A study conducted with satellite data from 1985 to 2023 recorded a decline in heat released from the ocean to the atmosphere: the AMOC has slowed by 14% in thirty years, with serious risks for mild winters in Europe.
The system of ocean currents that mitigates the European climate – the AMOC (acronym for Atlantic southern overturning circulationmeridional overturning of the Atlantic circulation) is weakening: this time the proof comes… from above, and not from measurements taken on the surface or in the depths of the oceans.
A study based on satellite observations has observed a reduction in heat exchange between Atlantic waters and the Earth’s atmosphere over the last 40 years. This heat release, of which the AMOC circulation is the engine, is the reason why winters in Western Europe are much milder than those of Canada and North America.
The analysis, awaiting review and in preprint on Research Squareis considered by some to be some of the most direct and convincing evidence of the weakening of the AMOC, a conveyor belt of warm surface water from the Tropics across the Atlantic, and cold deep water from the North Atlantic southward.
AMOC: how it works and the effects on the planet’s climate
The AMOC circulation is driven by differences in water temperature and salinity, which create differences in density: we can think of it as a “hydraulic pump” that collects warm, salty water from the Tropics and transports it towards the North Atlantic Ocean, where the water cools and becomes denser, sinking deeper and moving south again, to start the cycle again.
This system distributes heat throughout the planet and is an important regulator of the Earth’s climate: when water cools in the North Atlantic, it releases part of the thermal energy to the atmosphere, mitigating the climate of Western and Northern Europe. The AMOC is the reason for the difference of about 11 °C between the average winter temperatures of Rome and Chicago, which are also located at similar latitudes.
The hypothesis of a slow collapse of the AMOC
The enormous input of fresh water into the North Atlantic due to the melting of Greenland’s ice is diluting the waters of the North Atlantic, making them less salty and less dense, slowing down their sinking and thus weakening the entire AMOC mechanism.
According to some models, the continuous injection of fresh water into the Atlantic could slow down this vital current until it collapses, with potentially disastrous consequences for Europe’s climate, which would face drastic and rapid cooling, and for the world’s climate.
Less heat to give off
The new study, by scientists at the University of Reading in the UK, is the first to suggest a slowdown of the AMOC by measuring heat fluxes detected by satellites. Between 1985 and 2023, heat lost from the Atlantic Ocean would have decreased by 18%. After estimating the average flow of the AMOC per decade by comparing data from the first decade to subsequent ones, the researchers concluded that the AMOC slowed by 14% over 30 years.
A data in line with those that emerged in other studies carried out with different methods, for example by measuring surface ocean temperatures or variations in water pressure on buoys distributed in the North Atlantic.
Since satellites can measure the evolution of a trend over time, if validated, the new method could serve to monitor the “vital parameters” of the AMOC, complementing measurements of ocean surface temperature.
