National Geographic : 2009 Apr
• as in the three decades between 1950 and 1980. "It happens not just spatially, but also in time," says Brian Soden, a professor of marine and atmospheric science at the University of Miami. "And so the dry periods become drier, and the wet periods become wetter." Quantifying the effects of global warming on rainfall patterns is challenging. Rain is what scientists call a "noisy" phenomenon, meaning that there is a great deal of natural variability from year to year. Experts say that it may not be until the middle of this century that some long-term changes in precipitation emerge from the background clatter of year-to-year uctuations. But others are already discernible. Between 1925 and 1999, the area between 40 and 70 degrees north latitude grew rainier, while the area between zero and 30 degrees north grew drier. In keeping with this broad trend, north- ern Europe seems to be growing wetter, while the southern part of the continent grows more arid. e Spanish Environment Ministry has estimated that, owing to the combined e ects of climate change and poor land-use practices, fully a third of the country is at risk of deserti- cation. Meanwhile, the island of Cyprus has become so parched that in the summer of 2008, with its reservoir levels at just 7 percent, it was forced to start shipping in water from Greece. "I worry," says Cyprus s environment com- missioner, Charalambos eopemptou. " e IPCC is talking about a 20 or 30 percent reduc- tion of rainfall in this area, which means that the problem is here to stay. And this combined with higher temperatures---I think it is going to make life very hard in the whole of the Mediterranean." Other problems could follow from changes not so much in the amount of precipitation as in the type. It is estimated that more than a billion people---about a sixth of the world s population---live in regions whose water sup- ply depends, at least in part, on runoff from glaciers or seasonal snowmelt. As the world warms, more precipitation will fall as rain and less as snow, so this storage system may break down. e Peruvian city of Cusco, for instance, relies in part on runo from the glaciers of the SUDAN Drought scars the earth in Northern Darfur in October 2005. The UN calls this region "a tragic example of the social breakdown that can result from ecological collapse."