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  The capacity of a physical system for doing work. All species harness energy in some form, but the ability of human beings to harness, convert, release and (re)deploy energy to do work and yield goods and services has been the basis for the material evolution of human societies. Energy forms available to humans include mechanical, solar radiant, geothermal, elastic, heat, chemical and nuclear energy. Mechanical energy includes the kinetic energy of a body in motion and the potential energy that is stored and may be converted into free form by movement, friction and the expenditure of heat. Solar radiant energy is energy transferred from the sun, which sustains the biogeochemical cycles of the global ecosphere that maintain life on earth. Geothermal energy is the energy in volcanoes and geysers that is derived from within the earth. Elastic energy is derived from a condition of mechanical strain (i.e. elastic rebound), while heat energy is the internal random motion of molecules. Chemical energy is stored in the molecules of compounds and nuclear energy is contained in the nucleus of an atom, which may be released through either fission or fusion.

Life on earth is ultimately dependent upon the sun. Odum (1971) defines the primary productivity of an ecosystem as the rate at which producer organisms (usually green plants) store radiant energy in forms which may be usable food material for other species. Humans have eaten some of these plants, as well as some of the animals higher up the food chain, to derive energy for life. However, humans have moved from using their own muscle power to satisfy their energy needs (walking, hunting, fishing) through to other sources of energy (initially animals, then machines) to meet their needs, increasing per capita energy use, particularly by exploiting fossil-fuels, to create an energy-intensive economy.

World-wide energy consumption is increasing, partly because of increases in global population. However, per capita energy consumption varies significantly throughout the world. When considering energy consumption, it is important to recognize patterns of economic activity, e.g. energy is consumed in developing countries to make cheap products for export to wealthier countries. While there have been significant increased efficiencies in energy consumption in most developed countries (see von Weizsacker et al., 1997), there has also been a redistribution, rather than a reduction, in energy consumption. This work is important for geographers working on modelling human ecosystems, agricultural geography, geography of food, political ecology and for geographers questioning the high energy use model of development that currently operates in, and increasingly for, wealthier countries.

Since the industrial revolution, energy has increasingly been derived from non-renewable resources. Recent activities under the rubric of sustainable development offer the promise of increased use of energy from renewable resources, and a reduction in energy demand which may be achieved through a reduction in consumption, re-using existing materials and recycling. However, in Third World countries undergoing development, trends are to greater energy use and higher levels of pollution, as countries attempt to rapidly raise their gross national product. This trend in large economies, such as China, is a concern to many countries that previously have been through an extensive phase of capital accumulation involving high levels of energy expenditure.

Reduction in the use of energy is one of the most achievable aspects of sustainable development, because it reduces economic costs to a corporation or institution. Energy consumption may be reduced by improved design, greater insulation, changes in use patterns and so on, and does not challenge entities in a threatening way. However, some ecological economics work traces the use of energy and materials throughout the production process (i.e. they trace the \'throughput\'). Economists such as Herman Daly call for a \'steady state economy\' based on the stabilization of \'throughput\' (Daly, 1991). This notion was extended by Wackernagel and Rees (1996) to a measurement of the \'ecological footprint\' of a particular lifestyle, i.e. the consumption of materials and energy measured in hectares required to provide those materials and energy. The high use of non-renewable fossil fuels, and their throughput, is frequently challenged by the environmental movement including the high use of energy in road-based transport (Newman and Kenworthy, 1989), the use of nuclear power and the damming of rivers in wilderness areas to generate hydro-electric power.

Business interests have worked together with some parts of the environmental movement on energy efficiency since the oil crises of 1973 and 1979 which signalled the end of cheap, abundant supplies of non-renewable energy forms, such as oil. They highlighted the dependency of highly industrialized countries upon the Organization of Petroleum Exporting Countries (OPEC), and the vulnerability of economies in some developing countries. While concerns about limits to growth being in the form of a limit of natural resources, such as oil, have been reduced, there is greater concern now about the cumulative impacts of fossil-fuel consumption (see global warming). Ironically, OPEC\'s price rises may have provided the greatest spur towards sustainable development, despite their intention being about economic and political power. One concern is the finite character of the earth\'s sinks, e.g. the atmosphere, to assimilate wastes from energy and material use.

The transitions from non-renewable to renewable forms of energy, from highly polluting to less polluting forms of energy and from an energy-intensive economy to a energy-conserving economy vary over space and time. The reasons for this include availability of resources, capacity to change, the strength of vested interests, political interests (see Pickering and Owen, 1994) and issues of social justice. The availability of cheap energy sources provides an incentive for their use, especially when the environmental issues of resource extraction and pollution (see also acid rain) may be felt in a \'distant elsewhere\'. Natural gas is increasing in importance as a source of energy. However, the dependence on fossil fuel inhibits the transition to renewable forms of energy such as solar, wind and wave power. It means that the cost of renewable energy forms is not reduced because of inadequate research funding, and that the energy involved in their construction costs is also disproportionately high, often because they cannot achieve economies of scale in their production processes. Total world energy demand is anticipated to grow in the future, therefore, despite some efforts at reduction through greater efficiency, so that sustainable development programmes which encourage the use of appropriate technologies and energy sources in developing countries, transitions to more benign energy sources throughout the world, and reductions in energy use in developed countries, are crucial. (PM)

References Daly, H. 1991: Steady-state economics, 2nd edn. Washington, D.C. and Covelo, CA: Island Press. Newman, P. and Kenworthy, J. 1989: Cities and automobile dependence: An international sourcebook. Aldershot: Gower. Odum, E.P. 1971: Fundamentals of ecology, 3rd edn. Philadelphia: W.B. Saunders. Pickering, K. and Owen, L. 1994: An introduction to global environmental issues. London and New York: Routledge. von Weizsacker, E., Lovins, A. and Lovins, L.H. 1997: Factor 4: doubling wealth — halving resource use. London: Earthscan; St Leonards: Allen and Unwin. Wackernagel, M. and Rees, W. 1996: Our ecological footprint: reducing human impact on the earth. Gabriola Island, British Columbia: New Society Publishers.

Suggested Reading Hill, R., O\'Keefe, P. and Snape, C. 1995: Energy planning. In J. Kirkby, P. O\'Keefe and L. Timberlake, eds, The Earthscan reader in sustainable development. London: Earthscan, 78-98. World Resources Institute, 1994: Energy. In World Resources Institute, World resources 1994-95. New York and Oxford: Oxford University Press, 165-80.



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