Forestation's Climate Benefits Offset by Chemistry, Albedo Feedbacks

Roughly a third of the climate cooling that forests achieve by removing carbon dioxide (CO₂) from the atmosphere is offset through changes to atmospheric composition and decreased surface reflectivity, researchers report. The findings suggest that the benefits of wide-scale forestation efforts may be overestimated and do not represent a single solution for addressing climate change. They also highlight the urgency of simultaneously focusing on emissions reductions. Planting trees has been widely promoted as a nature-based solution to remove anthropogenic CO₂ from the atmosphere to help mitigate ongoing climate warming. However, wide-scale forest expansion may drive feedbacks within the Earth system that lead to warming. For example, changes to atmospheric composition (via the release of biogenic volatile organic compounds that affect greenhouse gasses and organic aerosols) and decreased surface albedo could offset the cooling effects of atmospheric CO₂ removal (CDR) by forestation. However, the effects and their impact on climate remain relatively unexplored. Using two state-of-the-art climate models – UKESEM1 and CESM2 – James Weber and colleagues compared extensive global forestation scenarios and evaluated their impact on climate and the climatic benefit from CDR. According to the findings, when forestation is simulated under a scenario with few other climate change mitigation strategies, the higher volatile organic compound emissions from vegetation increase greenhouse gasses ozone and methane in the atmosphere. This, combined with changes to surface albedo and light-scattering organic aerosols, results in a net warming effect, which offsets ~31% of the cooling benefit from CDR. However, Weber et al. show that tree planting still is an important climate change mitigation strategy to pursue when combined with greenhouse gas emission reductions. In a related Perspective, Garry Hayman discusses the study in greater detail.