Irreversibility of permafrost region carbon pool changes under temperature overshoot scenarios

Most pathways that meet the Paris Agreement goal of limiting the global temperature increase to well below 2 °C above preindustrial levels will require a temporary exceedance (“overshoot”) of the target temperature and subsequent restoration of the target with net negative carbon dioxide emissions. If the target temperature is exceeded, a larger proportion of frozen soils in the northern high-latitude permafrost region is expected to thaw, releasing additional carbon into the atmosphere through microbial respiration. This study investigates whether permafrost soil carbon loss during the temperature overshoot phase is reversible if the temperature is restored to its target level. To attain this goal, we force an Earth system model of intermediate complexity that includes representation of permafrost carbon processes with a set of future scenarios with varying magnitudes and durations of cumulative CO2 emissions overshoot. Results show that high-latitude soil carbon loss and recovery in response to overshoot is dependent on peak warming and the duration of time excess warming is held. Continued decline of the permafrost region soil carbon pool following restoration of the target temperature suggests that changes are irreversible for at least several centuries.