Ocean dynamics amplify non-local warming effects of forestation

Large-scale forestation, including reforestation, afforestation, and forest restoration, is prevalent in net zero climate strategies due to the carbon sequestration potential of forests. In addition to capturing carbon, forestation has biogeophysical effects, such as changes in albedo, that can influence surface temperatures locally (local effects), and at distant locations (non-local effects). Biogeophysical effects may offset the cooling benefits of carbon sequestration, hence requiring a robust understanding of their mechanisms to adequately integrate forestation into climate mitigation strategies and carbon accounting frameworks. Yet, the role of ocean dynamics, such as ocean circulation, ocean-atmosphere interactions, and ocean-sea ice interactions in driving non-local effects remains underexplored. In this study, we investigate the impact of ocean dynamics on the magnitude and geographic patterns of the non-local biogeophysical effects of large-scale forestation over a multicentury timescale using the University of Victoria Earth System Climate Model (UVic ESCM), an Earth System Model of Intermediate Complexity (EMIC). We conduct multicentury paired global forestation simulations, with the first simulation using a dynamic ocean (Dynamic Ocean Simulation) and the second using prescribed sea surface temperatures (Prescribed SST Simulation). To be able to separate local from non-local effects, we use the checkerboard approach in both simulations, alternating grid cells undergoing forestation (subject to local and non-local effects) with grid cells remaining deforested (subject to non-local effects only), and compare land surface temperature to a control simulation where all grid cells remain deforested. Using the model simulation data, we perform a surface energy balance decomposition for each simulation at multiple points in time. After a 500-year period, we find a non-local warming effect on land surface temperature in both the Dynamic Ocean Simulation and the Prescribed SST Simulation. However, these non-local warming effects are of much greater magnitude and encompass a greater geographic area, particularly at high latitudes, in the Dynamic Ocean Simulation compared to the Prescribed SST Simulation. Moreover, in the Dynamic Ocean Simulation, non-local warming effects on land continue to strengthen for multiple centuries after most of the forest has regrown and local effects have stabilized. This prolonged land surface warming is the result of a gradual increase in sea surface temperature over multiple centuries caused by ocean-atmosphere interactions combined with the ocean’s thermal inertia. Furthermore, the ocean warming is amplified by climate feedback mechanisms, including the water vapor feedback, fueled by ocean evaporation, and the sea ice-albedo feedback. Consequently, forestation has non-local warming effects that develop gradually and intensify over multiple centuries due to interactions within the Earth system. Net zero policies and carbon accounting frameworks must therefore consider the complete Earth system response over a sufficiently long timeframe to include the slow ocean’s response. Without the consideration of the full Earth system response, net zero policies relying heavily on forestation may not deliver on their climate objectives.