Two of Asia's 'water towers', the Brahmaputra and Indus river basins, are likely to be severely affected by climate change while others will be less affected and could even benefit, research on Asia's rivers shows.
One-fifth of the world's population is dependent on water from the Brahmaputra, Indus, Ganges, Yangtze and Yellow rivers — often referred to as Asia's water towers — which are fed by melt water from the Himalayas. Until now it was thought that all the Asian river basins would be similarly impacted by climate change, which would both reduce the amount of water available and harm food security.
But scientists from the Netherlands, believing that estimates were being made without good data, used computer simulations to assess the effects of climate change.
'We started to look into this problem because we were observing discussions about the importance of glacial melt from the Himalayas for 1.5 billion people, without seeing any hydrological assessments of the actual contribution of melt water to water availability,' said Mark Bierkens, chair of the department of physical geography at Utrecht University, and one of the study authors.
Their simulation shows that climate change will hasten the melting of upstream snow and ice reserves, leading to faster and heavier water flow and flooding downstream — although the exact rate and extent of the impact is still unclear. They also predicted that climate change would dry up major rivers because of higher evaporation of water (see World's major rivers 'drying up'). But their work which, for the first time, analysed the impact of climate change on melt water both upstream (more than 2000 metres above sea level) and downstream, where people live and grow crops, found that .
the 'effects of climate change on water availability and food security in Asia differ substantially among basins and cannot be generalised'.
The effects on the Indus and Brahmaputra basins are likely to be 'severe' owing to the large numbers of people living in the area and their heavy dependence on irrigated agriculture.
But people in the Yellow River basin, China, could benefit, as they depend less on irrigation, and also because the projected higher rains upstream could be retained in reservoirs for use later for crops.
'It turns out that downstream water availability is most dependent on upstream melt water for the Indus, moderately important for the Bramhaputra and less important for the other rivers (where water availability depends on downstream monsoon rainfall),' Bierkens, whose work was published in Science.
Overall, the scientists estimate, the food security of 4.5 per cent of the region's population — about 63 million people will decrease because there is less water. They stress the 'strong need' for prioritising adaptation options and increasing water availability.
Satellite studies from India indicate Himalayan snow has started melting in winter too, which will affect regional river flows (see Himalayan snow melting in winter too, say scientists).