Exploring the biogeophysical and biogeochemical impacts of an Arctic poleward expansion of the boreal forest

A poleward migration of vegetation is expected in response to rising surface temperatures in the boreal zone and the Arctic regions. This land cover change is proposed to have important impacts on the climate through biogeophysical and biogeochemical feedback mechanisms (Brovkin, 2002; Bonan, 2015; Spracklen et al., 2008). While the benefits of CO2 uptake from forest expansion are better known, further exploration is needed to disentangle the full spectrum of consequences that such a significant land cover change can lead to, including changes in surface albedo, Biogenic Volatile Organic Compounds (BVOC) emissions, evapotranspiration, and turbulent fluxes. These might have an even larger quantitative impact on the climate than carbon sequestration itself at these latitudes (Bonan, 2015).

In this work, we use the Norwegian Earth System Model (NorESM2) to simulate the climatic impacts of both idealized and more realistic (forecast using a Dynamic Global Vegetation Model) patterns of high-latitude vegetation expansion with a specific focus on quantifying the radiative forcing of the albedo and BVOC emissions-related changes affecting aerosols and clouds.

The results show that the expected vegetation migration in the northern latitudes can have a substantial impact on a global scale. In total, the surface albedo changes dominate at these high latitudes (+0.48 W/m2, as average value over 50°N) over the effects from changes in aerosols and clouds related to changes in BVOC emissions, resulting in a total of +0.43 W/m2. Surprising results are found regarding the BVOC emission-related impacts. We expected that the indirect impact on cloud radiative effect from increasing BVOC emissions and subsequent aerosol formation, would lead to a cooling effect. However, the BVOC-related contribution to the total radiative forcing is +0.01 W/m2, and to the forcing related to changes in cloud properties is +0.05 W/m2. This signal is small but positive. This is consistent with the observed decrease in aerosol and cloud droplet number concentrations (respectively -0.18% and -1.00%), which affect the cloud albedo by lowering it. The processes that take place from the increase in BVOC emissions to its impact on the radiative forcing revealed complexity that needs to be further disentangled. A deeper understanding of atmospheric chemistry, aerosol and cloud formation, dynamics, and interactions at these latitudes is necessary to fully understand the role of large-scale ecosystem shifts in the coupled climate system.