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 acknowledgment that methane (CH₄) and nitrous oxide (N₂O) are also important greenhouse gases (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 combined CH₄-N₂O effect. Three of the five forested biomes we studied followed this pattern: subtropical/tropical forest, subtropical/tropical savanna and temperate conifer forest (0.60 ± 0.30, 0.15 ± 0.06, and 0.83 ± 0.24 Mg CO₂e ha⁻¹ yr⁻¹, respectively). Results suggest that even after 100 years of regeneration, the radiative cooling of the climate from CO₂ sequestration in aboveground biomass exceeds the radiative warming driven by the net CH₄-N₂O effect among all ecosystems on average globally. We also found that the “climate opportunity benefit” of ecosystem regeneration—the difference in the net CH₄-N₂O effects of agriculture versus regeneration—yields a net cooling effect for all biomes. However, because the CH₄-N₂O effect diminishes the cooling effect of ecosystem regeneration, our results underscore that it is unsound to use ecosystem regeneration as a justification for continuing fossil fuel emissions.