the average temperature by 1.1–3.6°C, and the maximum and minimum monthly flow are projected to increase and decrease, respectively, and are ''likely'' to result in a high risk of food during the wet season and water shortages during the dry season ( [[#Wang--2021a|Wang et al., 2021a]] ).

Researchers have found that the southern Tibetan Plateau has been consistently melting from 1998–2007 and is projected to continue melting until 2050 ( [[#Lutz--2014b|Lutz et al., 2014b]] ) ( ''high confidence'' ). In HMA, glacier ice is projected to decrease by 49 ± 7% and 64 ± 5% by the end of the century under RCP4.5 and RCP8.5 scenarios, respectively ( [[#Kraaijenbrink--2017|Kraaijenbrink et al., 2017]] ). Local- and regional-scale projections suggest that peak water will generally be reached around the middle of the century, followed by steadily declining glacier runoff thereafter ( [[#Hock--2019|Hock et al., 2019]] ). A global-scale projection suggests that a decline in glacier runoff by 2100 (RCP8.5) may reduce basin runoff by about 10% for at least 1 month of the melt season ( [[#Huss--2018|Huss and Hock, 2018]] ). Significantly, research on climate change and its impact across Asia remains inconclusive and requires an assessment at the sub-regional scale ( [[#IPCC--2014a|IPCC, 2014a]] ; [[#Wester--2019|Wester et al., 2019]] ).

There is a projection of an increase in runoff until the 2050s mainly due to an increase in precipitation in the upper Ganges, Brahmaputra, Salween and Mekong basins, where it could be due to accelerated melting in the upper Indus basin. The runoff could increase in the range of 3–27% (7–12% in Indus, 10–27% in Ganges and 3–8% in Brahmaputra) by mid-century compared with the reference period (1998–2008) for Himalayan river basins depending on the different RCP scenarios ( [[#Lutz--2014a|Lutz et al., 2014a]] ). Likewise, [[#Khanal--2021|Khanal et al. (2021)]] suggest contrasting responses to climate change for HMA rivers in which, on the seasonal scale, the earlier onset of melting causes a shift in magnitude and peak of water availability, whereas on the annual scale, total water availability increases for the headwaters. The future flow would increase in Nepal’s Central Himalaya region ( [[#Nepal--2016|Nepal, 2016]] ; [[#Ragettli--2016|Ragettli et al., 2016]] ; [[#Bajracharya--2018|Bajracharya et al., 2018]] ). These changes in water availability in space and time will have serious consequences in downstream water availability for various sectoral uses and ecosystem functioning in Asia ( [[#Nepal--2014|Nepal et al., 2014]] ; [[#Green--2015|Green et al., 2015]] ; [[#Arfanuzzaman--2018|Arfanuzzaman, 2018]] ; [[#Wijngaard--2018|Wijngaard et al., 2018]] ; [[#Rasul--2019|Rasul and Molden, 2019]] ); however, future water availability is largely uncertain due to significant variation in climate-change projections among different global climate models ( [[#Nepal--2015|Nepal and Shrestha, 2015]] ; [[#Lutz--2016|Lutz et al., 2016]] ; [[#Li--2019a|Li et al., 2019a]] ).

A recent study ( [[#Didovets--2021|Didovets et al., 2021]] ) covering eight river catchments having diverse natural conditions within Central Asia, where water availability or scarcity is also a major developmental concern, and using the eco-hydrological model SWIM (including scenarios from five bias-corrected GCMs under RCP4.5 and RCP8.5) has show an increase in mean annual temperature in all catchments for both RCPs to the end of the 21st century. The projected changes in annual precipitation indicate a clear trend to increase in the Zhabay and decrease in the Murghab catchments, and for other catchments, they were smaller. Both the projected trends for river discharge and precipitation show an increase in the northern and decrease in the southern parts of the study region, whereas seasonal changes include a shift in the peak of river discharge up to one month, a shortening of the snow accumulation period and a reduction in discharge during the summer months.

