A carbon offset is a credit for negating or reducing the impact of emitting a tonne of carbon dioxide by paying someone else to absorb or avoid the release of a tonne of CO2 somewhere else. Carbon offsets are measured in metric tonnes and one carbon offset represents the reduction of one metric tonne of carbon dioxide or its equivalent in other greenhouse gases. Offsets usually involve financial support for projects that reduce the emission of greenhouse gases in the short- or long-term, such as wind farms, tree planting or hydroelectric dams. See also Carbon and Greenhouse Gases.