Those magnificent pictures of Earth from space, among the most humanly important and evocative results from the placing of manned and unmanned satellites in orbit during the last several decades, have shown us that underneath the clouds most of the world is blue. Viewed in this way from space, Earth is seen as a planet covered mostly by water, and most of that water is seawater.

This thin layer of water, covering 71% of Earth, affects or controls much of its climate and chemistry. The blue color of the sea tells us that most sunlight on the ocean is absorbed and not reflected. The absorption of sunlight warms the planet.

The warmth evaporates water, especially from the tropical ocean. Water in the atmosphere is a greenhouse gas, and this also helps to warm Earth. Some atmospheric water forms clouds; by reflecting sunlight clouds help to cool Earth. The balance between heating and cooling is always changing as clouds form and dissipate, so exact calculation of the balance is difficult. The water in the ocean is a vast reservoir of heat, which buffers and slows global change. Both the currents in the ocean and the winds in the atmosphere carry heat from low latitudes, where there is a net input, towards the poles, where there is a net loss of heat to space. These vast currents, simple in general concept, immensely complex in detail, profoundly affect both the local distribution of climate and the overall climate of Earth. The ocean contains an enormous reservoir of carbon dioxide; it both absorbs and releases this greenhouse gas from and to the atmosphere. Living things in the sea influence both the release and the uptake. Physical, chemical, biological, and geological processes interact with each other in the ocean and with the atmosphere to create the environment of the planet we live on.