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

Climate models show large differences in simulating Atlantic Ocean circulation and associated carbon uptake. Here the authors use sea surface salinity as an emergent constraint of these processes to show that Northern Hemisphere surface temperatures warm more slowly than expected. 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.

Automated summary

Climate models vary in simulating Atlantic Ocean circulation and carbon uptake. This study uses sea surface salinity (SSS) as a constraint to show that Northern Hemisphere surface temperatures warm slower than expected. The present-day SSS in the North Atlantic subpolar region significantly influences future carbon uptake, reducing uncertainty in warming and carbon accumulation.