Taxes that are used, or proposed, to achieve environmental objectives. For example, an energy tax, such as a tax on motor fuels, will lead to a reduction in emissions and contribute to the control of CO₂. Environmental taxes can be used instead of, or alongside, direct controls on quantity. The use of environmental taxes is justified by the theory of externalities and they are a special case of Pigouvian taxes. An important example is the carbon tax which has been proposed as a means of combating global warming. See also carbon tax; green tax.