Viewed from space, the earth appears blue to the human eye. This is because more than two-thirds of its surface are covered by sea water, which absorbs long-wave red, yellow and green light, but reflects short-wave blue light.
The total quantity of water in the world is almost impossible to visualize. There are 1.38 billion cubic kilometres of water, 97 per cent of which swirls in mighty currents in the basins of the world’s oceans. Only three per cent of this, or 35 million cubic kilometres, consists of fresh water, and of this, over 24 million cubic kilometres are in the form of ice and glaciers in the high-mountain regions and polar icecaps. The fresh water in the lakes, streams and rivers of the continents amounts to just 190,000 cubic kilometres.
