The physical capacity for doing work. Nearly all our energy derives from the sun, and technical progress has reflected more and more sophisticated uses of energy, from wind and water, through fossil fuels, to nuclear power.
In the early stages of industrialization, energy consumption is closely related to levels of economic development, and per capita GNP—see Zeng et al. (2008) Science 319, 5864 on China. Mature economies tend to be more energy efficient, perhaps because technology improves, and the emphasis shifts to service industries (M. Carr 1997). Even so, the advanced economies still account for most of the world’s energy consumption.
World demand for energy has increased so much that an energy crisis (a potential shortage of energy) has been identified; see the Roosevelt Institution’s 25 ways of solving the energy crisis. This crisis, together with the adverse environmental effects associated with the burning of fossil fuels (greenhouse effect, acid rain) has led to increased emphasis on energy conservation. Energy intensity is energy consumption per unit GDP. AAAG 101 (2011), issue 4 is devoted to new geographies of energy.