In a recent article in The Nation, arguing [unpersuasively—Ed.] that the Tennessee Valley Authority should repeat its 1970s fervid enthusiasm for nuclear power to combat global warming, writer Fred Stafford begins with a rhapsodic description of driving by TVA’s Watts Bar nuke.
In that description, he writes, “As I drove across State Highway 68, a pair of white plumes rose above the lush landscape of Eastern Tennessee. A clearing in the trees revealed their source: the two hulking cooling towers of the Watts Bar Nuclear Plant.
“These puffy vertical clouds aren’t greenhouse gasses like carbon dioxide; they’re water vapor.”
What’s wrong with this statement?
Water vapor is a greenhouse gas, responsible for about half of the warmth of our planet. It is, in fact, the most common greenhouse gas and most consequential.
Kerry Emanuel, MIT emeritus science professor, explains, “It’s the most important greenhouse gas in our climate system, because of its relatively high concentrations. It can vary from almost nothing to as much as 3% of a volume of air.”
Carbon dioxide – the chief villain in the climate change drama – constitutes some 420 parts per million in the atmosphere. That works out to about 0.04%, not much in comparison.
This greenhouse gas is necessary, NASA explains: “Greenhouse gases keep our planet livable. Without them, Earth’s surface temperature would be about 59 degrees Fahrenheit (33 degrees Celsius) colder.”
NASA adds, “Data from satellites, weather balloons, and ground measurements confirm the amount of atmospheric water vapor is increasing as the climate warms. (The United Nations’ Intergovernmental Panel on Climate Change Sixth Assessment Report states total atmospheric water vapor is increasing 1 to 2% per decade.) For every degree Celsius that Earth’s atmospheric temperature rises, the amount of water vapor in the atmosphere can increase by about 7%, according to the laws of thermodynamics.”
But water vapor isn’t the culprit in the current climate change conditions. Increasing water vapor isn’t causing global temperatures to rise. It’s a consequence of the warming and amplifies it. It’s also important to note that most water vapor emissions are not man-made, but the result of natural forces such as evaporation of water, sublimation of ice, and transpiration of plant life. About 90% comes from evaporation from oceans, seas, lakes, and rivers.
Water vapor also differs from the other greenhouse gases, and that’s a key to why it isn’t a target in climate change policies. Unlike CO2, methane, and other greenhouse gas emissions, water vapor doesn’t remain a gas. It quickly becomes a liquid or a solid: water and ice. At the temperatures and pressures in our atmosphere, gaseous water can quickly turn into a liquid or solid and fall out of the atmosphere.
MIT’s Emanuel says, ““This process is so rapid that, on average, a molecule of water resides in the atmosphere for only about two weeks.” Students of chemical reactions will recall that residence time is a key to understanding the processes. American astrophysicist Raymond Pierrehumbert at Oxford University notes, “If you put a kilogram of carbon dioxide in the atmosphere it has a warming effect that lasts millennia. You put a kilogram of methane in the atmosphere, the warming effect will disappear almost entirely after 20 years.”
Just how long CO2 remains in the atmosphere is head-scratching. The MIT Center for Global Change Science observes that once a CO2 molecule enters the atmosphere, lots of things at lots of different time frames can happen to it. “CO2 mostly does not break down into smaller molecules while in the atmosphere. Methane…reacts with oxygen to turn into CO2 and water within a matter of years, and that process can be observed and measured. But CO2 molecules typically linger until something absorbs them from the air. Some are quickly taken up by the ocean, plants, and soil. Other atmospheric carbon sticks around for generations.”
Ed Boyle, an MIT ocean geochemist, says of CO2, “The first 10% goes quickly, but it’s not very much of it. The second part goes on a scale of centuries to millennia, but that only gets 80% of it.” What’s left can last for tens of thousands of years.
The next time you drive by a large steam electric power station and see those rising clouds of steam, say thanks to a very important greenhouse gas: water vapor.
–Kennedy Maize
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