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Century-old climate myth still making the rounds

Carbon dioxide or water vapor, pick a side in this global warming debate. The fourth myth in the climate change series keeps the conversation going.

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Myth 2: Increased carbon dioxide (CO2) can’t contribute to global warming: It’s already maxed out as a factor and besides, water vapor is more consequential.

Some climate skeptics claim that the carbon dioxide (CO2) currently in the atmosphere is already “saturated” in its ability to absorb longwave radiation from Earth and therefore additional CO2 in the air won’t make a difference –won’t, that is, absorb more heat. They also argue that water vapor is a more potent greenhouse gas and therefore increases in CO2 shouldn’t be a concern. These claims have been made in recent years by Hungarian physicist Ferenc Miskoczi and other scientists. They were repeated in the Skeptic Handbook, published in 2009 by science writer Joanne Nova. Yet the seed of the argument actually goes back more than a century.

In 1900, scientists published results of a laboratory experiment interpreted at the time to signify that all of the long wavelength radiation emitted by Earth is absorbed by the atmosphere already, and that therefore, adding more CO2 couldn’t possibly make a difference.

Here’s what the scientists did in that early experiment: They sent infrared light through a foot long (30 cm) tube containing a small concentration of CO2 meant to simulate Earth’s atmosphere and then measured how much radiation made it through to the other end. Next, they cut the amount of CO2 by a third and measured how much radiation made it through. As it turned out, the results were nearly the same. Therefore, they reasoned, CO2 is already maxed out in its ability to further warm the planet.

Steam rising out of a tea pot

Image: Westy Ford on Flickr/CC

The flaw, noted climate modeling expert Charles Jackson, was simplifying the atmosphere down into something like a short tube or a thin sheet of glass. In reality, the atmosphere is thick with many layers. As radiation makes its way up through the atmosphere, it gets absorbed and re-emitted many times. And it’s re-emitted in all directions. More CO2 near the surface doesn’t make a big difference, but higher up in the atmosphere, more CO2 means more heat is absorbed and re-emitted (both up and down). The net effect is that it becomes harder for Earth to shed its heat back out to space.

Then there’s water vapor. It absorbs a wider range of wavelengths of radiation than CO2 and is more abundant overall in the atmosphere. So it seems logical that water vapor would have a larger role in climate change than CO2.

But, Jackson noted, Air Force experiments in the 1940s showed that in the upper atmosphere — where Earth’s heat is released into space — there is little water vapor and at lower pressures, it is less able to absorb radiation. So CO2 turns out to be more important than water vapor in the region that counts.

That isn’t to say that water vapor doesn’t matter. All climate models incorporate its effects in their simulations. The difference is that climate scientists consider it a feedback rather than a main driver of climate change. That’s because of observations showing that regardless of changes in global temperatures, global relative humidity stays fairly constant.

You are invited to post comments and follow-up questions on this site. You can also e-mail climate scientists questions. The scientists cannot respond to all questions individually but will address recurring themes with new entries.