To assess what a warmer world might mean for the human race, 30 groups from 12 countries have run thousands of simulations, using a standardized set of scenarios for greenhouse-gas emissions. They made projections of future water availability from a set of global hydrological models in conjunction with five state-of-the-art climate models that combined projections of changes in temperature and precipitation with data on variables such as regional water cycles, river run-off and population.
The multi-model assessment suggests that, in vulnerable regions, climate change will significantly add to the problem of water scarcity that is already arising from population growth. The modellers found that climate-driven changes in evaporation, precipitation and run-off will result in a 40% increase in the number of people worldwide who must make do with less than 500 cubic metres of water per year — a commonly used threshold to signify ‘absolute’ water scarcity.
Regions most at risk from water scarcity include parts of the southern United States, the Mediterranean and the Middle East. By contrast, India, tropical Africa and high latitudes in the Northern Hemisphere can expect to receive more water in a warming world.These are models, of course, because (like population projections) they provide us with the only way of trying to decipher the future. Despite variability in the models, the results consistently point to a future in which weather and water availability will be different. The sooner we own up to that and start planning our responses, the better off we will be. That just seems common-sensical to me.
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