Water vapour is the primary greenhouse gas in the Earth’s atmosphere. The contribution of water vapour to the
natural greenhouse effect relative to that of carbon dioxide (CO2
) depends on the accounting method, but can
be considered to be approximately two to three times greater. Additional water vapour is injected into the atmosphere from anthropogenic activities, mostly through increased evaporation from irrigated crops, but also through
power plant cooling, and marginally through the combustion of fossil fuel. One may therefore question why there
is so much focus on CO
2
, and not on water vapour, as a forcing to climate change.