Introduction

Can we think of life without water at all? There is a reason people say water is life — it is an essential component of every life form that exists on the surface of the earth. In this, the creatures of our planet are remarkably fortunate: earth is a water planet, and without that fact, none of us would exist.

Water, it turns out, is a rare commodity in our solar system. There is no water on the sun, and none has been found anywhere else in the solar system in the abundance our planet enjoys. Earth alone has been blessed with a plentiful supply of water on its surface — oceans, seas, rivers, lakes, ice, and the water vapour in its atmosphere. That abundance is exactly why our planet is known as the 'Blue Planet' — when viewed from space, it is water, more than anything else, that gives earth its distinctive colour.

This chapter, and the unit it belongs to, takes up that water in detail: how it circulates through the hydrological cycle, the character of the ocean basins that hold it, and the movements — waves, tides, and currents — that keep ocean water in constant motion.