A decline in atmospheric CO2 levels under negative emissions may enhance carbon retention in the terrestrial biosphere

Abstract

Abstract Negative emissions are a key mitigation measure in emission scenarios consistent with Paris agreement targets. The terrestrial biosphere is a carbon sink that regulates atmospheric carbon dioxide (CO 2 ) concentration and climate, but its role under negative emissions is highly uncertain. Here, we investigate the reversibility of the terrestrial carbon cycle to idealized CO 2 ramp-up and ramp-down forcing using an ensemble of CMIP6 Earth system models. We find a strong lag in the response of the terrestrial carbon cycle to CO 2 forcing. The terrestrial biosphere retains more carbon after CO 2 removal starts, even at equivalent CO 2 levels. This lagged response is greatest at high latitudes due to long carbon residence time and enhanced vegetation productivity. However, in the pan-Arctic region, terrestrial carbon dynamics under negative emissions are highly dependent on permafrost processes. We suggest that irreversible carbon emissions may occur in permafrost even after achieving net-zero emissions, which offsets ~30% of enhanced land C retention and could hinder climate mitigation.