Can ambitious forestation mitigate temperature overshoot?

Even though it has been estimated that country-level commitments on Afforestation/Reforestation (AR) are quite ambitious, amounting globally to 633Mha by 2060, typically modelling studies either apply only moderate levels of AR or are fully idealized, thus not taking into account technoeconomic and biodiversity considerations. Typically, high-end emission trajectories are also employed, yielding strong fertilization of vegetation by elevated CO2 levels and thus enhanced terrestrial carbon stocks, while the CDR potentials over more strongly mitigated pathways remain understudied. This is especially the case for overshoot pathways that are gaining research interest recently, given their relevance for reaching the more ambitious 1.5oC goal.

Here, with the fully coupled MPI-Earth System Model we investigate the mitigation potential of an ambitious yet spatiotemporally plausible AR scenario under an overshoot emission trajectory (SSP5-3.4os). The developed AR scenario employed here is commensurate with country commitments in 2060 and extends to 2100 reaching 935 Mha globally and is constrained by technoeconomic considerations based on a multitude of 1,259 Integrated Assessment Model-generated pathways. To further constrain the scenario, we consider biodiversity and restoration priority maps.

Based on a big ensemble member approach allowing for robust probabilistic analysis, our results demonstrate that ambitious AR can robustly mitigate global temperature in 2100 by 0.2oC, peak temperature by 0.09oC, and reduce temperature overshoot duration by 13 years, while also delaying the land carbon sink-to-source transition by ~10 years, compared to a reference scenario with constant land-use at 2015 levels. Temperature mitigation emerges also at the local scale, where biogeochemically-induced cooling compensates for any biogeophysically-induced local warming.

Overall, ambitious AR should be considered as a useful mitigation tool complementary to drastic emissions reduction even under more strongly mitigated pathways, despite potentially weaker CO2 fertilization.