| 英文摘要: | Management practices applied to existing types of land cover can influence the local climate as much as a conversion to a different type of plant cover. Approximately 83% of land on Earth has been appropriated for human use1. To prevent the further displacement of natural habitats, land-use will probably become more intensive over the coming years. Writing in Nature Climate Change, Sebastiaan Luyssaert et al.2 report that land management choices can have as large an impact on the local temperature as the impact due to a change in the type of land cover. The team demonstrates the power of extracting information from modern flux networks to quantify energy-balance differences for diverse sites across North America and Europe; these paired sites differ in management and span a range of climates and plant functional types (grasslands, forests and crops). The authors augment their analysis by using satellite remote sensing to extend the time series back in time. They also apply a coupled-surface-layer/planetary-boundary-layer model to quantify the changes in surface temperature and attribute them to particular biophysical factors. A broad understanding that human-induced changes in land cover, through deforestation and land management (for example, through grazing) can have an impact on the local climate has existed for centuries, and extends as far back as the Roman era and the time of the Greek philosophers. The mechanism for this land–climate interaction occurs through alterations to the surface energy balance and the carbon cycle3, 4. Historically, a misunderstanding of the feedbacks between land management and climate has promulgated the idea that 'rain follows the plow'; a myth that helped to entice the westward migration of pioneers in North America, who converted the prairie, west of the Missouri River, to farms5. These farmers were surprised, years later, when periodic droughts returned, causing many to abandon their farms. Analyses of these historical events led modern scholars to recognize that the experiences of these pioneers were more in-line with an alternative theory: 'drought follows the plow' on marginal lands6. These historical misconceptions illustrate why we need a firm understanding of how changes in land cover and management interact with the climate system, if we are to make effective land policy and management decisions. The work of Luyssaert et al.2 is a step in this direction. The impacts of changing land cover and management on the surface-energy balance are very complex and are full of negative (stabilizing) and positive (reinforcing) feedbacks (Fig. 1). In principle, the impacts of land use and management on the local climate revolve around the perturbation of a suite of nonlinear functions that are dependent climate variables, like sunlight, temperature and humidity. These functions depend on a set of biological and physical parameters that can be viewed as 'knobs' that control aspects of the system and can be 'turned' by changes in management practice and land cover. Key parameters include the reflectance of incoming solar radiation (albedo), leaf pore capacity (stomatal resistance) and plant canopy structure (leaf area index). These parameters affect the way that energy is partitioned into sensible heat (which warms the air) and latent heat exchange (which may cool the land surface through evaporative cooling), and in turn affect the temperature of the ground surface. Of additional importance are positive and negative feedbacks associated with: (1) the growth of the planetary boundary layer, which may dampen the warming of the atmosphere, (2) changes in the surface temperature of the land, which will alter the amount of available energy and (3) changes in surface roughness, that regulate the transfer of heat to the atmosphere7, 8.
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