The Role of Vegetation-Climate Feedbacks in Regional Earth System Dynamics
Summary, in English
The hypothesis of this thesis is that climate models should take structural vegetation changes that operate on interannual to centennial time scales into account in order to simulate climate change in a more realistic manner. To evaluate this hypothesis, a regional Earth System Model (ESM) has been developed, validated and applied over Europe. The model has enabled the identification of a number of regions where vegetation dynamics affect future climate, lending support to the hypothesis. The model, RCA-GUESS, combines the regional climate model RCA with the dynamic vegetation model LPJ-GUESS. RCA is continuously updated with dynamic albedo, leaf area index (LAI) and vegetated tile fractions for broadleaved and needleleaved forest and open land vegetation. These factors influence the radiation balance and the ratio between sensible and latent heat surface fluxes. The results suggest that it is important to account for vegetation-climate feedbacks in the analysis both of changes in mean climate and climate variability. Regarding mean climate, results indicate an accentuated temperature increase (winter, spring) in the Scandinavian Mountains as a result of tree line advance in response to warming. An increased forest fraction masks snow-covered areas, with a resulting albedo reduction. A significant temperature increase (summer) in southern Europe leads to a decline in LAI, resulting in a reduced evapotranspiration that reinforces the temperature increase. A less pronounced temperature rise in central Europe was attributed to the positive effect of "CO2 fertilization" on the simulated vegetation: a favourable effect on LAI positively impacts evapotranspiration, which dampens the temperature increase. The albedo effect in the Scandinavian Mountains dampens temperature variability, since variations in snow characteristics lose their significance beneath an increased forest fraction. In southern and central Europe, variability is strengthened, due to greater variations in temperature and water availability around bioclimatic limits governing establishment, growth and survival. Traditional climate models consider fast energy exchanges between the land surface and the atmosphere, utilizing a prescribed vegetation cover over time. A new generation of global ESMs are now including vegetation and the carbon cycling of the biosphere as interactive components. Such models have demonstrated that vegetation changes can result in strong feedbacks impacting on global climate. The course spatial resolution in global ESMs may, however, miss significant regional feedbacks. Regional ESMs are therefore important tools to consider for more accurate scenarios of future climate change when the focus is on the regional scale of, for example, Sweden or Europe.