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.
- The geosphere - this is the part of the planet composed of rock and minerals; it includes the solid crust, the molten mantle and the liquid and solid parts of the earth's core. In many places, the geosphere develops a layer of soil in which nutrients become available to living organisms, and which thus provides an important ecological habitat and the basis of many forms of life. The surface of the geosphere is subject to processes of erosion, weathering and transport, as well as to tectonic forces and volcanic activity, which result in the formation of landforms such as mountains, hills and plateaux.
- The atmosphere - this is the gaseous layer surrounding the earth and held to its surface by gravity. The atmosphere receives energy from solar radiation which warms the earth's surface and is re-emitted and conducted to the atmosphere. The atmosphere also absorbs water from the earth's surface via the process of evaporation; it then acts to redistribute heat and moisture across the earth's surface. In addition, the atmosphere contains substances that are essential for life, including carbon, nitrogen, oxygen and hydrogen.
- The hydrosphere - this consists of those parts of the earth system composed of water in its liquid, gaseous (vapour) and solid (ice) phases. The hydrosphere includes: the earth's oceans and seas; its ice sheets, sea ice and glaciers; its lakes, rivers and streams; its atmospheric moisture and ice crystals; and its areas of permafrost. The hydrosphere includes both saltwater and freshwater systems, and it also includes the moisture found in the soil (soil water) and within rocks (groundwater). Water is essential for the existence and maintenance of life on earth. In some classifications, the hydrosphere is sub-divided into the fluid water systems and the cryosphere (the ice systems).
- The biosphere - this contains all living organisms and it is intimately related to the other three spheres: most living organisms require gases from the atmosphere, water from the hydrosphere and nutrients and minerals from the geosphere. Living organisms also require a medium for life, and are adapted to inhabit one or more of the other three spheres. However, much of the biosphere is contained within a shallow surface layer encompassing the lower part of the atmosphere, the surface of the geosphere and approximately the upper 100 metres of the ocean. Humans are part of the biosphere, although they are increasingly responsible for the creation of systems that may be largely artificial (such as cities).
2.2.1 The earth system as a set of four overlapping, interacting spheres
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.
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:
- radiation - this is the process by which energy is transmitted through space, typically in the form of electromagnetic waves
- convection - this is the physical movement of fluids (such as water or air) that contain energy in the form of heat; convection does not occur in solids
- conduction - this is the transfer of energy in the form of heat through the substance of a medium (from molecule to molecule)
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:
- the horizontal transfer of sensible heat by the movement of warm air masses
- the transfer of latent heat in the form of atmospheric moisture
- the horizontal convection of sensible heat by ocean currents
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).
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.
- The nitrogen cycle - the nitrogen cycle is a relatively fast and highly complex cycle. Most of the atmosphere consists of gaseous nitrogen which is 'fixed' (in other words, made available for use by plants) biologically in soils. Soil bacteria convert nitrogen to ammonia; this, together with inorganic nitrate, is absorbed by plant roots and converted to organic compounds (such as proteins) in plant tissues. These compounds are eaten by herbivores; in turn, nitrogenous compounds are passed to carnivores, and they are ultimately returned to the soil in the form of nitrogenous waste products (such as urine and faeces) and as a result of the death and decomposition of organisms. Bacteria then convert the organic nitrogen compounds into ammonia and ammonium compounds, which are then converted by bacteria into nitrites and then nitrates, which are then available for re-uptake by plants. Some of the nitrogenous compounds that are not absorbed by plants are leached from the soil into groundwater, surface water and ultimately into seas and oceans. Of that nitrogenous material, some is used by aquatic plants, some accumulates as organic sediment, and some evaporates into the atmosphere. The cycle is completed by denitrifying bacteria which eventually convert nitrates and nitrites to ammonia, nitrogen and nitrogen oxides.
- The carbon cycle - carbon is stored in the atmosphere in the form of carbon dioxide, which is absorbed by plants and converted to carbohydrates by the process of photosynthesis. The cycle then follows food chains, with carbohydrates being consumed by herbivores and then carnivores, being metabolised during the process of respiration. Carbon dioxide is returned to the atmosphere as animals exhale and when organic waste and dead organisms decay. Vegetation and animals are thus important stores of carbon, although that carbon may be rapidly returned to the atmosphere if vegetation is burned. Soils are also important reservoirs for carbon. Atmospheric carbon dioxide is soluble in water, in which it forms carbonic acid, which forms bicarbonate ions and carbonate ions, which in turn form salts (such as the insoluble calcium carbonate, which accumulates in marine sediments, marine organisms and carbonate rocks, such as limestone). Carbon is typically stored in these forms until it is released to the atmosphere by chemical weathering. A diagrammatic representation of the carbon cycle is presented in 2.2.3.
2.2.3 Carbon cycle
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Source: UNEP/GRID-Arendal (2005a)
- The sulphur cycle - sulphur is released into the atmosphere during volcanic eruptions (in the forms of sulphurous gas, dust and particles) and as a result of the weathering of rocks. The oceans also play an important role in the sulphur cycle, as marine phytoplankton produce dimethyl sulphide, some of which enters the atmosphere and is converted to sulphur dioxide and sulphate aerosols. These compounds are ultimately converted to sulphuric acid and are deposited on the earth's surface in precipitation. In terrestrial ecosystems, bacteria break down sulphurous compounds and release the sulphur to the atmosphere again, mainly in the form of hydrogen sulphide.
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.