Present-day North Atlantic salinity constrains future warming of the Northern Hemisphere

Abstract Earth system models exhibit considerable intermodel spread in Atlantic Meridional Overturning Circulation intensity and its carbon uptake, resulting in great uncertainty in future climate. Here we show that present-day sea surface salinity (SSS) in the North Atlantic subpolar region modulates anthropogenic carbon uptake in the North Atlantic, and thus can be used to constrain future warming. Specifically, models that generate a present-day higher SSS in the North Atlantic subpolar region generate a greater uptake of anthropogenic carbon in the future, suppressing the greenhouse effect and resulting in slower warming, and vice versa in models with a present-day lower SSS. Emergent constraints based on the observed SSS greatly reduce the uncertainty of the Northern Hemisphere surface temperature warming and accumulative carbon uptake by about 30% and 53%, respectively, by the end of the twenty-first century under the Shared Socioeconomic Pathways 5–8.5 scenario.