CAUSES OF CLIMATIC CHANGE

The fact that climate influences terrestrial life is readily apparent. At the same time, it is all too clear that humans and other constituents of the biosphere affect climate significantly. Although many details about the interdependence of climate and the biosphere remain unknown, it has been firmly determined that there are relatively fixed supplies of certain elements essential to life (i.e., nutrients) that circulate in the environment. These materials are transported (by wind or water, for example) and transformed chemically into forms that can be used by living things.

Nutrients move in biogeochemical cycles. Climate, through atmospheric circulation, influences life by its effect on the flow of nutrients through these cycles. Many of these materials help to determine the composition of the atmosphere, which in turn affects climate. Water vapour is one such material. When it forms clouds, the Earth's albedo changes and more of the Sun's rays are reflected back into space. There is less heating, and so the climate is altered. (Water vapour and clouds also are important elements in the greenhouse warming effect; see below.)

Water is one of the most important nutrients for sustaining life, and its movement is termed the hydrologic cycle. What kinds of vegetation will grow in a given location is largely determined by the availability of water. Water is transferred to the air by means of evaporation and transpiration. Evaporation is the process whereby liquid water both from bodies of water (e.g., rivers and lakes) and from the soil is transformed to water vapour. Transpiration involves the discharge of water vapour to the atmosphere from the surfaces of plants, particularly from their leaves. The total transfer of moisture to the air via both these processes is termed evapotranspiration. On a global average, evapotranspiration on land is about six times less than evaporation of water from the ocean. Evapotranspiration, however, is often the principal local source of water vapour at the centres of continents.

Precipitation in the hydrologic cycle interacts with the so-called sedimentary cycle. Water helps to transport materials from the land to the sea, where they are incorporated into sediments. In the short term, this cycle consists of the processes of erosion, nutrient transport, and sediment formation. In the geologic, long-term sense, the sedimentary cycle utilizes mechanisms other than water flow. The processes involved are sedimentation, uplift, seafloor spreading, and continental drift.

The hydrologic and sedimentary cycles work together in the distribution of hydrogen, carbon, oxygen, nitrogen, phosphorus, and sulfur. These elements are the macronutrients, elements that account for 95 percent of the composition of all living organisms; they are necessary for sustaining life. The natural supply of these nutrients is fairly constant, with ample amounts existing within the Earth's crust. They are not always accessible, however, and must be recycled for life to continue.