The Southern Ocean as the freight train of the global climate under zero-emission scenarios with ACCESS-ESM1.5

Climate projection experiments presented here explore how the slow response in the Southern Ocean drives ongoing global warming even with zero CO2 emissions and declining atmospheric CO2 concentrations. These projections were simulated by the Earth System Model version of Australian Community Climate and Earth System Simulator (ACCESS-ESM1.5) and motivated by the Zero Emission Commitment Multi-model Intercomparison Project. ZECMIP simulations branch from the idealised warming with the "1-percent CO2" CMIP experiment onto a trajectory of zero carbon emissions. The original ZECMIP experiments simulated the zero-emission trajectories after emitting 1000 Pg of carbon into the climate, and optionally 750 and 2000 PgC; here we show extra trajectories after 1250, 1500 and 1750 PgC, and simulate climates to 300 years after branching to demonstrate long-term trends. In each of these experiments that switch to zero emissions after emitting 1000 PgC or more, the global climate continues to warm. In the case of the experiment that branched after 2000 PgC, or after 3.5 °C of warming from a pre-industrial climate, there is 0.37 °C of extra warming after 50 years of zero emissions and further warming continues for at least several centuries. From early in the 1-percent CO2 experiment, the circulation of the Southern Ocean is modified by the warming climate which drives changes in the distribution of both physical and biogeochemical subsurface ocean tracers that are ongoing in all zero-emission branches. We replicate the global climate warming in 1-percent CO2 and ZECMIP experiments with a simple slab model that contains regions that respond with different time scales to atmospheric CO2 and climate forcing, demonstrating the global climate response is due primarily to the slow response of the ocean, the Southern Ocean in particular. In these zero emission trajectories, the simulated climate moves from a Transient Climate Response (TCR) state towards a Equilibrium Climate Sensitivity (ECS) state. Since ECS is substantially greater than TCR, the global temperature can increase while CO2 decreases.