Evaporated H2O otherwise known as water vapor is a know greenhouse gas, but like the article states we do not think of it as a "key" greenhouse gas. What most people fail to account for is the fact that water vapor absorbs and then re-admits infrared radiation, which warms up the temperatures, especially those of the oceans, as the cover the majority of the earths surface. There have been indications that the small changes in temperature, starting from all the combined effects of greenhouses gases put more water vapor into the atmosphere. which rises the surface temperature even more. "As temperatures rise, the carbon dioxide equilibrium in the water changes, and this releases more carbon dioxide into the atmosphere" a continues cycle.