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

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 (CO2) concentration and climate, but its role under negative emissions is highly uncertain. Here, we investigate the reversibility of the terrestrial carbon cycle to idealized CO2 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 CO2 forcing. The terrestrial biosphere retains more carbon after CO2 removal starts, even at equivalent CO2 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.