Posted by: Barry Bickmore | February 24, 2012

Water Vapor Feedback Infographic

A common contrarian argument is that “water vapor is a more important greenhouse gas than CO2, but we don’t regulate evaporation”.  For instance, Roger Helmer, a British member of the European Parliament put it this way.

But the Warmists have the bizarre idea that only CO2 matters.  Certainly CO2 is a greenhouse gas, but it’s not even the most important one.  That’s water vapour, and there’s nothing we can do about it (as long as the wind blows over the ocean).

You can read a good rebuttal about this over at Skeptical Science, so I won’t reinvent the wheel here.  However, I made a nice infographic about why this “argument” is ridiculous… if you know any physics.  Here it is, and use it however you like.


  1. “… if you know any physics”

    That being the key requirement. If you know any physics and the most basic climate science, you won’t make such an ignorant argument to begin with. And honestly, do the ignoramuses really think climate scientists just up and forgot about the effects of water vapor?

    Dunning-Kruger at its finest.

  2. Except unlike CO2, water vapor is not a homogeneous distribution. It’s in fact by far the most dynamic component of the whole atmosphere, which is hardly consistent with it being a positive feedback.

    • Hi RW,

      FACT: Warmer temperatures cause more evaporation.

      FACT: Warmer air can hold more water without it condensing.

      • FACT: More evaporation causes more heat to be removed from the surface as the latent heat of evaporation.

        FACT: Evaporation ultimately drives cloud formation.

        FACT: On global average, clouds reflect more energy back out to space than is ‘blocked’ or delayed beneath them (about 20 W/m^2 more).

        • RW,

          1. Your first fact is correct, but is irrelevant unless the latent heat effect is greater than the increased greenhouse effect. What evidence do you have that it is?

          2. Your second fact is correct, but irrelevant since climatologists treat cloud feedbacks separately.

          3. Your third fact may be correct (I’m not motivated to check), but it says nothing about what the temperature gradient of the cloud effect is around the present temperature. We’re talking about climate CHANGE here, not a static description of the climate system.

          4. My main point was simply that water vapor IS an important greenhouse gas, but it CAN’T BE a significant climate driver because of its short residence time. It CAN BE a significant feedback mechanism, though.

          Don’t you agree with this last point? (You seem to be saying you think it is a net negative feedback, which would go against one point on my infographic, but hopefully you agree with the rest of what I said.)

        • BTW, it occurs to me that even if there is evaporative cooling at the surface, in a few days the water vapor condenses again, releasing that heat. That may be higher up in the atmosphere, but the lower atmosphere is pretty well mixed around.

        • FACT: that heat doesn’t leave the atmosphere, therefore that heat increases the back radiation from the atmosphere, heating the ground.

          FACT: Without condensation, clouds DO NOT FORM

          FACT: On average, clouds stop the radiation of energy from the earth through the IR window to space more than they reduce the incoming radiation.

          FACT: You haven’t read the science.

    • Rather an amusing “come-back” from a denier there. The have been decades of “Water us a MUCH MUCH MUCH bigger greenhouse gas than CO2!!!” from deniers.

  3. […] […]

  4. “Roger Helmer, a British member of the European Parliament …”

    LOL Is this Helmer the scientist? Isn’t this a bit of a strawman argument, Dr. Bickmore? I don’t think even your nemesis Lord Monckton uses this position.

    “do the ignoramuses really think….”

    Could you be a little more shrill Mr. “Skeptical”?

    • Dan,

      I made the graphic because friend of mine (a scientist) was asking for ideas on how to explain this concept simply to someone. The fact is that it’s a very common misconception that, for instance, is constantly being promulgated by the editors of my local newspaper, no matter how often they are told it’s ridiculous.

      And Dana’s frustration is pretty reasonable sounding to me. I don’t know how many times I’ve been told that climate scientists are ignoring water vapor. Right.

  5. Which “local newspaper”? Both the Trib and Des News have long since guzzled the Kool-Aid on CAGW.

    • The Daily Herald. In fact, a few months ago they quoted Craig Idso about this, but I found out that it was really Helmer they were quoting.

  6. There is some indication that irrigation and deforestation both have enough of an impact on water vapor to actually make a difference globally – the IPCC addressed “Tropospheric Water Vapour from Anthropogenic Sources” on pg. 185 of the AR4 (chapter 2.5.6) and found that evaporative cooling effects dominated the GHG effect from water vapor from these sources. Water vapor production from combustion was found to be a much smaller volume than from irrigation and deforestation, and so was not addressed either.


    • Good point. I don’t think we can compete with the ocean very well, though.

      • Oh, certainly, global humidity is dominated by ocean evaporation, but over land masses there is room for irrigation and deforestation to change humidity levels sufficiently to cause radiative forcing changes that, though small, can be calculated…

        (I guess all that would change about the graphic would be the last bullet, from “nothing changes, nobody cares” to “very little changes, almost nobody cares”)

  7. Er, there was supposed to be a however – eg, “water vapor from irrigation does have a global GHG effect, HOWEVER, that warming effect is smaller than the simultaneous cooling effect from evaporative cooling” (the IPCC estimated +0.03 W/m2 from GHG effects of irrigation H2O over Asia, but presumably less in terms of global average)

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