Ocean and climate
Swell, waves, currents, tides… the climate of our planet and the life it harbours originate from the continual movement of the sea, though only some of that movement is visible on the surface. The flows are much greater in deep water, stretching over immense distances and reaching staggering proportions.
An ocean in motion
On the surface, the movement of the water is caused mainly by the wind. In the final analysis, it is therefore influenced by the amount of sunshine across the planet. The direction of the water’s movement is affected by the seabed relief and the Coriolis force caused by the earth’s rotation. Moreover, because they are linked to the wind, these currents vary in direction and intensity, depending on the season or even the time of day.
Surface currents and bottom currents are closely linked, although bottom currents are not influenced by the wind, or at least hardly at all. In fact, and as their name suggests, they exist hundreds of metres beneath the surface. That is why they are also more stable or consistent. The power driving these currents results from differences in temperature and salinity between the water masses for, beneath their apparent uniformity, the oceans are actually a combination of very different waters in perpetual motion, with unique properties determined by their place of origin. A sea formed in the tropics is therefore warmer and saltier, due to higher temperatures and rates of evaporation. On the other hand, the water of the Antarctic is cold and less salty due to the massive input of fresh water from the frozen continent. Each of these water masses forms, moves and melts slowly during a round-the-world journey that can take five hundred to a thousand years.
The main underwater currents form a cycle called “thermohaline circulation”, which plays a major role in the life of the oceans. The journey begins in the tropics, where the trade winds push the warm water, heated on the surface by the sun, northwards to the Atlantic Ocean. In the form of two strong currents, the North Atlantic drift and the Gulf Stream, the water can achieve a flow of 30 million cubic metres per second off the coast of Florida. When it arrives in the seas of Greenland, Norway, Iceland and Labrador, the surface water cools on contact with the cold dry air from the Arctic.
When it cools, the water becomes denser and sinks down to the depths of the ocean, pulled down by its own weight. At the same time, it starts to return towards the south at a rate of 15 to 20 million cubic metres per second. The deep layers of the ocean are re-oxygenated by the massive supply of oxygen-rich surface water. The southward movement continues until it reaches the Antarctic Circumpolar Current, which enables these deep water masses to reach the Indian ocean and the Pacific.
The journey continues with a new movement northwards. By mixing with the tropical water, the deep water returns to the surface. The loop ends with its return to the Atlantic via Cape Horn and the Cape of Good Hope, which is the start of the next round-the-world cycle.
These currents play an important role in combining the water masses and have an impact on the ocean’s major bio-geochemical cycles. What is more, they form a sort of conveyor belt, a means of transport for the migratory species or those which are unable to move independently, such as microalgae or jellyfish. The surface currents also encourage the dispersion of eggs and larvae, thus ensuring species propagation.
Global ocean movements do not only control the dynamic of nutrients; they enable exchanges between the ocean and the atmosphere which account for our climate. Their significant thermal capacity enables the ocean to trap large quantities of heat, which are then distributed, thus determining the effects of the climate. One of the best known examples is the effect of the Gulf Stream on the European climate: as it moves north, the Gulf Stream approaches the European coasts, with the warm current heating the westerly wind, which then warms Europe’s winter season. As a result of this, Europe benefits from a much milder climate than the east coast of North America, which is on the same latitude. This means that, in winter, the temperature in Paris might be 10 °C higher than that in Montreal, sometimes even more, although the French capital is further north than the city of Quebec.
Climate system in danger
For some years, the scientific community has been scrutinising changes in sea level with growing concern. After several millennia of stability, the world’s oceans have been rising since the 1900s. This rise, in the region of 1.7 mm per year during the 20th century, is accelerating and currently exceeds 3.2 mm per year according to the satellites. These are the only tools really able to measure the global ocean, with their data confirmed by the worldwide network of tide gauges, instruments attached to the seabed which measure the depth of the water.
The explanation for this worrying trend comes down to two words: global warming. And although the details of the calculation and measurements are complex, the basic mechanisms of the rise are easy to understand. There are two processes at work. The first is thermal expansion: heating the ocean increases its volume (as with most solids, liquids or gases). The ocean has now absorbed most of the global warming and is therefore in the process of expanding, causing its levels to rise. The second process: the melting of continental ice. Enormous quantities of water are effectively immobilised in the form of ice in the two large polar ice sheets (Greenland and the Antarctic) and in the thousands of continental glaciers distributed around our planet, from Siberia to the Himalayas by way of the Alps or the Andes – collectively known as the “cryosphere”. However, within the cryosphere, only that part of the ice sheet east of the South Pole still appears to remain stable: the vast majority of glaciers on land (over 90%) are in the process of melting, just like the Greenland sheet and the west Antarctic, resulting in a rise in the level of the ocean, which is the end recipient of all this water. If this rise continues at its current rate, there will undoubtedly be few major problems before the end of this century.
However, in view of the rapid decline of the cryosphere, and particularly the accelerated melting of the Greenland and west Antarctic glaciers, experts expect this rate to accelerate significantly. The opinion now predominant in the scientific community is that sea levels will rise by over one metre during the 21st century, possibly by a significant amount.