Continued permafrost ecosystem carbon loss under net-zero and negative emissions.

The loss of ecosystem carbon (the sum of vegetation, litter, and soil carbon) may occur in a permafrost region under mitigation pathways, which could reduce the efficiency of carbon dioxide removal. Here, we investigate changes in permafrost under net-zero and negative emissions, based on idealized emission-driven simulations using a state-of-the-art Earth system model. While acting as a net ecosystem carbon sink during most of the positive emission phase, permafrost becomes a net ecosystem carbon source just before reaching net-zero and negative emissions. Permafrost slowly recovers, especially in regions with high organic carbon content, and net ecosystem carbon loss persists until the end of simulations, resulting in a cumulative net ecosystem carbon loss of approximately 14 petagrams of carbon (PgC) in both scenarios. In addition, methane emissions increase under net-zero and negative emissions, due to the irreversibility of the inundated areas. We conclude that the permafrost ecosystem carbon loss may continue under net-zero and negative emissions, which could hinder climate change mitigation efforts.