Assessing the lifetime of anthropogenic CO₂ and its sensitivity to different carbon cycle processes

Abstract. Although it is well-established that anthropogenic CO₂ emitted into the atmosphere will persist for a long time, the duration of the anthropogenic climate perturbation will depend on how rapidly the excess CO₂ is removed from the climate system by different biogeochemical processes. The uncertainty around the long-term climate evolution is therefore not only linked to the future of anthropogenic CO₂ emissions, but to our insufficient understanding of the long-term carbon cycle. Here, we use the fast Earth system model CLIMBER-X, which features a comprehensive carbon cycle, to examine the lifetime of anthropogenic CO₂ and its effects on the long-term evolution of atmospheric CO₂ concentration. This is done through an ensemble of 100,000 year long simulations, each driven by idealized CO₂ emission pulses. Our findings indicate that, depending on the magnitude of the emission, 75 % of anthropogenic CO₂ is removed within 197–1,820 years after emissions end. Approximately 4.3 % of anthropogenic CO₂ will remain beyond 100 kyr. We find that the uptake of carbon by land, which has only been marginally considered in previous studies, has a significant long-term effect, storing approximately 4–13 % of anthropogenic carbon by the end of the simulation. For the first time, we have quantified the effect of dynamically changing methane concentrations on the long-term carbon cycle, showing that its effects are likely negligible over long timescales. The timescale of carbon removal via silicate weathering is also reassessed here, providing an estimate (80–105 kyrs) that is significantly shorter than some previous studies due to higher climate sensitivity, stronger weathering feedbacks, and the use of a spatially explicit weathering scheme, leading to a faster removal of anthropogenic CO₂ in the long-term. Our study highlights the importance of adding model complexity to the global carbon cycle in Earth system models, as to accurately represent the long-term future evolution of atmospheric CO₂.