Reforestation and afforestation increase the amount of carbon stored in the biosphere, thereby reducing atmospheric carbon dioxide (CO2). However, bio-geophysical changes also occur due to modifications in the physical characteristics of land, which affect the surface energy balance. Early differences in climate impact estimates largely arose from variations in models and scenarios. Trees help cool the land by increasing latent heat flux (LE) and evapotranspiration (ET), which increases humidity and lowers temperatures.
Higher humidity levels, however, reduce downwelling surface shortwave radiation (SWd) as more radiation is absorbed by the atmosphere and clouds. The production of clouds due to increased aerosol and ET concentrations raises the longwave downwelling radiation (LWd). Additionally, changes in wind speed, vertical mixing, and latent fluxes caused by increased surface roughness can result in either local warming or cooling. More deforestation in the SSP370 experiment will create land-use gaps to support population growth and economic expansion.
By contrast, the ssp370 experiment minimizes deforestation by converting large areas of cropland and grassland into forests. While this significantly increases carbon stores in plants and soils, it does not directly affect atmospheric CO2 concentrations or global air temperatures. The effects of fire-related carbon emissions on land cover and climate were evaluated by comparing ssp370 with ssp126Lu.
Although aerosols produced by burning biomass were the same in both models, the effects of fires on the climate were not fully represented in this experimental design. With the help of the surface energy balance decomposition (SEBD) approach, researchers evaluated how the addition of surface energy fluxes translates into surface temperature changes.
Using the 12 models from Coupled Model Intercomparison Project Phase 6 (CMIP6), researchers examined the impact of net forestation on both carbon cycle and non-carbon cycle processes. Results showed that the land carbon storage is not directly affected by fire-related changes. Bio-geophysical effects, however, produced a global mean cooling (-0.002 ± 0.041 K) with up to 50% of simulations indicating significant cooling. Eight out of twelve models suggested tropical cooling of -0.058 ± 0.058 K, consistent with earlier findings. Overall, bio-geophysical responses to net forestation included cooling in the tropics and warming in the Northern Hemisphere.
Only five of the models incorporated interactive fire modules, and three of those relied on the same fire module, limiting the robustness of fire-related outcomes. In models projecting net global warming, the Northern Hemisphere warmed more strongly due to increased reflection, while the equatorial regions experienced reduced latent cooling. The changes in atmospheric circulation also influenced near-surface air temperature over the ocean under net deforestation.
Most models revealed major climate-specific disadvantages at low latitudes, with the exception of regions outside the tropics, where fire activity increased. Cooling was primarily attributed to reduced downwelling solar radiation and enhanced ET from cloud and aerosol interactions.
Reference: Gomez JL, Allen RJ, Horowitz LW, et al. Climate effects of a future net forestation scenario in CMIP6 models. npj Clim Atmos Sci. 2025;8:297. doi:10.1038/s41612-025-01127-4
