Atlantic sea current system is slowing down due to climate change. And the result, a « cold drop » off Greenland, will probably aggravate the disruption of the European climate
At the end of June, France was suffocating under a heat dome. On June 21, the average temperature on the surface of the oceans was around 21°C, above previous records. However, in the south of Greenland, a pocket of water resisted, alone, the planetary furnace: the « cold blob », or « cold spot » of the North Atlantic. This paradox is not a weather whim. It is even the symbol of a problem that affects the AMOC, an underwater current. And its slowdown could redraw the European climate.
What is the AMOC? This is the English acronym for « Atlantic turning meridian circulation ». It designates a vast system of ocean currents, which acts as a « rolling belt »: hot water from the tropics circulates on the surface to the north, cools, falls to the bottom of the oceans and travels in the other direction. This movement helps to regulate the climate, especially in Europe.
However, it seems that the treadmill is slowing down. On April 8, a team of researchers published in the journal Science Advances an upward revised estimate of a weakening of the AMOC: it projects a loss of about 50% of the force of the current by the end of the century. Worrying, even if the hypothesis of a complete stop still remains to be nuanced. In its sixth report, the IPCC considered a complete collapse of the AMOC in the 21st century « very unlikely ».
The climate of Boston in Western Europe
For Davide Faranda, CNRS research director at the Pierre-Simon-Laplace Institute and Paris-Saclay University, this observation still joins the consensus that has been emerging for several years. « Observents and models converge towards the same observation: the AMOC has probably weakened since the mid-20th century and is expected to continue to slow down during the 21st century under the effect of global warming, » he notes. But precisely quantifying this decline remains a challenge. « Direct measurements cover only about twenty years and reconstructions of the past still contain uncertainties, » adds the researcher.
For France, a marked weakening of the arrival of hot waters would not cancel global warming. On the contrary, it could make it more contrasting. « Summers would continue to be very hot under the effect of global warming, while winters could occasionally become colder in some regions, » explains Davide Faranda. It evokes a climate close to that of New York or Boston, where hot and humid summers rub shoulders with much more harsh winters than in Western Europe.
Effects of the slowdown in the AMOC to be anticipated in the long term
Is this famous cold spot, regularly pointed out, proof of the slowdown? It is « cohistent, » replies the researcher, « since lower circulation transports less hot water to high latitudes. » It is in addition to other signals compatible with a slowdown, without any isolated weather event that can yet be directly attributed to the slowdown of the AMOC. « The effects are mainly manifested on long-term climate trends, » says Davide Faranda.
However, this change is not inevitable. « The more limited global warming is, the more the risk of a significant weakening or a possible changeover decreases, » insists Davide Faranda. The two mechanisms that cause the slowdown in currents, the warming of the ocean and the arrival of fresh water from the melting of Greenland, depend directly on greenhouse gas emissions. But if a point of no return were to be crossed, some changes would become very difficult to reverse for several centuries.
source : 20 minutes
