The next time your throat is as dry as a bone and the Sun is beating down, take a glass of clean, cool water. Savour it. Sip by sip. Vital and appreciated as that water is, it will be even more precious to those who will follow you. By the end of this century, billions are likely to grip by water stress and the stuff of life could be an unseen driver of conflict. So say hydrologists who forecast that on present trends, freshwater faces a double crunch ? from a population explosion, which will drive up demand for food and energy, and the impact of climate change.

"Approximately 80 per cent of the world's population already suffers serious threats to its water security, as measured by indicators including water availability, water demand and pollution," the Nobel-winning Intergovernmental Panel on Climate Change (IPCC) warned in a landmark report in March. "Climate change can alter the availability of water and therefore threaten water security." Already today, around 768 million people do not have access to a safe, reliable source of water and 2.5 billion do not have decent sanitation. Around a fifth of the world's aquifers are depleted.

Jump forward in your imagination to mid-century, when the world's population of about 7.2 billion is expected to swell to around 9.6 billion.

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