As Arctic ice melts, massive amounts of freshwater flow into the ocean. The seawater becomes diluted and less saline, a development that reduces its density and causes it to sink more slowly—and one that will likely slow AMOC.
Now, Schaumann and his colleague Eduardo Alastrué de Asenjo, from the Max Planck Institute for Meteorology and the Climate Modeling group of the University of Hamburg, have confirmed: as a result of this process, less CO₂ is transported from the surface ocean to the deep ocean. More CO₂ remains in the atmosphere, which accelerates global warming.
“Our findings indicate that previous studies on AMOC weakening most likely underestimated the economic impacts,” says Schaumann. In global terms, the acceleration of climate change would produce more frequent and extreme weather events like heat waves, droughts and floods, leading to an increase in the social cost of carbon. This cost represents the damage caused by additional CO₂ emissions, and the increase in the social cost of carbon could offset the economic benefits of the cooling that accompanies a weaker AMOC.