Global and regional hydrologic cycle impacts of forestation

As nations plan to plant billions to trillions of trees to mitigate against climate change, it is essential to understand how large-scale re- or afforestation  will impact the Earth system. Trees remove carbon dioxide (CO2) from the atmosphere through photosynthesis and can store the sequestered carbon for centuries if not disturbed. This has climate benefits, as CO2 removal contributes to reduced atmospheric CO2 concentration and is a key measure for limiting global average temperature increase to 1.5 ⁰C or 2 ⁰C relative to pre-industrial conditions. However, despite this favorable biochemical effect of net tree cover increase, there are global and regional biophysical effects which remain understudied. One example of this is the impact of afforestation, reforestation, and avoided deforestation (referred to as forestation henceforth) on the atmospheric and terrestrial portions of the hydrologic cycle at the global and regional scales. This study uses a process-based modelling framework and relevant simulations from the World Climate Research Programme's Sixth Coupled Model Intercomparison Project (CMIP6) to quantify the global and regional impacts of realistic forestation on the global hydrologic cycle for a high-emissions shared socio-economic pathway to 2100 (SSP3-7.0). To accomplish this, the CMIP6 Land Use Model Intercomparison Project's afforestation experiment is leveraged. Changes in key hydrologic cycle variables and metrics such as precipitation recycling and soil moisture deficit are investigated. While the global impact of large-scale forestation on the hydrologic cycle is difficult to detect, regional impacts—often but not exclusively within the regions where forestation occurs—are apparent. Impacts on atmospheric and terrestrial hydrologic cycle variables can be seen with potential implications for water availability in some regions. Findings highlight the potential unintended consequences of including forestation in climate mitigation strategies.