A naturally occurring gas that is also a by-product of burning fossil fuels (fossil carbon deposits such as oil, gas, and coal), of burning biomass, of land-use changes, and of several industrial processes. It is the principal anthropogenic greenhouse gas that affects the Earth’s radiative balance. It is the reference gas against which other greenhouse gases are measured and therefore has a Global Warming Potential of 1.

(World Bank November 10, 2009)