ScienceDaily (Nov. 17, 2008) Water vapor is known to be Earth's most abundant greenhouse gas, but the extent of its contribution to global warming has been debated. Using recent NASA satellite data, researchers have estimated more precisely than ever the heat-trapping effect of water in the air, validating the role of the gas as a critical component of climate change.
http://www.sciencedaily.com/releases/2008/11/081117193013.htm