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.
Dessler’s work does seem to agree with a number of other studies I’ve been following. I believe, however, the 3 degrees Celsius of future warming unnecessarily confuses concerns regarding CO2. A doubling of CO2 would increase global temperatures by +2C. (not 3 deg. as inferred) based on Dessler’s finding. Beyond that, the increase is significantly less because of the exponentially decreasing effect CO2 can assert at higher concentrations.
Most of the climate warming that CO2 can force has already occurred. Based on a number of other studies looking at this issue, current science estimates CO2 might assert approximately another +0.4 to +0.6 deg. rise by the end of the century if we continue our current CO2 emission trends. That estimate doesn’t factor in the albedo effect of tropospheric clouds and cloud modulators (not to be confused with water vapor) which serves to cool the planet.
So the best guesstimate would range from somewhere between +0.2C to +0.4C of warming above present levels by the end of the century with very little warming beyond that providing mother nature doesn’t have come cooling tricks up her sleeve. It should be mentioned that the modulators for tropospheric clouds and the degree of albedo feedback is very poorly understood presently so all numbers presented by me or by others regarding predicted climate change is for entertainment purposes only.
Is Andrew Dessler Really confirming that measurements show that more water vapor actually causes net surface temperatures to increase or only that warmer air will usually will have more water vapor and thus the two correlate.
Does more water vapor cause secondary effects such as more clouds that cause cooling and lower water vapor?
In central Minnesota for example, average water vapor increases 600% from winter to summer, tracks the average temperature in a few days, but there is less seasonal temperature rise that just the change in solar heat can do alone after accounting for the delayed response from the thermal inertia of the soil and atmosphere. So overall net feedback is negative!
But quite a lot of he 2500 seem to be ONS data sets which have been around for a while. I’d like to pick apart the new data that might be of a different nature!
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Yes I truly agree with this water vapor has a big part on global warming. Studies shows that water vapor feedback roughly doubles the amount of warming caused by CO2. So if there is a 1°C change caused by CO2, the water vapor will cause the temperature to go up another 1°C.
[...] energy back out to space. Has there been a break through? Check out the work of Andrew Dessler Andrew Dessler says water vapor’s role in warming now understood | Water | EarthSky CO2 is a climate forcer. The resulting increase in temperature results in a positive feedback loop [...]
“Yes I truly agree with this water vapor has a big part on global warming. Studies shows that water vapor feedback roughly doubles the amount of warming caused by CO2. So if there is a 1°C change caused by CO2, the water vapor will cause the temperature to go up another 1°C.” by Janna
I also agree with her, as we can see the more the CO2 produces the greater the warm we feel, I think it’s really the time to act and do something in our mother earth like planting trees to offset the CO2 and reduce the CO2 by limiting the production of vehicles and find alternative ways. Thanks for sharing this informative article.