Marine currents, the regulators of the climate

Life on Earth cannot exist without the ocean. Every other breath is made possible by the oxygen produced by oceanic plant plankton. The ocean, the gigantic volume of water in perpetual motion that covers 71% of the globe, determines the climatic conditions that prevail on the Earth's surface. The ocean has absorbed about 25% of the CO2 emitted into the atmosphere by human activities. By absorbing and storing the main greenhouse gas, the ocean is playing a fundamental role in regulating the climate.

The ocean and the atmosphere together form a gigantic natural thermal machine. In effect, the ocean and the atmosphere are constantly exchanging gases, water and heat, which the ocean redistributes around the globe through marine currents.

These exchanges determine the climate around the world.

The ocean warms by absorbing solar energy and cools by evaporation and by releasing heat into the atmosphere.

However, as water has a high thermal inertia, it heats up and cools down very slowly. This means that the world's oceans store enormous quantities of heat that they release over very long periods of time.

Heat is redistributed across the planet by powerful currents that flow from the bottom to the surface and from one region to another.

Marine water moves between the equator and the polar regions, and between the surface and the depths. Vast masses of water are pushed by the currents, which create immense "rivers" in the sea.

These oceanic movements, whether horizontal or vertical, are determined by:

• the winds,
• forces related to the rotation of the Earth
• and differences in density or temperature between the bodies of water.

Through the surface currents, the heat accumulated by the sea in the tropics will warm up the polar zones, thereby reducing the differences in temperature.

In the North Atlantic, the Gulf Stream carries heat from the tropics to Europe. It then splits into two branches that flow northwards where the water cools. Sea ice forms and salt is released into the seawater.

These waters are more salty, colder and denser and sink to the bottom. They then flow back towards the equator and after a long journey around the planet, they come back up to the surface.

The waters only sink to the depths in some very specific points around the globe, which are mainly located in the North Atlantic and the Southern Ocean. The largest source is situated in the North Sea, between Greenland and northern Europe.

"We know more about the surface of the Moon than we do about the seabed!"

This lack of knowledge does not mean that it is not coveted, as the potential of the high seas is immense: such as living resources, mineral resources, new molecules.

Even if it is clearly understood that the ocean's resources are not infinite, the IUCN estimates that there are between 500,000 and 100 million species living in the oceans and only about 250,000 species have been recorded.

Moreover, the deep sea, such as the abyssal plains or the deep ocean trenches, are extreme places where marine life can nevertheless thrive, particularly around hydrothermal vents, where temperatures can reach 400 degrees.

The ability of species to adapt is a clear indication of the immense biological diversity that exists in the high seas.

By 2030, the Organisation for Economic Co-operation and Development (OECD) predicts that profits generated at sea will exceed those generated on land.

The ocean is the primary source of protein for 3 billion people and 90% of goods are transported by sea!

• The high seas are used for fishing, for transporting 90% of international trade, and for laying the submarine cables through which our telephone and Internet communications pass.

• Engineers seek to harness the tremendous power of waves and currents to provide clean, renewable energy.

• The deep seabed is home to minerals and rare earths that are essential to our technologies and are already being explored.

• The aim of the biodiversity census is to discover new molecules that will be the source of medicines and innovation, particularly in the chemical industry. The number of patents derived from marine genetic resources is substantially increasing.

Antarctica also plays an important role. Along the coasts, near Adelie Land, there is an area without any ice where warmer water flows upwards, exchanges heat and forms cold water that sinks to the bottom.

The Antarctic Circumpolar Current (ACC), which is the most powerful, largest and fastest moving current on the planet, agitates the waters of three oceans: the Atlantic, Indian and Pacific. It is 20,000 km long, 200 to 100 km wide and can go down to depths of 4,000 m. It carries 130 million m3 per second and has a surface speed of 0.9 to 3.7 km/h.

Due to its geographical position and the particularities of the ACC, the Southern Ocean plays a major role in ocean circulation around the globe and in climate regulation.

Changes in water temperature and salinity (as land and polar ice melts) affect the density of the water masses, which could slow the descent of cold waters into the deep sea. The marine currents could be affected.

Changes in current and wind patterns can cause climatic disturbances such as severe storms.

In its latest Special Report on the Ocean and Cryosphere in a Changing Climate, published in 2019, the IPCC warns that melting ice and rising temperatures are altering the workings of ocean currents, which have an extremely long global cycle.

In particular, the slowing of the North Atlantic Current reduces marine productivity, causes winter storms in Europe, and reduces rainfall in the Sahel and South Asia.

The Ocean & Climate platform provides scientific fact sheets on the role of climate in climate change.