Weaker ocean currents could cost trillions

By weakening the North Atlantic overturning circulation, the ocean absorbs less carbon dioxide - resulting in consequential costs and further warming of the climate

The ocean is the largest CO₂ reservoir on earth. Cold seawater, including the water of the Arctic Ocean, is particularly absorbent. The problem is that the North Atlantic overturning current, a powerful driver of the global climate that also includes the Gulf Stream, is weakening. This current would actually transport the water enriched with CO₂ southwards. This means that more of the greenhouse gas remains in the atmosphere, which in turn exacerbates global warming. A global modeling of the climate and economic effects now shows that the slowdown could result in follow-up costs of several trillion euros by 2100. These findings once again dampen hopes of the supposedly positive effects of a Gulf Stream collapse: even if Europe were to cool down as a result of a weaker Gulf Stream, the global climate would continue to heat up.

The North Atlantic Overturning Circulation - Amoc for Atlantic Meridional Overturning Circulation - channels warm water from the tropics to the north and lets cold water flow back to the south. In this way, it acts as a radiator for Europe’s climate. The fact that it threatens to lose strength in the course of climate change has so far been seen as beneficial by economic research, as this could cool the northern hemisphere in times of global warming. -However, the Amoc slowdown would further accelerate climate change," explains Felix Schaumann, PhD student in sustainability economics at the University of Hamburg and the Max Planck Institute for Meteorology.

The CO₂ content of the atmosphere is rising

It was already known that the melting ice in the Arctic is releasing huge quantities of fresh water into the oceans. The diluted and less salty seawater is less dense and no longer sinks as easily, which is likely to slow down the Amoc. The ocean is also the largest carbon reservoir on earth. It absorbs more carbon than the atmosphere and organisms on land. The cold waters of the Arctic Ocean are particularly absorbent. Schaumann and his colleague Eduardo Alastrué de Asenjo have now been able to demonstrate this: Due to a weaker Amoc, less carbon dioxide (CO₂) is also transported from the sea surface to the depths of the ocean. This means that more CO₂ remains in the atmosphere and this increases global warming.

Climate warming continues

-Our results suggest that previous studies on AMOC mitigation have probably underestimated the consequences," says Schaumann.

From a global perspective, more frequent and more extreme weather events such as heat, drought and flooding would occur as climate change continues to accelerate. This would increase the social costs of CO₂. These costs reflect the damage caused by the emission of CO₂. This effect could outweigh the positive economic effects of the cooling caused by Amoc mitigation.

The social costs

The results are based on a global climate model combined with a further model specialized in economic consequential costs. This calculates the consequential costs of climate damage based on the expected CO₂ content of the atmosphere. The researchers used this to create a scenario that assumes CO₂ development without the Amoc effect and another that includes different degrees of mitigation. They were thus able to show that there is a direct correlation between the strength of the Amoc and the amount of carbon that the oceans absorb.

In the run-up to the World Climate Conference: an interview with Jochem Marotzke, Director at the Max Planck Institute for Meteorology in Hamburg