Carbon pricing is a method for governments to mitigate climate change, in which a monetary cost is applied to greenhouse gas emissions. This is done to encourage polluters to reduce fossil fuel combustion, the main driver of climate change. A carbon price usually takes the form of a carbon tax, or an emissions trading scheme (ETS) that requires firms to purchase allowances to emit. The method is widely agreed to be an efficient policy for reducing greenhouse gas emissions. Carbon pricing seeks to address the economic problem that emissions of CO2 and other greenhouse gases are a negative externality – a detrimental product that is not charged for by any market.