Vegetation-cloud feedbacks to future vegetation changes in the Arctic regions

Abstract

This study investigates future changes in the Arctic region and vegetation-cloud feedbacks simulated using the National Center for Atmospheric Research Community Atmosphere Model Version 3 (NCAR-CAM3) coupled with a mixed layer ocean model. Impacts of future greening of the Arctic region are tested using altered surface boundary conditions for hypothetical vegetation distributions: 1) grasslands poleward of 60°N replaced by boreal forests (GtoBF) and 2) both grasslands and shrubs replaced by boreal forests (GStoBF). Surface energy budget analysis reveals that future greening induces a considerable surface warming effect locally and warming is largely driven by an increase in short wave radiation. Both upward and downward shortwave radiation contribute to positive surface warming: upward shortwave radiation decreases mainly due to the decreased surface albedo (a darker surface) and downward shortwave radiation increases due to reduced cloud cover. The contribution of downward shortwave radiation at surface due to cloud cover reduction is larger than the contribution from surface albedo alone. The increased roughness length also transported surface fluxes to upper layer more efficiently and induce more heating and dry lower atmosphere. A relatively smaller increase in water vapor compared to the large increase in low-level air temperature in the simulation reduces relative humidity and results in reduced cloud cover. Therefore, vegetation-cloud feedbacks induced from land cover change significantly amplify Arctic warming. In addition to previously suggested feedback mechanisms, we propose that the vegetation-cloud feedback should be considered as one of major components that will give rise to an additional positive feedback to Arctic amplification.