Flipping the cost of tipping? Economic impacts of reduced AMOC carbon drawdown

Social cost of carbon (SCC) research has paid increasing attention to climate tipping points and feedback mechanisms. The weakening of the Atlantic Meridional Overturning Circulation (AMOC) is currently treated as a global benefit, as it would lower Northern Hemisphere surface temperatures and thereby offset parts of global warming and its associated economic damages. We add to the literature on economic impacts of AMOC weakening by, for the first time, adding a second impact channel which acts through carbon cycle changes. A weaker AMOC directly leads to a reduced export of carbon-rich surface waters to the deep ocean, such that, conversely, more carbon remains in the atmosphere and acts to increase global temperatures and associated economic damages. By drawing on carbon cycle feedback and freshwater hosing experiments, we provide climate modelling evidence on the magnitude of this AMOC-induced carbon feedback, and develop an emulator with which to include these estimates into a simple integrated assessment model. Based on these IAM calculations calibrated to ESM results, we find that carbon cycle feedbacks lead to an SCC increase of around 1%, which is in the same order of magnitude as the SCC decrease caused by AMOC-induced temperature changes. Taking into account this carbon effect could thus flip the overall economic effect of AMOC weakening from a net benefit into a net cost.