Continued global warming after CO2 emissions stoppage

Recent studies have suggested that global mean surface temperature would remain approximately constant on multicentury timescales after CO2 emissions1–9 are stopped. Here we use Earth system model simulations of such a stoppage to demonstrate that in some models, surface temperature may actually increase on multi-century timescales after an initial century-long decrease. This occurs in spite of a decline in radiative forcing that exceeds the decline in ocean heat uptake—a circumstance that would otherwise be expected to lead to a decline in global temperature. The reason is that the warming effect of decreasing ocean heat uptake together with feedback effects arising in response to the geographic structure of ocean heat uptake10–12 overcompensates the cooling effect of decreasing atmospheric CO2 on multi-century timescales. Our study also reveals that equilibrium climate sensitivity estimates based on a widely used method of regressing the Earth’s energy imbalance against surface temperature change13,14 are biased. Uncertainty in the magnitude of the feedback effects associated with the magnitude and geographic distribution of ocean heat uptake therefore contributes substantially to the uncertainty in allowable carbon emissions for a given multicentury warming target. A large body of studies using simplified climate models1–4,6–8 and more sophisticated Earth system models5,9 find that global mean surface temperature stays roughly constant for a couple of centuries at the value attained when carbon emissions are stopped. These studies suggest that the cooling effect of reduction in radiative forcing R due to the decrease in atmospheric CO2 is roughly balanced by the warming effect of reduction in ocean heat uptake N , such that the difference R−N remains approximately constant7. This effect is a consequence of the fact that: the ocean heat and carbon uptake are both controlled in large part by the physical mixing of shallow oceanic waters into the deeper oceans; and under higher atmospheric CO2, the reduction of the radiative forcing sensitivity to atmospheric CO2 is roughly compensated by the higher airborne fraction of anthropogenic CO2. In this paper we show that feedback effects associated with the magnitude and geographical distribution of ocean heat uptake can lead to increasing temperatures, even if the differenceR−N decreases. We performed multi-century simulations using the Geophysical Fluid Dynamics Laboratory Earth System Model15,16 (GFDL ESM2M) and the National Centre for Atmospheric Research Climate System Model17,18 (NCAR CSM1; Methods). Both models are forced with a 1,800GtC pulse so that the atmospheric CO2 concentration is instantaneously quadrupled frompreindustrial conditions. Both models simulate a rapid atmospheric