The Gulf Stream that stops: this is the scenario of the film The next day, released in 2004. In this blockbuster, the arrest of the ocean currents of the North Atlantic is at the origin of a series of catastrophes. While this scenario is greatly exaggerated, the fear of a significant slowdown in ocean currents in the Atlantic linked to global warming has been a cause for concern for scientists for several decades. A published study February 25 in Nature Geoscience indicates that these currents are already slowing down, having reached their lowest level in at least a millennium.
- The circuit of the great Atlantic current, called the Southern Atlantic Overturning Circulation (AMOC).
Oceans are not inert water bodies. They are constantly moved by the sea currents, which bind the oceans together. This large-scale continuous movement, called thermohaline circulation or MOC (southern overturning circulation), plays a very important role in regulating the climate system. “ The ocean transfers enormous amounts of water, carbon and heat from the surface to the depths through the thermohaline circulation. It therefore really plays the role of a thermostat at the level of the global climate. “, explains a Reporter Jean-Baptiste Sallée, researcher at laboratory THE OCEAN of the Sorbonne University of Paris.
The stability of this thermostat is therefore a good indicator when trying to understand past climate changes, to try to anticipate those to come. “ What the study shows us, which is based on the observation of several indirect factors, is that the Atlantic thermohaline circulation [Amoc] it has been very stable for about a millennium. But in the past century there has been a marked reduction in these Atlantic currents. “, says Jean-Baptiste Sallée. The planet’s climate thermostat would then be in motion, probably under the effect of global warming.
- The melting of the glaciers contributes to the slowing of the Amoc.
The difference in density between surface water and deep water drives the thermohaline circulation. The denser the surface water, the more it can immerse and feed the thermohaline circulation. Deep waters have a density of about 1,050 kg / m3 against 1,029 for surface waters near the poles and 1,020 for tropical waters. However, global warming will cause a decrease in the density of surface water in the oceans. On the one hand, surface waters that are warmer due to warming will also be less dense due to their thermal expansion. Additionally, melting glaciers and ice packs and changing precipitation patterns at high latitudes will inject large quantities of freshwater into the ocean’s surface waters. This addition of fresh water will make the water less salty and therefore less dense.
What consequences would the accentuation of the slowdown in Amoc have? ? Bringing the softness and humidity of the ocean, the Amoc deeply rules the climate of the continents. A significant slowdown would have very negative consequences on a global scale. “ In the Sahel region, a decrease in the Amoc would lead to a significant drop in rainfall, with significant anthropogenic repercussions in this densely populated region. “, She said Reporter Didier Swingedouw, researcher CNRS at the University of Bordeaux.
“ In northern Europe, a decrease in the Amoc translates into an increase in the frequency of storms and a rise in sea level in North America and Europe. It will also produce a significant decline in marine organic production. “, says Jean-Baptiste Sallée. “ It is also possible that in Europe the contrast between the seasons is more marked, with more severe winters and very hot summers. “adds Didier Swingedouw.
But this slowdown is certain ? For Didier Swingedouw the question is rather to know in what proportion the Amoc will decrease: “ We know that a decrease in the Amoc is very likely. Rather, it is a question of quantifying the extent of this decrease and its time horizon. “
Source: Fabien Houy-Delalande for Reporter
Photo: NASA satellite image of Amoc. Eurekalert