Effects of shrub cover increase on the near surface atmosphere in northern Fennoscand

Abstract

Shrub expansion in high latitudes is a widely observed response to climate change. Extensive evidence has shown that shrub expansion can lead to positive feedbacks to the regional climate. In this study we evaluate the sensitivity to a potential expansion in shrub and tree cover in the northern Fennoscandia region. Two perturbation experiments are performed in which we prescribe a gradual increase of vegetation height in the alpine shrub and tree cover according to empirically established climatic zones within the study region. The first experiment is based on present day climate, and the second is based on a future 1 K increase in temperature. To evaluate the sensitivity of the atmospheric response to inter-annual variations, simulations were conducted for two different years, one with warmer and one with colder spring and summer conditions. We have applied the Weather Research and Forecasting model (WRF) with the Noah-UA land surface module in evaluating biophysical effects of increased shrub cover on the near surface atmosphere on a fine resolution (5.4 km x 5.4 km). We find that shrub cover increase leads to a general increase in near surface temperatures with the peak influence occurring during the snow melting season. It has the largest effect in spring, by advancing the onset of the melting season, and more moderate effect on summer temperature. We find that the net SW absorbed by the surface is sensitive to the shrub and tree heights, which act to strengthen the albedo decrease. Counteracting effects include increased snow cover and enhanced evapotranspiration causing increased cloud cover and precipitation. We find that the strength of the feedback effects resulting from increased shrub cover is more sensitive to snow cover variations than summer temperatures. Taller vegetation has a stronger influence on both spring and summer temperatures. Our results show that the positive feedback to high latitudes warming induced by increased shrub and tree cover is a robust feature across inter-annual differences in meteorological conditions, and will likely play an important role in the future.