Quantifying the Surface Energy and Water Balance Impacts of Forestation Using an Earth System Multi-Model Ensemble

Forestation has gained prominence as a nature-based climate solution considering international commitments to limit global warming to well-below 2°C above pre-industrial levels. In addition to sequestering carbon dioxide from the atmosphere, forestation affects energy and water fluxes between the land surface and atmosphere, ultimately impacting the hydrologic cycle. Using a multi-model ensemble of Earth system model simulations for scenarios from the sixth phase of the Coupled Model Intercomparison Project wherein all forcings except land use change are identical, we examine forestation impacts on surface energy and water balance across five climatically diverse study regions. Surface and cloud albedo, turbulent heat fluxes, and longwave radiation fluxes are altered with implications for surface temperature where tree cover increase is ≥ 10% of grid cell area. Surface temperature decreases in the tropics and subtropics while slight warming occurs in the highest latitude study region, consistent with previous studies. Evapotranspiration (ET) and precipitation (P) increase in all study regions. P partitioned into ET increases and available water (P-ET) decreases in most study regions. Decreased runoff (R) often follows P-ET decrease and runoff ratio (R/P) decreases in all study regions, whereas subsurface soil moisture decreases in some, all with implications for water management. Shifting transpiration-to-evapotranspiration ratio plays a role in these anomalies. Our findings highlight the need to consider hydrologic cycle impacts when implementing forestation as a climate solution.