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1.3.3. The Biosphere

Life may be found in almost any environment existing on Earth. Nevertheless, in a discussion on the climate system, it is convenient to regard the biosphere as a discrete component, like the atmosphere, oceans and cryosphere.

The biosphere, both on land and in the oceans, affects the albedo of the Earth's surface. Large areas of continental forest have relatively low albedos compared to barren regions such as deserts. The albedo of deciduous forests is about 0.15 to 0.18 whilst that of coniferous forests is 0.09 to 0.15 (Barry & Chorley, 1992). Tropical rainforest reflects even less energy, approximately 7 to 15% of that which it receives. In comparison, the albedo of a sandy desert is about 0.3. Clearly, the presence of the continental forests affect the energy budget of the climate system.

The biosphere also influences the fluxes of certain greenhouse gases such as carbon dioxide and methane. Plankton in the surface oceans utilise the dissolved carbon dioxide for photosynthesis. This establishes a flux of carbon dioxide, with the oceans effectively "sucking" down the gas from the atmosphere. On death, the plankton sink, transporting the carbon dioxide to the deep ocean. Such primary productivity reduces by at least four-fold the atmospheric concentration of carbon dioxide (Broecker, 1982), weakening significantly the Earth's natural greenhouse effect.

The biosphere also influences the amount of aerosols in the atmosphere. Millions of spores, viruses, bacteria, pollen and other minute organic species are transported into the atmosphere by winds, where they can scatter incoming solar radiation, and so influence the global energy budget (see section 1.2.4). Primary productivity in the oceans results in the emission of compounds known as dimethyl sulphides (DMSs). In the atmosphere these compounds oxidise to form sulphate aerosols called marine non-sea-salt (nss) sulphate (Charlson et al., 1987). These nss sulphates act as condensation nuclei for water vapour in the atmosphere, thus allowing the formation of clouds. Clouds have a highly complex effect on the energy budget of the climate system (see section 2.7). Thus changes in primary productivity in the oceans can affect, indirectly, the global climate system.

There are, of course, many other mechanisms and processes which couple the biosphere with the rest of the climate system, but the discussion has illustrated the major influences of the biosphere upon the global climate system.