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© Thierry Boussa
c, Fotolia
1. The water cycle, in perpetual movement
There is a permanent system of exchanges between Earth’s different
reservoirs of water and water changes form as it moves through the
stages of its cycle. Water of the oceans, the rivers, lakes, the soil and plants
evaporates under the heat of the sun. The water vapour rises up into the
Distribution of the water on Earth in the
various reservoirs
Water vapour in the atmosphere = 0,001%
Lakes, rivers = 0.01%
Underground water = 0.6%
Glaciers
and polar ice caps = 2.3%
Oceans and seas = 97%
atmosphere, cools and then condensation begins. Vapour transforms into
tiny particles which gather in their thousands to form clouds. Water then
falls to earth as rain, hail or snow, feeds water courses, lakes, oceans
and the water table underground or accumulates on glaciers. This cycle
is continuous. The ocean takes part in this because it moves heat and
humidity around on the planet.
After Oceans et atmosphère, Hachette Éducation, 1996.
2. Exchanges between sky and sea
e poles
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would be several
The ocean exchanges water with the atmosphere continually, in the
form of vapour and precipitations, but it also exchanges heat. The
atmosphere also acts, setting the ocean in motion under the force of
winds, driving currents and waves which propagate at the surface.
The relationships between ocean and atmosphere are complex: winds
and currents interact and give rise to cyclic variations in climate; these
factors are highly active in the regions close to the Equator.
3. A powerful heat regulator
The ocean absorbs the sun’s radiation and stores the various forms as heat, building an immense heat
reservoir, more than 1,000 times as great as the atmosphere ! The tropics capture more heat than the other
regions of the globe, because they are directly, vertically below the sun. The ocean surface currents, drawn
by the winds, displace the warm waters towards the polar regions, where they plunge to the depths before
slowly returning to the tropical regions. The mechanism is like that of a central heating system.
The circulation routes of warm and cold currents at world scale
Deep-ocean
cold current
Atlantic
Ocean
Pacific
Ocean
Surface
warm current
Indian
Ocean
20 °
10 °
0°
After Troisième rapport d’évaluation du GIEC, 2001.