The ocean exerts a major control on the climate response to greenhouse gas emissions by taking up a large fraction of anthropogenic heat and CO₂ from the atmosphere, thus moderating atmospheric climate feedbacks and ultimately the warming we experience at the Earth’s surface. However, projections of future climate change using coupled climate models suffer from large uncertainties due to long-standing issues in resolving crucial processes such as cloud feedback and ocean heat uptake.

In this seminar, I will present some recent results from my PhD at LOCEAN in Paris concerning the mechanisms of ocean heat uptake in the current generation of climate models (CMIP6).

Starting from an oceanic perspective, I will review the processes controlling the efficiency at which heat is transferred from the surface to the deep ocean in CMIP6 climate models. I will clarify the inter-model relations and biases in upper ocean stratification, mixed layer depth and AMOC strength, which have all been previously proposed as controls on ocean heat uptake efficiency. Adopting a more coupled climate system viewpoint, I will then present a study in which we identify a mechanistic emergent constraint across the CMIP5/6 ensemble which allows us to obtain improved projections of future global ocean heat uptake. This constraint is based on the coupled model representation of deep ocean temperatures, Southern Ocean SSTs, and shortwave cloud feedback. Using Antarctic sea ice extent as a predictor, we obtain future heat uptake projections that are larger than unconstrained CMIP6 values, contrasting with previous studies and highlighting the role of the (Southern) ocean for transient global climate change.