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Water vapor

October 2, 2006 | A version of this story appeared in Volume 84, Issue 40

With regard to A. Frank Leo's question of why the effects of water vapor have not been considered in the discussion about global warming, I would suggest that it is because the amounts of water vapor that are naturally present in the atmosphere due to evaporation from the oceans and other bodies of water are so vast that they dwarf the amounts of water vapor created by combustion of various materials, including fossil fuels (C&EN, June 19, page 9). Approximately 71% of Earth's surface is covered by water. This creates a huge reservoir of water vapor in the atmosphere that does not appear to be affected to any significant extent by water vapor produced by anthropogenic activities. On the other hand, human activities such as combustion have had a profound effect on the smaller amounts of carbon dioxide present in the atmosphere from natural sources, as would be expected.

How this situation would be expected to change if all the motor vehicles in the world were to switch from carbon-based to hydrogen fuels cannot be predicted with certainty; however, I would suggest that there would be very little effect on water vapor emissions. If there were any effect at all, it seems likely to be beneficial. The combustion of hydrogen produces 1 mole of water vapor for every mole of hydrogen gas used. The combustion of 1 mole of an eight-carbon atom containing hydrocarbon (representative of the hydrocarbons found in gasoline), on the other hand, produces 9 moles of water vapor per mole of hydrocarbon-nine times as much as burning hydrogen. Not only would there be a beneficial decrease in carbon dioxide emissions in switching to hydrogen fuels, but also there would likely be a decrease in water vapor emissions.

Claybourne C. Snead
Arlington, Va.

Regarding Leo's letter, all legitimate climate scientists are well aware of the important roles played by water vapor and clouds in global warming and climate change. All modern climate-change models strive to include realistic parameterizations of those effects and to separate changes due to human activities from natural climate variations. A simple Web search for "water vapor climate" yields dozens of websites from news media, academic institutions, and government agencies on this topic. Current models suggest that global warming due to anthropogenic greenhouse gases alone will be compounded 50-100% by warming due to increased water vapor in a warmer future atmosphere.

Obviously, many uncertainties are associated with the predictions, not the least of which is how society will respond to the multiple threats posed by anthropogenic climate change. But global climate change is not the only problem we will face from rising carbon dioxide concentrations. Leaders in the petrochemical industries might want to look into the future and decide that it is prudent to preserve our nonrenewable oil and gas supplies to produce the millions of everyday products we make from these resources, rather than simply pumping them out of the ground and burning them.

William M. Landing
Tallahassee, Fla.

In his letter, Leo says "water vapor contributes as much as 95% of the greenhouse gases," implying the effect of carbon dioxide on global warming is insignificant. His statement ignores the well-known facts that the vapor pressure of water is temperature dependent and that water vapor condenses at its dew point. Hence, water vapor may be the dominant greenhouse gas in the tropics and at Earth's surface, but at the low temperatures found at the polar regions and at high altitudes, the effect of noncondensable greenhouse gases (such as carbon dioxide) may be significant. Second, global warming will increase the amount of water vapor in the atmosphere, thereby amplifying the warming effect of noncondensable greenhouse gases.

The claims of alarmists vis-à-vis global warming may well be exaggerated, but the water vapor argument does not justify dismissal of carbon dioxide without further study.

Paul E. Eckler
Wildwood, Mo.

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