In the Scandinavian mountains, feedbacks related to surface albedo amplify the warming in winter and spring. Over continental Europe, feedbacks related to summertime evapotranspiration both dampen (central Europe) and amplify (southern Europe) the warming. Climate-vegetation feedbacks also affect the variability of temperature. As for the seasonal mean conditions, whether feedbacks act to dampen or enhance temperature variability depends on the region and the season (Wramneby et al., 2010b).
The study is made with a coupled model system RCA-GUESS (Smith et al., 2010). It consists of the Rossby Centre regional climate model RCA3 (Kjellström et al., 2005; Samuelsson et al., 2006) and the dynamic vegetation model LPJ-GUESS (Smith et al., 2001; Wramneby et al., 2008).
Without coupling RCA3 simulates regional climate changes in response to changes in sea-surface temperature, greenhouse gas concentrations and global climate conditions including feedback mechanisms within the atmosphere-surface system. Included is the effect vegetation has on e.g. the surface albedo, the evapotranspiration and the roughness of the surface. Only the annual cycle of vegetation density (phenology, expressed in term of Leaf-Area Index, LAI) is represented. Longer-term vegetation dynamic processes that affect the distribution of different vegetation classes (amount of forest and open land areas) are not simulated.
LPJ-GUESS is an offline vegetation model that simulates how vegetation evolves in response to climate conditions. Depending on their characteristics with respect to local climate conditions, vegetation elements – known as plant functional types (PFTs) – are established and compete for limited resources (light, soil water and living space). PFTs are also affected by stochastic disturbances (e.g. fire) that destroy biomass and kill individual plants. During a simulation the vegetation density varies continuously with time as a response to available resources. The proportional cover of forest and open land areas also varies as a response to changing competitive balance between PFTs and how disturbances affect the structure of the vegetation. However, LPJ-GUESS alone cannot account for how these changes may feed back on the climate.
With the coupled system RCA-GUESS, we can study the dynamic of the atmosphere and the vegetation together and how interactions in the system may act to create, strengthen or dampen feedbacks that in turn affect the atmosphere and vegetation.
Here we present results based on two scenario simulations where RCA-GUESS has been forced by output from the GCM ECHAM5 and the A1B emission scenario. In the first simulation, interactive vegetation was allowed for the whole period 1961-2100. In the second simulation (1991-2100), the state of the vegetation reached at 1990 in the first simulation was thereafter held constant. By comparing the two simulations we can quantify the impact of interactive vegetation on the climate and back on the vegetation through feedbacks (Wramneby et al. 2010a).
Feedbacks dominated by albedo
A warmer climate causes forest to move into present tundra areas as e.g. in the Scandinavian mountains. Figure 1a shows that woody species (forest) expand northward at high latitudes and upward in northern mountain areas. Due to decreasing snow amounts there is a corresponding decrease in albedo since vegetation has a lower reflectivity than snow. This decrease will be further enhanced with interactive vegetation since the expanding forest, having very low albedo, will tend to shadow the remaining snow (Figure 1b). Where this happens the warming is further enhanced (Figure 1c).
Feedbacks dominated by evapotranspiration
As reported by Wramneby et al. (2010a), continental Europe can be associated with contrasting evapotranspiration feedbacks during future summers. Evapotranspiration is affected by available soil water and by vegetation density (level of LAI). Evapotranspiration decreases during summer over continental Europe due to limited soil water. However, with interactive vegetation this decrease is suppressed over central Europe due to an increased density of vegetation (LAI) (Figure 2a), which leads to a reduced warming over this region (Figure 2b). In southern Europe on the other hand the interactive vegetation gives a general reduction in vegetation density (LAI and areal coverage) which further enhances the reduction in evapotranspiration (Figure 2a) and leads to a stronger warming (Figure 2b).
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Samuelsson, P., Gollvik, S., Ullerstig, A. 2006. The land-surface scheme of the Rossby Centre regional atmospheric climate model (RCA3). Rep. Meteorol. 122, 25 pp., Swed. Meteorol. and Hydrol. Inst., Norrkoping, Sweden.
Smith, B., Prentice I.C., and Sykes M.T. 2001. Representation of vegetation dynamics in modelling of European ecosystems: comparison of two contrasting approaches. Global Ecology and Biogeography 10: 621-637.
Smith, B., Samuelsson, P., Wramneby, A., Rummukainen, M. 2010. A model of the coupled dynamics of climate, vegetation and terrestrial ecosystem biogeochemistry for regional applications. Submitted to Tellus Series A.
Wramneby, A. 2010. The role of vegetation-climate feedbacks in regional earth system dynamics. PhD thesis. Dept. Earth and Ecosystem Sciences. Lund University. ISSN 0346-6787.
Wramneby, A., Smith, B., Samuelsson, P. 2010a. Hotspots of vegetation-climate feedbacks under future greenhouse forcing in Europe. Submitted to Journal of Geophysical Research – Atmospheres.
Wramneby, A., Smith, B., Samuelsson, P., Nikulin, G. 2010b. Are current climate models sufficient to address climate variability? – The role of vegetation dynamics. Submitted to Climate Dynamics.