S3A global 'climate opportunity benefit' of forest regeneration: meta-analysis shows warming from soil CH4 and N2O is small relative to agriculture

Global assessments of ecosystem regeneration as a climate mitigation strategy have traditionally focused on CO2, despite the acknowledgement that methane (CH4) and nitrous oxide (N2O) are also important greenhouse gasses (GHGs). We conducted a meta-analysis of studies that measured soil CH4 and N2O fluxes in unmanaged regenerating forested and savanna ecosystems, with a focus on understanding biome-specific differences in these GHG fluxes compared to a counterfactual of agricultural land use. We expected most regenerating ecosystems to act as small CH4 sinks and relatively larger N2O sources, with a net warming CH4-N2O effect. Three of the five forested biomes we studied followed this pattern: subtropical/tropical forest, subtropical/tropical savanna and temperate conifer forest (1.64 ± 0.56, 0.24 ± 0.07, and 0.65 ± 0.38 Mg CO2e ha-1 yr-1, respectively). Results suggest that after 100 years of regeneration, the radiative cooling of the climate from CO2 sequestration still exceeds the radiative warming driven by the net CH4-N2O effect among non-boreal ecosystems. We also found that the “climate opportunity benefit” of ecosystem regeneration—the subtraction of the net CH4-N2O effect of agriculture from the net CH4-N2O effect of regeneration—is positive (i.e., a net cooling effect) for all non-boreal biomes. However, our results underscore that it is unsound to use ecosystem regeneration as a justification for the proliferation of emissions-heavy activities. Ecosystem regeneration must be paired with swift and meaningful decarbonization to be an effective nature-based climate solution.