Water Vapor, The Other Greenhouse Gas
Water vapor is the most abundant greenhouse gas, but it is more often than not a result of climate changes rather than man-made emissions. Water or moisture on the Earth's surface absorbs heat from the sun and the surroundings. When enough heat has been absorbed, some of the liquid's molecules may have enough energy to escape from the liquid and begin to rise into the atmosphere as a vapor. As the vapor rises higher and higher, the temperature of the surrounding air becomes lower and lower. Eventually, the vapor loses enough heat to the surrounding air to allow it to turn back into a liquid. Earth's gravitational pull then causes the liquid to "fall" back down, completing the cycle. This cycle is also called a "positive feedback loop."
Water vapor is more difficult to measure than the other greenhouse gases and scientists are uncertain as to the exact part that it plays in global warming. Scientists believe there is a correlation between the increase of carbon dioxide in our atmosphere and the increase of water vapor. The NOAA Web site has this to say:
As water vapor increases in the atmosphere, more of it will eventually also condense into clouds, which are more able to reflect incoming solar radiation (thus allowing less energy to reach the Earth's surface and heat it up).