Land availability and policy commitments limit global climate mitigation from forestation.

Forestation (afforestation and reforestation) could mitigate climate change by sequestering carbon within biomass and soils. However, global mitigation from forestation remains uncertain owing to varying estimates of carbon sequestration rates (notably in soil) and land availability. In this study, we developed global maps of soil carbon change that reveal carbon gains and losses with forestation, primarily in the topsoil. Constraining land availability to avoid unintended albedo-induced warming and safeguard water and biodiversity (389 million hectares available for forestation globally) would sequester 39.9 petagrams of carbon by 2050, substantially below previous estimates. This estimate drops to 12.5 petagrams of carbon with land further limited to existing policy commitments (120 million hectares). Achieving greater mitigation requires expanding dedicated forestation areas and strengthening commitments from nations with considerable but untapped potential. Editor's summary: Cutting emissions is necessary to curb climate change, but storing carbon in forest biomass and soils can also contribute to climate mitigation. Estimating the potential increase in forest carbon is hindered by limited data on soil carbon sequestration rates and unrealistic assumptions about the land available for forestation. Wang et al. addressed these issues by synthesizing soil data and considering potential effects of forestation on warming, water quality, and biodiversity to produce a more realistic estimate: <40 petagrams of carbon by 2050 from reforestation and afforestation, which is much lower than previous estimates. Considering only the land that has been pledged for forestation in existing policy commitments further reduces the estimate. —Bianca Lopez [ABSTRACT FROM AUTHOR]