CO2 is the most important anthropogenic greenhouse gas. It accounts for around three-quarters of the warming impact of current human greenhouse-gas emissions. The key source of CO2 is the burning of fossil fuels such as coal, oil and gas, though deforestation is also a very significant contributor. It persists in the atmosphere for 100 years and has a global warming potential = 1, since it is the baseline unit to which all other greenhouse gases are compared.