Source: AAAS Atlas of Population & Environment

The world isn't running out of water. Nature keeps purifying it and bringing it around one more time. But too often it's not where we need it. It is in the oceans or the glaciers or deep underground or in raging rivers far from where people live.

Greenland's 60 000 inhabitants have more water than anyone else. They don’t need it. But people in the driest countries do. They need huge amounts of water to irrigate their crops as well as to quench their thirst.

And water is hard to get to where it is needed. Think how much a single bucket of the stuff weighs. Think how many buckets a village or a town or a city uses in a day. To make matters worse, our precious water is often dirty. More than a billion people in the world can't drink a glass of clean water when they are thirsty.

During the 21st century, water is destined to become as precious as oil. We will need to take care of every drop. So here is your essential briefing guide to the new world of water.