The intensity and frequency of extreme discharges are ''very likely'' to increase towards the end of the century. The future of the upper Indus basin water availability is highly uncertain in the long term due to uncertainty surrounding precipitation projections ( [[#Lutz--2016|Lutz et al., 2016]] ). The future hydrological extremes of the upper Indus, Ganges and Brahmaputra river basins suggest an increase in the magnitude of extremes towards the end of the 21st century by applying RCP4.5 and RCP8.5 scenarios, mainly due to an increase in precipitation extremes ( [[#Wijngaard--2017|Wijngaard et al., 2017]] ). In the Brahmaputra, Ganges and Meghna, including the downstream component, the runoff is projected to increase by 16, 33 and 40%, respectively, under the climate-change scenarios by the end of the century during which the changes in runoff are larger in the wet seasons than the dry seasons ( [[#Masood--2015|Masood et al., 2015]] ). In the Mekong River basin also, extremely high-flow events are ''likely'' to increase in both magnitude and frequency, which can exacerbate flood risk in the basin ( [[#Hoang--2016|Hoang et al., 2016]] ); however, uncertainty is high regarding future hydrological response due to large variation in precipitation projections, modelling approaches and bias-correction methods ( [[#Nepal--2015|Nepal and Shrestha, 2015]] ; [[#Lutz--2016|Lutz et al., 2016]] ; [[#Li--2019a|Li et al., 2019a]] ).

Current research on the adverse relationship between climate change and river flows suggests that there is a high possibility that some of the river basins affected by floods could be Brahmaputra, Congo, Ganges, Lena and Mekong, with a return period of 10 years ( [[#Best--2018|Best, 2018]] ).

In most parts of the upper Ganges and Brahmaputra rivers, the 50-year return level flood is ''likely'' to increase and to a lesser degree in Indus River. Similarly, the extreme precipitation events are also expected to increase to a higher degree in the Indus than the Ganges and Brahmaputra basins ( [[#Wijngaard--2017|Wijngaard et al., 2017]] ). Increase in extreme precipitation events is ''likely'' to cause more flash-flood events in the future ( ''medium confidence'' ). In the case of the Indus, increasing temperature trend in the future may lead to accelerated snow and ice melting which may increase the frequency and intensity of floods in the downstream areas ( [[#Hayat--2019|Hayat et al., 2019]] ). The Ganges–Brahmaputra region also faces the threat of increased frequency of flood events ( [[#Lutz--2019|Lutz et al., 2019]] ). Additionally, the Ganges basin also shows a higher sensitivity to changes in temperature and precipitation ( [[#Mishra--2016|Mishra and Lilhare, 2016]] ).

Assessing the impact of climate change on water resources in nine alpine catchments in arid and semiarid Xinjiang of China ( [[#Li--2019a|Li et al., 2019a]] ), it has been noted that even though the total discharge revealed an overall increasing trend in the near future, the impact of climate change on different hydrological components indicated significant spatio-temporal heterogeneity in terms of the area, elevation and slope of catchments, which could be usefully factored into climate-adaptation strategies.

It was noted early on ( [[#Singh--2011|Singh et al., 2011]] ) that the main drivers that influence the provisioning of ecosystem services and human well-being in the HKH region are a mix of environmental change in general and climate change in particular, but much more data and knowledge on the HKH region are needed in order to develop either a regional or global understanding of climate-change processes.

Climate change impacts cryospheric water sources in the Hindu Kush, Karakoram and Himalayan ranges which in turn carry consequences for the Indus, Ganges and Brahmaputra basins. The impact of climate change on spring-fed rivers in the HKH is under-researched and therefore makes projections difficult. Further research is needed for understanding the impact of deforestation, urbanisation, development and introduction of water infrastructures, such as tube wells, in the hill region ( [[#Aayog--2017|Aayog, 2017]] ). This in turn calls for greater investment in research and development for the HKH by both the national and regional organisations. There is ''high confidence'' that due to global warming, Asian countries could experience an increase in drought conditions (5–20%) by the end of this century ( [[#Prudhomme--2014|Prudhomme et al., 2014]] ; [[#Satoh--2017|Satoh et al., 2017]] ).

Soil erosion in high-mountain areas is particularly sensitive to climate change. A recent study ( [[#Wang--2020|Wang et al., 2020]] ) that focused on the mid-Yarlung Tsangpo River, located in the southern part of the Tibetan Plateau, has revealed dramatic land surface environment changes due to climate change during recent decades. It has further shown that increasing precipitation and temperature would lead to increasing soil-erosion risk in ~2050 based on the Coupled Model Intercomparison Project (CMIP5) and RUSLE models.

High-resolution climate-change simulations suggest that due to deadly heatwaves projected in some of the densely populated agricultural regions of South Asia (i.e., the Ganges and Indus river basins), those regions are ''likely'' to exceed the critical threshold of wet-bulb temperature of 35°C under the business-as-usual scenario of future GHG emissions ( [[#Im--2017|Im et al., 2017]] ).

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