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In page Carbon tax:

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An ad valorem energy tax is levied according to the energy content of a fuel or the value of an energy product, which may or may not be consistent with the emitted greenhouse gas amounts and their respective global warming potentials. Studies indicate that to reduce emissions by a certain amount, ad valorem energy taxes would be more costly than carbon taxes.[3] However, although greenhouse gas emissions are an externality, using energy services may result in other negative externalities, e.g., air pollution not covered by the carbon tax (such as ammonia or fine particles). A combined carbon-energy tax may therefore be better at reducing air pollution than a carbon tax alone.[citation needed]