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Land cover and land use change

A major research focus for TSS is natural and human-mediated changes in land cover and ecosystem functions and their effects on climate, water resources, and biogeochemistry. TSS scientists worked on several projects to implement land cover and land use change in CLM and to use climate models to study the impact of these processes on climate.

Johan Feddema (University of Kansas), Gordon Bonan, and Keith Oleson studied the effects of historical and future land cover change on global climate. Adding the effects of changes in land cover to the A2 and B1 transient climate simulations described in the Special Report on Emissions Scenarios (SRES) by the Intergovernmental Panel on Climate Change leads to significantly different regional climates in 2100 as compared with climates resulting from atmospheric SRES forcings alone. Agricultural expansion in the A2 scenario results in significant additional warming over the Amazon. Agricultural expansion in the mid-latitudes produces cooling and decreases in the mean daily temperature range over many areas. (Figure 5.)

Oleson, Bonan, and Feddema developed and tested an urban land cover parameterization for CLM. The parameterization uses concepts from urban canyon models to simulate the radiative balance of a city, turbulent energy fluxes, and the hydrologic cycle. The model is designed to be compatible with structural and computational constraints of CLM for coupling to a global climate model, yet complex enough to explore physically-based processes known to be important in determining urban climatology. The city representation is based upon the urban canyon concept which consists of roofs, sunlit and shaded walls, and canyon floor. The canyon floor is divided into pervious (e.g., residential lawns, parks) and impervious (e.g., roads, parking lots, sidewalks) fractions. Trapping of longwave radiation by canyon surfaces and solar radiation absorption and reflection is determined by accounting for multiple reflections. Separate energy balances and surface temperatures are determined for each canyon surface. A one-dimensional heat conduction equation is solved numerically for a ten-layer column to determine conduction fluxes into and out of canyon surfaces. The urban model was compared to observed fluxes and temperatures for Mexico City and Vancouver in collaboration with Sue Grimmond (King's College London). The model captures the behavior of urban land cover compared with rural land cover and gives insights to the urban heat island. (Figure 6.)

Figure 5. (High resolution image) Temperature differences. June-July-August temperature differences in 2050 and 2100 due to land-cover change in the B1 and A2 scenarios. Values were calculated by subtracting the greenhouse gas–only forcing scenarios from a simulation including land-cover and greenhouse gas forcings.

Figure 6. (High resolution image) Heat fluxes. Average diurnal cycle of simulated and observed heat fluxes for the Mexico City site (Me93) for days 336-341 (Dec 2-7, 1993).