Global warming continues to have surprises in store for us. The latest one? An imminent risk of seeing the collapse of one of the main drivers of our climate: the Atlantic Southern Overturning Circulation (AMOC). This gigantic ocean treadmill, which transports heat from the tropics to Europe, could indeed stop, which would plunge many regions of the globe into climatic chaos. The consequences would be particularly dramatic for tropical regions where monsoons, vital for agriculture and water supplies, would be radically disrupted for at least a hundred years.
AMOC, a climate engine in danger
The AMOC (or Atlantic Southern Overturning Circulation) is much more than a simple “air conditioner”: it is a complex system of ocean currents that plays a crucial role in regulating the global climate. Think of it as a gigantic liquid treadmill which transports heat and nutrients across the Atlantic Ocean.
The AMOC is driven by the difference in water density: the warm, salty waters of the tropical Atlantic, less dense, rise north to the surface, while the cold, denser waters of the Arctic plunge into the depths. This incessant movement, comparable to an immense pump, redistributes heat on the surface of the planet and influences precipitation patterns.
This fragile balance is, however, undermined by the global warming. The accelerated melting of glaciers and ice caps, particularly in Greenland, is leading to a massive influx of fresh water into the North Atlantic. This fresh water, less dense than salt water, dilutes surface water and thus slows down the diving process of cold water. It is then as if we poured fresh water into a glass of salt water: the density decreases and the water tends to remain on the surface. Similarly, the influx of fresh water into the North Atlantic reduces the density of surface waters, thereby weakening the engine of the AMOC.
Devastating consequences for tropical monsoons
The consequences of a collapse of the AMOC would be considerable, particularly for Europe. Recently, however, a study focused more on tropical regions. THE monsoonsthese seasonal winds which bring abundant rain, are in fact closely linked to ocean circulation. The heat transported by the AMOC influences ocean temperatures and therefore the formation of intertropical convergence zones (ITCZs) where precipitation is concentrated.
If the AMOC weakens, heat transported northward will decrease, which will then cause the North Atlantic to cool. The ITCZ will then move southwards, thus modifying precipitation patterns in many regions of the globe. According to scientists, rainy seasons could become shorter and less intense in West Africa, India and East Asia, which would jeopardize agriculture and the water supply of millions of people. In addition, the Amazon rainforest, the lungs of the planet, could experience a delayed and less abundant rainy season with potentially disastrous consequences for this fragile ecosystem.
The researchers also pointed out that the effects of an AMOC collapse could be irreversible for at least a century. Even if greenhouse gas emissions were drastically reduced, it would take a long time for the climate system to return to balance.
An irreversible shift on the horizon?
Scientists also warn of a critical tipping point that could be reached sooner than expected. According to some recent modeling, the collapse of the AMOC could occur as early as the middle of this century if greenhouse gas emissions continue to grow at the current rate. This pessimistic scenario could precipitate sudden and unpredictable climate changes, affecting not only tropical regions, but also marine ecosystems, rising oceans and the stability of atmospheric currents. This shift would not only be a challenge for science, but also an urgent call to review global climate policies to mitigate the impacts of such a catastrophe.
