Climate Change 2001: The Scientific Basis

Figure 7.5: Schematic showing relationships between a simulation of the Atmospheric Boundary Layer (ABL), a Land-Surface Parametrization (LSP), vegetation and soil properties, and anthropogenic change. Interactions are shown by broad white arrows marked with capital letters, fluxes by grey arrows, and dependencies by dotted lines. (A) Diurnal-seasonal interactions between the ABL and the LSP; the ABL variables of air temperature, humidity, downward short-wave radiation, downward long-wave radiation, wind speed and precipitation (T, q, S ,L , u, P) are used to force the LSP which calculates net radiation minus ground heat, sensible heat, and latent heat fluxes ( R_n �G, H, lE), which in turn feed back to the atmosphere. Three surface parameters in the LSP are critical to these calculations: Albedo and surface roughness (a,Z₀ ) determine the radiative balance and turbulent exchange regime, and in third generation LSPs, the canopy conductance term, g_c (equivalent to the summation of all the leaf stomatal conductances) determines the vegetation evapotranspiration rate (lE) and net photosynthetic rate (P_net ). On time-scales of minutes to hours, g_c is a direct function of T, q, S ,CO₂ concentration and soil moisture (W). Increasing CO₂ concentration can act to signifi-cantly reduce gc and hence limit lE. The maximum value of g_c is determined by parameters related to vegetation density or leaf area index (LAI), and biochemical capacity (V_max ). Long-term climatic forcing (B) and land-use change (C) can alter the vegetation type and density, soil properties and ecosystem respiration rates, R_d , by which carbon is returned to the atmosphere from the vegetation and soil. (D) Changes in vegetation properties affect V_max and LAI, and changes in soil properties affect soil moisture (W) and runoff (R₀ ).

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