timescales. Biophysical feedbacks (such as transpiration) operate on time-scales of minutes, while biogeochemical feedbacks (such as plant growth) become significant only after several days and longer. Biogeographic changes (such as species composition) occur over years and longer.

Land cover affects the amount of aerosols that can be lifted by wind into the atmosphere. In general, the less dense the vegetation and the more intense the human activity on the land, the more aerosols can be lifted by dust storms and other updrafts. The impact of aerosols is discussed in detail in other sections of this report.

Snow has a very significant impact on the land-cover albedo and, as a result, on the radiation balance of the land cover. However, the same amount of snow at a specific location and time has a different synergistic effect with different land-cover types. For example, the albedo of an evergreen coniferous tree canopy is lower than that of an adjacent snow-covered clearing. Land type also affects the duration of snow on the ground.

Partitioning Between Latent and Sensible Heat Fluxes on a Regional Scale

Changes in the partitioning of net radiative fluxes into sensible and latent can substantially alter the atmospheric circulation. Using the results from Chase et al. (2000a), where a conservative estimate of land-cover change by humans was specified, Pielke et al. (2002) reported a globally averaged redistribution of sensible and latent turbulent heat fluxes on the order of 1 W m−2. Therefore, the spatial redistribution of the surface turbulent fluxes indicates that the net radiation received at the surface is changed in how the energy is inserted into the atmosphere. The surface forcing of climate, as a result, is altered.

The effect on local climate can be substantial. Marshall et al. (2004a,b) documented major alterations in summer rainfall and temperature, and in freeze occurrence, due to land conversion in the twentieth century in Florida. The observed land change between the pre-1900 period and 1993 in Florida is shown in Figure 2-5. Figure 2-6 shows model results for the change in net radiation at the surface, averaged over July-August, in response to that land-use change. Over the land area shown in Figure 2-5, the two-month, area-averaged reduction in rainfall was 10-12 percent. Freeze intensity and duration also changed as a result of land-use conversion. In the agricultural area just south of Lake Okeechobee, the draining of marshes and their replacement with orchards and other agricultural crops resulted in greater radiation loss to space with a resultant longer and more severe freeze.

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