These indicators can be expressed as productivity or intensity ratios, by relating emissions to measures of production or consumption, or as decoupling trends showing the two variables separately. The ratio between CO2 emissions from production and GDP is a widely used indicator. It brings together several aspects related to the environmental efficiency of production and is a prime indicator of relative decoupling between carbon from fossil fuel use as an input and domestic production. The CO2 intensity of production is also representative of other environmental issues, in particular emissions of greenhouse gases and to some extent air pollution, which are correlated with the carbon intensity of economic production.