Comment on egusphere-2024-1810

Abstract. A growing body of evidence suggests that to achieve the temperature goals of the Paris Agreement, carbon dioxide removal (CDR) will likely be required in addition to massive carbon dioxide (CO2) emissions reductions. Nature-based CDR, which includes a range of strategies to sequester carbon in natural reservoirs, could play an important role in efforts to limit climate warming to well below 2 °C above preindustrial levels. Agricultural CDR could enhance soil carbon sequestration, though the climate efficacy of such methods remains uncertain. Here, we use an intermediate complexity climate model to perform simulations of agricultural CDR in the form of soil carbon sequestration at a range of possible rates for different costs under three future emissions scenarios. We found that plausible levels of agricultural CDR were able to reduce CO2 concentration by 5–19 ppm and global surface air temperature by 0.02–0.10 °C by the end of century. This temperature decrease was non-linear with respect to cumulative removals, as any carbon removed remained part of the active carbon cycle, lessening the climate benefit compared to if the removed carbon was permanently stored in geological reservoirs. CDR was found to be more effective at reducing surface air temperature in low emissions scenarios, but less effective at reducing atmospheric CO2, compared to high emissions scenarios. This was because the weaker CO2 sinks in a high CO2 world had a more muted response to removal, so a substantially higher proportion of carbon was removed from the atmosphere for a given amount of CDR in a higher emissions scenario. The enhanced temperature response to CDR in lower emissions scenarios was due to the logarithmic response of radiative effects to changes in CO2, where at low atmospheric CO2 concentrations, small changes in CO2 are more effective at changing the global radiative balance than at higher CO2 concentrations. CDR was substantially more effective when implemented at a higher rate, as CDR makes a proportionally larger difference in a climate with lower cumulative air fraction of CO2. Land and soil carbon responses were driven by the scenario-dependent balances between the impacts of CDR on primary productivity from CO2 fertilization, and the impacts on soil respiration from increased soil carbon availability and global temperatures.

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