Energy planning and policy
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World sources of energy are many and varied; they include solar, wind, geothermal, wood, peat, tides, human and animal effort, biological waste and so on. However these are either too small for inclusion in national figures or are too difficult to quantify and therefore, in international statistics, the only sources of energy which are aggregated are coal, oil, natural gas and electricity produced from hydro or nuclear sources. The following table shows how the world supply of energy has increased in the last four decades. Consumption of coal has barely doubled whereas consumption of crude oil has increased more than tenfold and the supply of natural gas has increased twentyfold. Nuclear and hydro electric power have also grown considerably but are still generated from fossil fuels — coal, oil and gas — and about a quarter of the 1972 production shown below was used for this purpose.