Global pattern of trends in streamflow and water availability in a changing climate

Climate change Health warning Nature this week includes reviews, original research and comment on a hot topic, the regional effects of climate change. The cover image — Chicago during the July 1995 heatwave — highlights one potential risk. Patz et al. consider the available evidence and suggest that climate warming already contributes to ill health and thousands of premature deaths across the world, and is likely to have serious health implications in the future. Recent work suggests that some regions are particularly at risk: areas where climate is dominated by El Niño/Southern Oscillation events, sub-Saharan Africa, and sprawling urban areas subject to the heat island effect are already suffering from climate impacts and these are projected to increase. In many cases, regions at high risk are those least responsible for causing climate change. Sure thing All currently available climate models predict a near-surface warming trend under the influence of rising levels of greenhouse gases in the atmosphere. Barnett et al. evaluate the effect of such a warming trend on regional hydrology, particularly in snowmelt-dominated environments. They suggest that warming will cause a change from snowfall to rain, diminishing natural water storage capacity, as well as earlier melting of winter snow, shifting peak river runoff away from the periods of highest demand in summer and autumn. The reduction in glaciers and snow-packs are likely to have severe consequences for the water supply of one-sixth of the Earth's population. Water supply It is generally assumed that climate change will alter the hydrological cycle, but predicting which parts of the world will be drier and which will be wetter is a difficult problem. Milly et al. focus on streamflow and water availability trends and find that an ensemble of twelve current climate models accurately accounts for twentieth-century changes. The same models project potentially crucial regional effects on streamflow in the future that could threaten the availability of freshwater in many regions of the world by the year 2050.