Global and Regional Hydroclimatic Responses to Alternative Global Reforestation Pathways

Abstract. Reforestation is a crucial component of climate change mitigation scenarios due to its potential to sequester CO₂ from the atmosphere. In addition to its effect on atmospheric CO₂, reforestation affects climate through changes in the surface energy and water balance. While the effects of large-scale reforestation on surface air temperature are well documented, its impact on the hydrological cycle remains underexplored. This study examines the global and regional hydrological consequences of two reforestation scenarios utilizing the CanESM5.1 Earth System Model: sustainable reforestation and a reversal of historical deforestation since 1850. Both scenarios are compared to a fixed land-cover baseline. We examine the components of the hydrologic cycle, cloud cover, and surface air temperature through simulations conducted from 2015 to 2200. Reforestation drives substantial regional variation in hydroclimatic responses. Higher latitudes experience warming in response to reforestation, whereas cooling dominates in low latitudes. Reforestation increases evapotranspiration in all regions, while precipitation and runoff responses vary by region. The results show that the hydrologic cycle continues to change in most regions long after tree cover has been restored, modulated by century-scale changes in large-scale atmosphere and ocean circulation. Our findings underscore the importance of reforestation strategies that jointly consider carbon sequestration and their effects on the water cycle.