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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Explaining how the water vapor greenhouse effect works

What the science says...

Select a level... Basic Intermediate

Increased CO2 makes more water vapor, a greenhouse gas which amplifies warming

Climate Myth...

Water vapor is the most powerful greenhouse gas

“Water vapour is the most important greenhouse gas. This is part of the difficulty with the public and the media in understanding that 95% of greenhouse gases are water vapour. The public understand it, in that if you get a fall evening or spring evening and the sky is clear the heat will escape and the temperature will drop and you get frost. If there is a cloud cover, the heat is trapped by water vapour as a greenhouse gas and the temperature stays quite warm. If you go to In Salah in southern Algeria, they recorded at one point a daytime or noon high of 52 degrees Celsius – by midnight that night it was -3.6 degree Celsius. […] That was caused because there is no, or very little, water vapour in the atmosphere and it is a demonstration of water vapour as the most important greenhouse gas.” (Tim Ball)

When skeptics use this argument, they are trying to imply that an increase in CO2 isn't a major problem. If CO2 isn't as powerful as water vapor, which there's already a lot of, adding a little more CO2 couldn't be that bad, right? What this argument misses is the fact that water vapor creates what scientists call a 'positive feedback loop' in the atmosphere — making any temperature changes larger than they would be otherwise.

How does this work? The amount of water vapor in the atmosphere exists in direct relation to the temperature. If you increase the temperature, more water evaporates and becomes vapor, and vice versa. So when something else causes a temperature increase (such as extra CO2 from fossil fuels), more water evaporates. Then, since water vapor is a greenhouse gas, this additional water vapor causes the temperature to go up even further—a positive feedback.

How much does water vapor amplify CO2 warming? Studies show 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. When other feedback loops are included, the total warming from a potential 1°C change caused by CO2 is, in reality, as much as 3°C.

The other factor to consider is that water is evaporated from the land and sea and falls as rain or snow all the time. Thus the amount held in the atmosphere as water vapour varies greatly in just hours and days as result of the prevailing weather in any location. So even though water vapour is the greatest greenhouse gas, it is relatively short-lived. On the other hand, CO2 is removed from the air by natural geological-scale processes and these take a long time to work. Consequently CO2 stays in our atmosphere for years and even centuries. A small additional amount has a much more long-term effect.

So skeptics are right in saying that water vapor is the dominant greenhouse gas. What they don't mention is that the water vapor feedback loop actually makes temperature changes caused by CO2 even bigger.

Basic rebuttal written by James Frank

Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial


Last updated on 5 July 2015 by pattimer. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

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Comments 376 to 382 out of 382:

  1. So if CO2 increases temperature that increases H2O and H20 increases temperature, why doesn't that increase temperature forever or is there a mechanism that cools the climate before that happens?

  2. Likeitwarm:

    You like asking questions that you think are "gotcha's", don't you?

    Tell you what: what you are describing is an infinite series. Put the infinite series into a mathematical form, including a variable that tells you how much H2O increases by for a given temperature increase, and then how much that temperature increase will increase H2O, and see how it behaves?

    You will probably find that not all infinite series lead to infinite increases. Some of them have finite limits and asymptotically approach a limiting value (as long as the appropriate mutiplliers fall within certain limits).

    When you have your mathematical expression of the problem you are asking about, get back to us.

  3. Oh, darn. The moderator has pointed Likeitwarm to a place where the answers to my homework assignment can be found.

    The infinite series I had in mind is:

    1 + x + x*x + x*x*x....


    1+ x + x2 + x3 ...

    where x is the additional single-step feedback temperature rise added to the initial 1 degree rise. At each subsequent step, x acts on the extra rise from the previous step, hence the x2, x3 terms.

    Brilliant mathematicians have managed to find a closed form (finite)  solution to that infinite sum, for x less than 1.

    sum = 1/(1-x)

    As long as x < 1, there is an eventual stable sum. If x = 1, the denominator becomes zero and the sum becomes infinite. If x>1 the sum at each step  represents an exponential increase.

  4. ScienceTruther @379 ,

    we have been over all this, with other threads and other monikers, so many times before.

    You do not even try to understand the very basic physics.  You do not get to be published in Nature journal, and you do not get a Prize in Stockholm.  At a long shot, you might have a chance at an IgNobel.


    [BL] Note that the comment being responded to no longer exists.

  5. The commenter @379 presently calling themselves ScienceTruther is entirely incorrect to cite either Pierrehumbert (2011) or Zhong & Haigh (2013) to support the assertion that "it was predicted from advanced spectroscopy calculations that a four-fold increase in CO2 would be needed for even a detectable, much less dangerous, change in temperature due to CO2." Indeed, Zhong & Haigh demonstrate the exact opposite.


    [BL] Note that the comment being responded to no longer exists.

  6. There hasn't been a single experiment comparing temperatures of infrared transparent airtight capsules with different CO2 concentrations, including on with current-atmosphere concentration... also, comparing with different infrared filters in the "crystal" and heights, so where's the real falsifiable science? It's all based on assumptions without being able to discriminate confounding variables!

  7. Prof Nazar:

    Are you trying to claim that there is no experimental evidence that CO2 will absorb IR radiation at certain wavelengths?

    Are you trying to claim that the amount of absorption does not vary with CO2 concentration?

    If so, then why is it that I can buy commercial off-the-shelf technology that measures CO2 concentrations in air using those principles?

    Are you arguing that the absorption of IR radiation by CO2 does not increase the energy level of that CO2 molecule? And thus the average temperature of the air?

    If so, you are taking a position that does not respect the conservation of energy.

    If you are just saying "I haven't seen anything that convinces me", then you are probably making an Argument from Incredulity.

    You are long on assertions, and really, really short on presenting any actual evidence for your assertion.

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