How much of the Earth's water is fresh and accessible?

About 97 per cent of the world's water is salt. Of the remaining three per cent, roughly two thirds is locked in ice caps and glaciers and most of the rest is groundwater, a good deal of it too deep to reach. What is left in lakes, rivers and shallow accessible aquifers is well under one per cent of the total - and rivers themselves are a rounding error, something like a hundredth of one per cent.

That thin slice is also badly distributed and moves on its own schedule. Nine countries hold about 60 per cent of the world's fresh water, and there are large populations sitting where the rain is not. Groundwater is the buffer everyone falls back on, and much of it is being drawn down far faster than it refills - aquifers under northern India, the North China Plain and the American High Plains are all in measurable decline, and the last of those holds water that fell during the last ice age and is not coming back.

The visible losses are the sharpest lesson. The Aral Sea was the fourth largest lake in the world in 1960 and is now mostly desert with ships sitting on it, because two rivers were diverted to irrigate cotton. It took about thirty years.