Boreal Afforestation's Underestimated Cloud Influence on Earth's Energy Imbalance

Abstract

Earth's Energy Imbalance (EEI) is accelerating, partly due to declining planetary albedo from reduced cloud cover. Boreal afforestation can either mitigate or exacerbate this trend through competing biophysical feedbacks. While snow masking reduces surface albedo (+0.5 to +2.5 $W/m^2$ warming), forests can enhance low-level cloud cover (0.1-0.5%) and increase cloud reflectivity via biogenic volatile organic compounds (BVOCs), producing potential cooling (-1.8 to -6.7 $W/m^2$). This BVOC-aerosol-cloud pathway remains poorly constrained but may dominate under warmer conditions, challenging carbon-centric mitigation paradigms. Large-scale initiatives (e.g., Canada's 2 Billion Tree Commitment) risk unintended warming if not climate-smartly sited. We present a tiered decision-support framework that integrates biogeochemical and biophysical processes, explicitly incorporating cloud-aerosol feedbacks. Urgent inclusion of these feedbacks in policy is essential to ensure boreal afforestation contributes to EEI stabilization.