When Ban Ki-moon, the UN secretary-general, was asked to ponder the future of the world before an audience of powerful businessmen and politicians, at a meeting in Switzerland earlier this year, he could have chosen any topic he liked. What he focused on was both a hoary old favourite, and a newly popular preoccupation, of debates on world affairs: the rising risk of wars over fresh water, as populations increase and the world gets drier. "As the global Economy grows, so will its thirst...many more conflicts lie over the horizon,' he said, after deploring the fact that "too often, where we need water, we find guns.' Mr Ban wasn't the first to sound the alarm. In 2006 John Reid, who was then British defence secretary, triggered headlines such as "Water wars loom' after disclosing that a unit at his ministry was preparing for a world of battles over life's most basic necessity. And warnings have been issued by people closer to the edge of contested waters. After making peace with Israel in 1979, the late President Anwar Sadat said that henceforth Egypt would not wage war, except over water. But is the idea of countries marching to battle to get water, or to defend it, really plausible? Researchers at Oregon State University say they have found evidence to the contrary, showing that the world's 263 trans-boundary rivers (whose basins cover nearly half the land surface of the world) generate more co-operation than conflict. Over the past half-century, 400 treaties had been concluded over the use of rivers. Of the 37 incidents that involved violence, 30 occurred in the dry and bitterly contested region formed by Israel and its neighbours, where the upper end of the Jordan river was hotly disputed, and skirmished over, before Israel took control in the 1967 war. And some inter-state water treaties are very robust. The Indus river pact between India and Pakistan survived two wars and the deep crisis of 2002. Where the doom-mongers do have a point is this: drought, desertification and food shortage are among the factors that foment conflict within states by tipping some areas, at least, into social collapse. The drying up of old grazing lands, once shared by Arab herders and African farmers, is one of the things that pushed Sudan's west into chaos and misery. But what about war between nations that more-or-less function? For anyone who takes a cynical view of the causes of war, water seems a less likely agent than say, oil or diamonds. For dictators or warlords (the sort who sponsored or prolonged ghastly wars in Congo and Angola), water is less enticing than minerals or gems. It is harder to steal and sell. But conflicts of interest over water can certainly poison inter-state relations, even when an imbalance of power is so great that the aggrieved party could never consider using force. Mark Zeitoun, a Canadian scholar at the London School of Economics, says rivers provide a perfect case of "asymmetrical co-operation' between countries that are forced to work together on terms dictated by the strongest. Take the Nile, where the main riparian states, Egypt, Sudan, Uganda and Ethiopia, or their colonial masters have been watching each other's water use closely for a century at least