What will happen to Europe if the Gulf Stream weakens significantly?

The Atlantic Meridional Overturning Circulation (AMOC), of which the Gulf Stream is a part, appears to be more unstable than previously thought. If the Gulf Stream weakens, Europe will become the odd one out on a warming planet: it will warm less than other regions, or may even become colder, especially in winter. And this will happen while the rest of the world continues to warm. How this could change the climate in Europe has been investigated in a new study.

The AMOC is the large-scale circulation in the Atlantic Ocean that plays a crucial role in regulating both the global and European climate  by redistributing heat through the ocean. The most recent IPCC report from 2021 assessed the risk of a collapse of the Atlantic Meridional Overturning Circulation (AMOC) this century as low. However, there is growing concern that the Atlantic Ocean circulation may be more sensitive than previously believed.  

As rainfall increases and the Greenland ice sheet continues to melt, the water in the North Atlantic becomes less salty, and therefore less dense. Under normal conditions, cold, salty water near Greenland sinks into the deep ocean, driving the AMOC. But as the water becomes lighter due to freshening, this sinking process weakens, disrupting the entire system. This weakening is being driven by climate change.

What this means for Europe

A new study by researchers from the Royal Netherlands Meteorological Institute (KNMI) and Utrecht University has examined how Europe’s climate could change if the AMOC were to substantially weaken under future climate. They use the same complex climate model to simulate a range of scenarios in which the circulation system substantially weakens by more than 80%, while also including the effects of global warming. 

The researchers simulated different levels of freshwater input into the North Atlantic, ranging from small to large, to test how sensitive the AMOC system is. They also considered two future emissions scenarios: a moderate one (RCP4.5) and a high one (RCP8.5). These scenarios project global warming by the year 2100 of 2.7°C and 5°C, respectively, compared to the late 19th century.

René van Westen, lead author of the study, said: “This study is the first to use a climate model of this complexity to examine in detail the potential temperature impacts on the European climate of an AMOC collapse scenario triggered by global warming.”

Expanding Arctic sea-ice pack

In one intermediate global warming scenario (RCP4.5), this still resulted in regional cooling over Europe due to a weakened circulation system. The Arctic sea-ice pack could expand southward and reach parts of Northwestern Europe, covering parts of Britain, Scandinavia and the Netherlands. Michiel Baatsen, the co-author of the study, explains: “Since sea ice reflects much more sunlight than the darker ocean surface, this expansion amplifies the cooling. As a result, winter temperatures in those regions could plummet.”

In the high emissions scenario (RCP8.5), although the AMOC is more likely to collapse, the impact on European temperatures is less dramatic. That’s because sea ice wouldn’t be able to expand as far south due to warmer ocean temperatures caused by higher greenhouse gas concentrations. So, in this case, the extreme cold would be less severe than in the scenario where sea ice reaches Europe’s coast. The climate impacts on winter temperatures in Northwestern Europe vary significantly depending on the scenario.

Since sea ice reflects much more sunlight than the darker ocean surface, this expansion amplifies the cooling. As a result, winter temperatures in those regions could plummet.

Cold winters

If the circulation weakens, Northwestern Europe would experience the most pronounced cooling. Under global warming of +2°C, a cold extreme that currently occurs once every ten years in the Netherlands could drop to -20°C — around fifteen degrees colder than in the pre-industrial climate, according to the same climate model. In Scotland the cold extreme could reach -30°C, a full 23 degrees colder than in the late 19th century.

“The relatively mild climate for a city such as Edinburgh would see drastic changes. For example, it would experience 164 days with minimum temperatures below zero, that’s almost 50% of the year, and an increase of 133 days compared to the pre-industrial climate,” said Baatsen. “The Scandinavian climate would become much colder under such a scenario, with even Norway’s typically mild west coast potentially experiencing extremes below -40°C — a drop of 25 degrees compared to the pre-industrial climate,” van Westen added. 

Other climate changes

Southern Europe’s climate is less influenced by a weakened circulation. The temperature contrast between northern and southern Europe would increase, particularly in winter. This would intensify pressure differences and could lead to stronger winter storms and greater day-to-day temperature variability.

Beyond temperature effects, previous studies show that a weaker circulation system would also lead to less precipitation in Europe, along with faster sea-level rise in the Atlantic Ocean. Clearly, changes to the AMOC could trigger significant shifts in the global and regional climate.

The Scandinavian climate would become much colder under such a scenario, with even Norway’s typically mild west coast potentially experiencing extremes below -40°C — a drop of 25 degrees compared to the pre-industrial climate

Initial estimate

This study provides an initial estimate of the potential temperature effects of a weakened Atlantic Ocean circulation under various climate change scenarios. The results depend on the specific climate model used (the CESM) and the method used to induce circulation changes — in this case, the addition of freshwater to the North Atlantic, which makes the ocean circulation more sensitive to weakening under climate change. Similar experiments with other climate models, and ideally withouth the aditional freshwater input, are needed to assess the European temperature impacts under a weaker Atlantic Ocean circulation. Climate models do project a consistent weakening of the Atlantic Ocean circulation as global warming intensifies.