Water vapor has role in global warming

Andrew Dessler is an atmospheric scientist at Texas A&M University. He’s an expert on water vapor, the most abundant greenhouse gas in Earth’s atmosphere. At a science meeting in late 2009, Dessler told EarthSky that the effect water vapor plays in global warming is now understood.

Andrew Dessler: One of the durable urban legends you hear is that the models don’t get water vapor right, or we don’t understand water vapor. And that’s no longer the case.

Dessler explained that water vapor has been proven to be a major contributor to global warming. He said carbon dioxide emissions provide the initial warming, by increasing surface temperatures on the planet. Warmer temperatures cause more water to be evaporated off the oceans, which increases the amount of water vapor, or humidity, in the air.

Andrew Dessler: The higher humidity in the atmosphere, because water vapor is a greenhouse gas, gives you additional warming. It’s that amplification that we call the ‘water vapor feedback.’

Dessler said data from AIRS, a NASA satellite instrument that measures water vapor across the world, confirmed this feedback.

Andrew Dessler: You get twice the warming with the water vapor feedback than you would without the water vapor feedback.

In other words, water vapor makes carbon dioxide twice as effective at warming the planet. Dessler has been studying the role of water vapor in the atmosphere for the past 10 years. He said today, the science of water vapor is well understood, but the public is still confused about it.

Andrew Dessler: If you go back 20 years, there was some credible arguments that there were holes in our understanding. But over the course of the last 20 years, we’ve really nailed down some of these problems. But for whatever reason, the knowledge that we’ve really solved a lot of these problems hasn’t made it into the public debate.

He said that almost all of the water vapor in the atmosphere comes from evaporation off oceans, not from human activities. Dessler added that water vapor doesn’t act like most other greenhouse gases.

Andrew Dessler: The important thing to realize with water vapor – it is the biggest greenhouse gas – but it’s tied tightly with surface temperature. If you know what the surface temperature of the planet is, you know how much water vapor is in the atmosphere. But it acts like a feedback, not a forcing. So it’s correct that it’s the most important greenhouse gas, but it’s not correct to think that therefore carbon dioxide is not important.

Dessler broke down the numbers for the common scientific assumption of 3 degrees Celsius of future warming.

Andrew Dessler: Most of that warming turns out to come from feedback, not the direct warming of carbon dioxide. Carbon dioxide alone would give you one degree, and then the water vapor feedback gives you another degree, and then there are a bunch of other feedbacks give you the last degree. But of the feedbacks, water vapor is the most important one.