Céline Heuzé , Martin Mohrmann, Ellen Andersson, Emelie Crafoord

Deep water formation is not only the driver of the global ocean circulation; by sending heat and carbon to the deep ocean, it is also crucial for climate change mitigation. Yet its future is uncertain: will it slow down as stratification increases, emerge in polar regions as the wind starts blowing over previously ice-covered waters, or intensify with increased evaporation? Here we present the first global study of the evolution of deep water formation as atmospheric CO2 concentration increases, using the latest generation of Earth System models (CMIP6). We show that open ocean deep convection stops globally shortly before 600 ppm, mostly in response to increased stratification, but that deep water formation continues under a different regime. Deep convection does not emerge in ice-freed regions. The mechanism is self-reinforcing, as less mixing also increases stratification and modifies heat fluxes, with most oceanic regions gaining even more heat.