Earth system effects, side-effects, and carbon cycle feedbacks of reforestation and ocean alkalinization
For the assessment of CDR methods side-effects and carbon cycle feedbacks play an important role as they may alter the methods’ mitigation potential. Here we comparatively assess a reforestation scenario and an ocean alkalinization scenario as examples for land- and ocean-based CDR methods with respect to their effects in the coupled Earth system. We perform model experiments using the Max Planck Institute Earth System Model (MPI-ESM) with prognostic carbon cycle, forced by fossil-fuel CO2 emissions according to the high-emission scenario RCP8.5. The two CDR scenarios are very different regarding their potential to reduce global warming. Yet, normalized to the same reduction in global warming, we find that more CDR is needed via reforestation than via ocean alkalinization. This lower cooling efficiency of reforestation is due to the biogeophysical effects of land cover change counteracting the cooling CDR signal and to the climate sensitivity possibly being state-dependent. Furthermore, we find a lower efficiency to remove carbon from the atmosphere for reforestation compared to ocean alkalinization, since the ocean releases more carbon in response to reforestation than the land in response to alkalinization. Overall our findings illustrate how side-effects and feedbacks in the Earth system alter the mitigation potential of CDR methods.
Sebastian Sonntag is a scientist at the Max Planck Institute for Meteorology, Hamburg, Germany. He has a background in physics, he holds a PhD in geoscience, and his research involves understanding interactions and feedbacks in the Earth system using models of different complexity. He is working on both SRM as well as land- and ocean-based CDR methods with a focus on the climate and carbon cycle response as simulated with an Earth system model.