Gulf Stream System at its weakest in over a millennium

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An [Atlantic meridional overturning circulation] AMOC slowdown has long been predicted by climate models as a response to global warming caused by greenhouse gases. According to a number of studies, this is likely the reason for the observed weakening. The Atlantic overturning is driven by what the scientists call deep convection, triggered by the differences in the density of the ocean water: Warm and salty water moves from the south to the north where it cools down and thus gets denser. When it is heavy enough the water sinks to deeper ocean layers and flows back to the south. Global warming disturbs this mechanism: Increased rainfall and enhanced melting of the Greenland Ice Sheet add fresh water to the surface ocean. This reduces the salinity and thus the density of the water, inhibiting the sinking and thus weakening the flow of the AMOC.

Its weakening has also been linked to a unique substantial cooling of the northern Atlantic over the past hundred years. This so-called cold blob was predicted by climate models as a result of a weakening AMOC, which transports less heat into this region.

The consequences of the AMOC slowdown could be manifold for people living on both sides of the Atlantic as Levke Caesar explains: "The northward surface flow of the AMOC leads to a deflection of water masses to the right, away from the US east coast. This is due to Earth's rotation that diverts moving objects such as currents to the right in the northern hemisphere and to the left in the southern hemisphere. As the current slows down, this effect weakens and more water can pile up at the US east coast, leading to an enhanced sea level rise." In Europe, a further slowdown of the AMOC could imply more extreme weather events like a change of the winter storm track coming off the Atlantic, possibly intensifying them. Other studies found possible consequences being extreme heat waves or a decrease in summer rainfall. Exactly what the further consequences are is the subject of current research; scientists also aim to resolve which components and pathways of the AMOC have changed how and for what reasons.

"If we continue to drive global warming, the Gulf Stream System will weaken further -- by 34 to 45 percent by 2100 according to the latest generation of climate models," concludes Rahmstorf. This could bring us dangerously close to the tipping point at which the flow becomes unstable.