Streams of blood, or streams of peace

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.