Sources of energy that use up the earth’s finite mineral resources; these include fossil fuels. Concern about the exhaustion of nonrenewable energy sources, together with the fact that burning fossil fuels contributes to air pollution, carbon emissions, and the greenhouse effect, has prompted increased use of renewable energy resources, which are not exhaustible. All depend directly or indirectly on the energy supplied by the sun, whether solar heating and photovoltaic cells, wind turbines, biofuels, or hydroelectric generators.