28 Sep 2023
 | 28 Sep 2023
Status: this preprint is currently under review for the journal BG.

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

Matthew A. Chamberlain, Tilo Ziehn, and Rachel M. Law

Abstract. 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.

Matthew A. Chamberlain et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2023-146', Andrew MacDougall, 06 Nov 2023
  • RC2: 'Comment on bg-2023-146', Anonymous Referee #2, 01 Dec 2023

Matthew A. Chamberlain et al.

Matthew A. Chamberlain et al.


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Short summary
This manuscript explores the climate processes that drive increasing global average temperatures in Zero-Emission Commitment (ZEC) simulations despite decreasing atmospheric CO2. The ACCESS-ESM1.5 shows the Southern Ocean to continue to warm locally in all ZEC simulations. In ZEC simulations that start after the emission of more than 1000 Pg of carbon, the influence of the Southern Ocean increases the global temperature.