Global and Regional Hydrological Impacts of Global-Scale Forest Expansion

Global-scale afforestation, reforestation, and forest restoration have gained significant attention as climate change mitigation strategies due to their significant carbon dioxide removal (CDR) potential. However, there has been limited research into the unintended consequences of increasing global forest cover from a biophysical perspective. Using the Community Earth System Model version 2 (CESM2), we apply a plausible global forestation scenario, which aligns with current net zero proposals and commitments, within a Paris Agreement-compatible warming scenario to investigate the land surface and hydroclimate response. Compared to a control scenario where land use is fixed to present-day levels, the forestation scenario is significantly cooler at low latitudes (0.8°C-3.0°C) by 2100, driven by a 10% increase in evaporative cooling in forested areas. However, shifts from grassland or shrubland to forest (afforestation) lead to a doubling of plant water demand in some regions, causing significant decreases in soil moisture (5% globally) and water availability (10% globally) in regions with increased forest cover. While there are some increases in low cloud and seasonal precipitation over these regions, with lower and in some places negative cloud radiative forcing, the impacts on large-scale precipitation and atmospheric circulation are limited. This contrasts with the response of precipitation to simulated large-scale deforestation reported in previous modelling studies, which significantly decreased low-latitude rainfall. The forestation scenario demonstrates local cooling benefits in low latitudes without major disruption to global hydrodynamics beyond those already projected to result from climate change, in addition to the cooling associated with CDR. However, the water demands of extensive afforestation of non-forest biomes have implications for its viability given uncertainty in future precipitation changes, especially in the tropics.