emand for freshwater is soaring as populations grow and personal consumption rises, yet there is no more of it on Earth than there was 2,000 years ago, when there were only 3 per cent as many people to use it. Water is being withdrawn from rivers, lakes and underground sources - for irrigated agriculture, industry and domestic consumption - faster than it can be replenished.

Just as seriously, freshwater supplies are threatened by pollution as sewage wastes, toxic industrial effluents, pesticides and fertilizers flow into lakes and rivers or leach into groundwater. Disease and death result; water-related diseases, like diarrhoea, kill millions of people every year. The environment is damaged, and wildlife species are endangered.

Thirty-one countries - mostly in Africa and the Middle East - are already chronically short of freshwater. This is expected to get very much worse. By 2025, it is forecast, more than 2.8 billion people in 48 countries will face water stress or scarcity. By 2050 this is expected to rise to 4 billion people in 54 countries - about 40 per cent of humanity. Ethiopia, India, Kenya, Nigeria and Peru are all likely to run short in the next quarter century. Parts of China already face chronic problems.

It may already be too late for some water-short countries with rapidly rising populations to avoid a crisis. But the world needs to conserve water, pollute less, manage supply and demand, and, where necessary, try to slow population growth and cut overconsumption.