Water is a greenhouse substance. Water also exerts a greenhouse effect, whether as water vapor (i.e., a gas) or a liquid (including droplets in clouds and fog). In this regard, atmospheric water differs in many ways from CO2. Its vapor concentration in air is much higher than that of CO2; at "room temperature" the capacity of water in air is about 25 parts per thousand (25,000 parts per million) whereas currently the content of CO2 is about 390 parts per million. For this reason, the greenhouse effect from atmospheric water is much stronger than that of atmospheric CO2. Without the greenhouse effect of water, ambient temperatures on the earth would be far below freezing. Second, locally the actual water vapor content can be anywhere from 0 to 100% of the upper limit (the relative humidity). Globally the long-term cycle of water between water vapor, clouds and fog, rain and snow, glaciers and groundwater, and the oceans remains at equilibrium, in the absence of global warming. But thirdly, the capacity of air to hold water vapor (as the gas) increases by about 7% per degree C (3.9% per degree F). Thus as the long-term global average temperature rises because of the CO2 greenhouse effect, the overall intensity of the global water cycle will grow.
Discussing the role of cirrus clouds in this same post, NASA points out that they emit only small amounts of radiation because of their cold temperature. Thus, being composed of (solid) water, cirrus clouds strongly absorb heat (infrared) radiation reaching them from below, and retain a significant fraction of that heat, leading to higher atmospheric temperature than would be the case if they were absent. NASA states that in a world with higher average global temperatures, the air would have more water content that leads to formation of more cirrus clouds. In this view CO2-induced greenhouse warming would be amplified by the presence of more heat-retaining cirrus clouds in the upper atmosphere.
© 2012 Henry Auer