The biosphere

The biosphere includes plants on land and in the sea and all animals. The response time of the biosphere is variable and not well defined. This reflects not just a lack of knowledge, but also the diversity of the biosphere. The land surface covers approximately 30 per cent of the Earth and, while some may be ice covered, much of the surface will be covered with vegetation. Therefore vegetation provides the boundary layer to a substantial proportion of the Earth's atmosphere. The vegetation will therefore determine the surface energy budget, with characteristics such as the albedo and latent heat transfer being modulated by the type of vegetation. The atmospheric boundary layer, the lowest part of the atmosphere, interacts directly with the surface vegetation. The drag on the atmosphere will be affected by the height of the vegetation. For a long time such issues have mainly been of interest to micro-climatologists but it is increasingly important to the global climate debate. The biosphere not only directly affects the radiation budget through changes in albedo but also mass exchange, in particular water and carbon. Plants affect soil structure through leaf litter and also their root system. This in turn affects the interception of precipitation and the way in which water runs off and infiltrates the soil. Some of the most disastrous localized impacts of extensive deforestation have been the mudslides caused by the changes in runoff. The catchment area of the Yangtze River in China has been extensively deforested and heavy rains during 1998 resulted in several devastating floods. The El NiƱo originally took the blame but the Chinese government has recently acknowledged that the more likely culprit is the removal of forest from the area.

The biosphere is not only a result of the climate, but may well play a vital modulating role. This is through the carbon dioxide cycle that was described in the section on the hydrosphere. The importance of the biosphere to this cycle cannot be stressed too strongly. All aspects of the biosphere regulate the cycle. For example, young trees take up more carbon dioxide than mature trees. Therefore changes in the biosphere, whether natural or human, may result in climatic change. Furthermore the sensitivity of the biosphere to the atmospheric climate means that past climates can be studied through their effect on the biosphere.

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