Greenhouse gases, water vapor and you

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Several readers pointed out an omission in last week’s post about the National Oceanic and Atmospheric Administration’s release of its Annual Greenhouse Gas Index, which showed that man-made gases that contribute to global warming continued a steady rise. The post -– and the AGGI –- mentioned carbon dioxide, methane, nitrous oxide and other gases, but failed to mention the biggest contributor to global warming: plain old water vapor.

“I want to comment that the way-dominant greenhouse gas in the atmosphere is not mentioned, namely water vapor,” writes Ken Saunders of Pacific Palisades. “Water vapor accounts for about 97 percent of the total (natural plus man-emitted) greenhouse warming of the planet. See, e.g., John Houghton’s ‘The Physics of Atmospheres, 3rd edition,’ Cambridge University Press, 2002.”


This is true, water vapor is the major player in the greenhouse effect and is often omitted from reports and reporting about global warming -– mostly because it is more of a symptom than a cause in global climate change, and cannot be easily mitigated.

Tom Boden, director of the U.S. Energy Department’s Carbon Dioxide Information Analysis Center at Oak Ridge National Laboratory, acknowledges in an email: “Folks are right when they state water vapor is a powerful greenhouse gas and not routinely measured directly in the atmosphere. Atmospheric water vapor is difficult to measure, highly reactive, and variable in amount due to meteorological conditions (i.e., atmospheric water vapor is continuously being generated from evaporation and continuously removed by condensation).”

“Water vapor is the most important greenhouse gas and natural levels of [carbon dioxide, methane and nitrous oxide] are also crucial to creating a habitable planet,” writes John Reilly, professor at MIT and co-director of the Joint Program on the Science and Policy of Global Change, Center for Environmental Policy Research, in an email.

That idea leads many to believe that global warming is natural and cannot be affected much by human activity. Reader Roy W. Rising of Valley Village writes: “Today’s report focuses on a bundle of gases that comprise a very small part of total of ‘greenhouse’ gases. It totally disregards the long-known fact that about 95% of all ‘greenhouse’ gases is WATER VAPOR! Spending billions of dollars to alter a few components of the 5% won’t affect the natural course of climate change.”

Reilly warns, however, that scientists don’t blame water vapor or clouds for global warming.

“Concerns about global warming are about how human beings are altering the radiative balance,” says Reilly. “While some of the things we do change water vapor directly, they are insignificant. Increasing ghg’s [greenhouse gases] through warming will increase water vapor and that is a big positive feedback [meaning: the more greenhouse gases, the more water vapor, the higher the temperature]. But the root cause are ghg’s. So in talking about what is changing the climate, changes in water vapor are not a root cause.”

Water vapor is, however, included in modeling used to study global warming. Boden adds: “We do measure water vapor fluxes routinely at the Earth’s surface in terrestrial systems. All climate models account for water vapor in the processes of evaporation, condensation and transpiration. Since water vapor is naturally occurring and mostly driven by natural processes it would be difficult to mitigate (e.g., cap on a lake) and thus does not enter into reduction discussions.”

So, when NOAA’s Jim Butler confirmed in our previous post that carbon dioxide, nitrous oxide, methane, and two CFCs cause 95% of global warming, he meant that these five gases are at the root of a complex reaction that also involves water vapor and any number of other factors. The fact that you and I are responsible for generating a bunch of those man-made gases makes them the five to watch.

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-- Dean Kuipers