A new scientific report by Dutch researchers and published in this week's issue of Science magazine assesses the likely effects of climate change on the major Asian rivers and deltas. This is a subject that has been much in the public eye--and rightly so, considering its importance--since the disclosure of a very sloppy error in one of the IPCC assessment reports, and with ongoing difficulties setting the record straight about the fate of the Himalayan glaciers. The Science article, "Climate Change Will Affect the Asian Water Towers," finds that annual meltwater from the glaciers is extremely important in Pakistan's Indus River and in the Brahmaputra, which originates in China and runs down through Tibet to its delta, near Dhaka, in Bangladesh. Meltwater plays "only a modest role," on the other hand, in the Ganges, Yangtze and Yellow River systems.
The key elements in the Dutch analysis were, on the one hand, an examination of all upstream hydrological processes at elevations higher than 2000 meters, and, on the other, an inventory of downstream precipitation patterns. The net impact of retreating glaciers depends on the importance of meltwater relative to rainfall. Meltwater was found to have a dominating influence in the Indus system and a strong influence in the Brahmaputra, affecting potentially the food supplies of 35 million people. In the Ganges, Yangtze and Yellow River systems, rainfall plays a relatively greater role.
"We conclude," say the authors, "that although considerable cryospheric changes are to be expected, their impact will be less than anticipated by . . . the Intergovernmental Panel on Climate Change." The IPCC had predicted that because of global warming and glacial retreat, rivers like the Ganges, Indus and Brahmaputra soon would become "seasonal rivers." But the authors argue they already are seasonal rivers, and in some ways in a benign sense: Melt and rainy seasons generally coincide, and so a decrease in meltwater from warming will be compensated for to some extent by an increase in rainfall, also caused by the warming. In some cases, in fact, "an accelerated melt peak may thus alleviate a shortage of irrigation water in the drought-prone early stages of the growing season."
Even in the extreme scenario in which all the Himalayan glaciers disappear (as an IPCC report erroneously said could happen by 2035), the effects are most pronounced by far in the Indus and Brahmaputra basins. So, while those effects are nothing to sneeze at, global warming will not have a drastic impact on hundreds of millions or billions of Asians as the IPCC implied and has been explicitly claimed in reports on climate change and human migration.