Carbon Cycle Response to Artificial Atmospheric Carbon Dioxide Removal

Artificial removal of carbon dioxide from the atmosphere is increasingly discussed as a means to mitigate climate change, particularly in the context of meeting stringent climate targets. The efficiency of atmospheric CO₂ removal is determined by the interplay between the natural carbon sinks and atmospheric CO₂ levels. Only a few studies have explored the response of the global carbon cycle to atmospheric CO₂ removal. Here, we use an Earth System model of intermediate complexity – the University of Victoria Earth System Climate Model (UVic ESCM) - to explore the response of the carbon cycle to atmospheric CO₂ removal under a range of idealized scenarios, which differ in the total amount and rate of negative emissions, and the initial state of the system. We perform two sets of model simulations: one where a drop in atmospheric CO₂ to a target level and maintenance at that level is prescribed (P), and one where an equivalent amount of negative CO₂ emissions is prescribed over a given period of time, with atmospheric CO₂ left to evolve freely thereafter (E).

Results indicate that for both simulation sets, CO₂ outgasses from the terrestrial biosphere and the ocean during the atmospheric CO₂ removal phase. The amount of outgassing is sensitive to the experimental setup (P versus E simulations) and the rate of CO₂ removal. We also find that a specific atmospheric CO₂ target level is reached earlier in the E simulations compared to the P simulations, with the time lag increasing with decreasing rates of CO₂ removal. For large amounts of CO₂ removed from the atmosphere (>150 ppm), maintenance of atmospheric CO₂ at the target level requires continued negative emissions to counter outgassing of CO₂ from the marine and terrestrial carbon sinks. In contrast, for low to moderate amounts of CO₂ removed (<100 ppm), maintenance of the target CO₂ level allows for emissions of CO₂ into the atmosphere, due to uptake of CO₂ by carbon sinks. Accordingly, if atmospheric CO₂ is left to evolve freely after the removal effort, atmospheric CO₂ rebounds for large amounts of CO₂ removed and continues to decline for moderate amounts removed.