The Role of Carbon Cycle Feedbacks in the Land and Ocean Response to Zero Emissions

The zero emissions commitment (ZEC) – change in global average temperature following a cessation of emissions – is determined by inertia in both physical and biogeochemical components of the climate system. The ZEC is commonly quantified from fully coupled model simulations in which the land and ocean respond to changes in both climate and atmospheric CO2 concentration. As a result, the role of carbon cycle feedbacks in zero emissions (ZE) simulations has not been explored in detail. This study uses an Earth system model to analyze the role of carbon cycle feedbacks in the land and ocean response to ZE. First, the model was forced with constant emissions of 10PgC yr-1 for 100 years (esm-flat10 experiment), then a series of zero emissions simulations were initialized from different time points along the esm-flat10 trajectory (esm-flat10-zec experiment). In each simulation, emissions were immediately halted, then the system was allowed to evolve. Simulations were run in fully coupled, biogeochemically coupled and radiatively coupled modes to isolate feedbacks. When the CO2 effect is isolated, atmospheric CO2 concentration declines more rapidly relative to the fully coupled mode due to continued land and ocean uptake. This decline in atmospheric CO2 concentration reduces the rate of carbon uptake, which in turn, reduces the rate of decline in atmospheric CO2 concentration. However, when the climate effect is isolated, warming results in land and ocean carbon loss. The continued warming exacerbates carbon loss, further amplifying warming. Overall, the concentration-carbon feedback acts to stabilize carbon sinks, resulting in a smaller ZEC, whereas the climate-carbon feedback acts to exacerbate carbon loss, resulting in a larger ZEC (relative to the ZEC in the fully coupled mode). Our results indicate that carbon cycle feedbacks are a key control on the ZEC, emphasizing the importance of disentangling and quantifying feedbacks in net-zero emissions pathways.