Ocean heat uptake and storage during climate stabilization at different global warming levels in GFDL-ESM2M

The ocean is storing the majority of excess heat in the Earth system resulting from the release of anthropogenic greenhouse gases into the atmosphere. This heat uptake will persist even after cessation of greenhouse gas emissions, and it will continue for centuries in scenarios where global warming is limited to levels set by the Paris Agreement. This continued heat uptake has important implications for regional climate, ecosystems and sea level rise. However, the dynamics of ocean heat uptake and the redistribution of this heat under stabilized global warming remain poorly understood, particularly the time scales involved. Here, we apply the Adaptive Emission Reduction Approach to a fully coupled Earth System Model to simulate different levels of stabilized global warming until the year 3000. We reveal significant differences between the transient phase when surface temperatures first reach the targeted warming level, and the near-stabilized state after close to 1000 years at the warming level. We explore non-linearities in the evolution of the ocean circulation and ventilation over these time scales, as well as the sensitivity of heat uptake and storage to different global warming levels. For example, the stabilization simulations reveal long-term differences across global warming levels in the vertical redistribution of heat, with a relatively warmer upper ocean and colder deep ocean with warmer surface temperatures. We also find a threshold effect between 1.5ºC and 2ºC of global warming, where surpassing this threshold triggers irreversible changes that profoundly impact the redistribution of heat in the ocean. Specifically, during the stabilization phase at 2ºC of global warming and above, the subpolar Southern Ocean shows a recovery of deep convection that leads to an export of colder bottom waters than under pre-industrial conditions, that is not present at 1.5ºC.