Energy policy is a key area in each of the world's economies. The oil shocks of the 1970's emphasised how important energy had become. In recent years a growing awareness of environmental issues has had a major impact on perceptions of energy use as growing numbers of people express concern at the relationship between energy and the greenhouse effect, acid rain and the depletion of the ozone layer. All of this has created a demand for more, and better, models of energy use.