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2.2 Defining the earth system

The main components of the earth system

The earth system is itself an integrated system, but it can be subdivided into four main components, sub-systems or spheres: the geosphere, atmosphere, hydrosphere and biosphere. These components are also systems in their own right and they are tightly interconnected. The four main components of the earth system may be described briefly in the following way.

2.2.1 The earth system as a set of four overlapping, interacting spheres

Legend for Biosphere

Source: unit author

The main components of the earth system are interconnected by flows (also known as pathways or fluxes) of energy and materials. The most important flows in the earth system are those concerned with the transfer of energy and the cycling of key materials in biogeochemical cycles.

Energy flows

The earth is a vast, complex system powered by two sources of energy: an internal source (the decay of radioactive elements in the geosphere, which generates geothermal heat) and an external source (the solar radiation received from the Sun); the vast majority of the energy in the earth system comes from the Sun. Whilst some variations in these two sources occur, their energy supplies are relatively constant and they power all of the planet's environmental systems. Indeed, energy both drives and flows through environmental systems, and energy pathways may be highly complex and difficult to identify. For instance, energy may take the form of latent heat which is absorbed or released when substances change state (for example, between the liquid and gaseous phases). An example of energy flow and transformation through an ecosystem is illustrated in 2.2.2. Energy is transferred within and between environmental systems in three main ways:

2.2.2 Simplified depiction of energy flows and transformations in terrestrial ecosystems
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Source: Smithson et al (2008) p. 41

As well as being transferred within environmental systems, energy may also be transformed from one form to another; for instance, a rock fall involves the conversion of potential energy (due to gravity) to kinetic energy (due to movement) and to thermal energy, or heat (due to friction). The transfer and transformation of energy are associated with the performance of work; hence the sun performs work in heating the earth by its radiation, and a glacier performs work in moving sediment down-slope using the kinetic energy of its ice, water and rock. When work is carried out within the earth system, energy is transferred from one body to another, and it may also be converted from one form to another in the process. Throughout environmental systems, as energy is transformed from one form to another in performing work, heat is released; that heat is subsequently exported from the system, usually into the atmosphere and then into space. Yet the total energy content of the earth system remains the same (it is conserved), for energy cannot be created or destroyed. It follows that the earth system is only able to continue to function because it is constantly replenished with a sufficient supply of energy (mainly from the sun).

The dominant flows of energy at the global scale occur as a result of the large discrepancies that occur between the amounts of solar radiation received (and re-emitted) at different points on the earth's surface. Such discrepancies are most clearly apparent in the wide variations in surface temperature that exist between the equator and the poles. Those temperature variations drive the global energy circulation which acts to redistribute heat from the warm to the cold parts of the earth's surface. An overall poleward transfer of energy occurs by means of a variety of processes: the transfer of heat by winds and warm air masses; the transfer of latent heat associated with water vapour; the movement of heat in ocean currents; and the returning counter-flows of cooler air and water. The three main processes of energy transfer at the global scale may be summarised as:

It is important to acknowledge that pronounced latitudinal variations occur in these three processes. Overall, however, these processes of energy transfer maintain a state of equilibrium in the earth system: they remove energy from areas of surplus (in lower latitudes) and transfer it to areas of deficit (in higher latitudes).

Biogeochemical cycles

The earth system contains several 'great cycles' in which key materials are transported through the environment. In general, cycles occur in closed systems; at the global scale, many systems may be assumed to be closed because the earth receives negligible quantities of minerals from space (as a result of meteorite impacts) and because only limited quantities of materials can escape the earth's atmosphere. The key materials that cycle through the major biogeochemical cycles are carbon, oxygen, hydrogen, nitrogen, phosphorous and sulphur - all of which are essential for life. The biogeochemical cycles operate at the global scale and involve all of the main components of the earth system; thus materials are transferred continually between the geosphere, atmosphere, hydrosphere and biosphere. However, since the biogeochemical cycles involve elements that are essential for life, organisms play a vital part in those cycles. Typically then, the biogeochemical cycles involve an inorganic component (the abiotic part of the cycle, including sedimentary and atmospheric phases) and an organic component (comprising plants and animals, both living and dead). Like other environmental systems, biogeochemical cycles involve the flow of substances between stores (also known as reservoirs) in the geosphere, atmosphere, hydrosphere and biosphere. Water plays a vital role in mediating many of the flows between stores.

Three of the key biogeochemical cycles are the nitrogen, carbon and sulphur cycles, whose main features are described here.

2.2.3 Carbon cycle
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Source: UNEP/GRID-Arendal (2005a)

Of course, biogeochemical cycles have been substantially modified by human activities - a fact that has enormous implications for the understanding and management of environmental issues.