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Freshwater will be in ever-shorter supply as climate change gathers pace. Veteran climate modeller Syukuro Manabe and his colleagues from the US-based Princeton University simulated what effect a quadrupling of atmospheric carbon dioxide from the pre-industrial levels would have on the global hydrological cycle over the next 300 years. They found that both evaporation and precipitation would increase. Evaporation will reduce the moisture content of soils in many semi-arid parts of the world, including northeast China, the grasslands of Africa, the Mediterranean and the southern and western coasts of Australia.
With increase in precipitation, the overall discharge of freshwater from rivers around the world will rise by almost 15 per cent. The biggest increases will be in the thinly populated tropics and the far north of Canada and Russia. For instance, the flow of the river Ob in Siberia is projected to increase by 42 per cent by the end of the 23rd century. But there will be lower flows in many mid-latitude rivers, which run through heavily populated regions. Those that will start to decline include the Mississippi, Mekong and especially the Nile