In fact, there is some evidence that such an increase in humidity in the lower atmosphere may already be happening. Greenhouse skeptics sometimes cling to the importance of water vapor in Earth s radiative budget in hopes that it somehow lessens the importance of manmade greenhouse gases. But water s importance offers no real reason to think that manmade emissions won t warm Earth. The increase in other greenhouse gases will be many times higher than the increase in water vapor. Because different greenhouse gases absorb radiation at different wavelengths, water vapor does not soak up all the infrared energy coming from Earth s surface, and never can.
Water also exists in liquid form in the atmosphere.
Water vapor condenses into clouds and rain when it encounters lower temperatures and pressures. Clouds themselves play a key role in the radiative regimen of Earth and atmosphere. Clouds can reflect the visible-range light coming toward Earth from the sun acting in effect as a sunshade, cooling the atmosphere and Earth below them. At the same time, clouds can also absorb the infrared radiation coming from Earth s surface acting as a kind of blanket holding heat in and warming the lower atmosphere. Clouds currently are believed to have a small net cooling effect.