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Sunday, 30 September, 2007

Evaporating the water vapor argument

A new study Identification of human-induced changes in atmospheric moisture content (Santer 2007) was published last week, inspiring me to revisit the water vapour argument. A popular skeptic argument (well, a ranking of #20 is no mean effort) is that water vapour is the most important greenhouse gas, rendering CO2 warming relatively ineffective. Water vapour is indeed the most dominant greenhouse gas. The radiative forcing for water is around 75 W/m2 while carbon dioxide contributes 32 W/m2 (Kiehl 1997). Water vapour is also the dominant positive feedback in our climate system and a major reason why temperature is so sensitive to changes in CO2.

Unlike external forcings such as CO2 which can be added to the atmosphere, the level of water vapour in the atmosphere is a function of temperature. Water vapour is brought into the atmosphere via evaporation - the rate depends on the ocean and air temperature and is governed by the Clausius-Clapeyron relation.

If extra water is added to the atmosphere, it condenses and falls as rain or snow within a week or two. Similarly, if somehow moisture was sucked out of the atmosphere, evaporation would restore water vapour levels to 'normal levels' in short time.

Water Vapour as a positive feedback

As water vapour is directly related to temperature, it's also a positive feedback - in fact, the largest positive feedback in the climate system (Soden 2005). As temperature rises, evaporation increases and more water vapour accumulates in the atmosphere. As a greenhouse gas, the water absorbs more heat, further warming the air and causing more evaporation.

How does water vapour fit in with CO2 emissions? When CO2 is added to the atmosphere, as a greenhouse gas it has a warming effect. This causes more water to evaporate and warm the air more to a higher (more or less) stabilized level. So CO2 warming has an amplified effect, beyond a purely CO2 effect.

How much does water vapour amplify CO2 warming? Without any feedbacks, a doubling of CO2 would warm the globe around 1°C. Taken on its own, water vapour feedback roughly doubles the amount of CO2 warming. When other feedbacks are included (eg - loss of albedo due to melting ice), the total warming from a doubling of CO2 is around 3°C (Held 2000).

Empirical observations of water vapour feedback and climate sensitivity

The amplifying effect of water vapor has been observed in empirical studies such as Soden 2001 which observed the global cooling after the eruption of Mount Pinatubo. The cooling led to atmospheric drying which amplified the temperature drop. A climate sensitivity of around 3°C is also confirmed by numerous empirical studies examining how climate has responded to various forcings in the past.

Satellites have observed an increase in atmospheric water vapour by about 0.41 kg/m² per decade since 1988. A detection and attribution study (Santer 2007), otherwise known as "fingerprinting", was employed to identify the cause of the rising water vapour levels. Fingerprinting involves rigorous statistical tests of the different possible explanations for a change in some property of the climate system.

Results from 22 different climate models (virtually all of the world's major climate models) were pooled and found the recent increase in moisture content over the bulk of the world's oceans is not due to solar forcing or gradual recovery from the 1991 eruption of Mount Pinatubo. The primary driver of 'atmospheric moistening' was found to be the increase in CO2 caused by the burning of fossil fuels.

Basic theory, observations and climate models all show the increase in water vapor is around 6 to 7.5% per degree Celsius warming of the lower atmosphere. The observed changes in temperature, moisture, and atmospheric circulation fit together in an internally and physically consistent way. When skeptics cite water vapour as the most dominant greenhouse gas, they are actually invoking the positive feedback that makes our climate so sensitive to CO2 as well as another line of evidence for anthropogenic global warming.

Posted by John Cook at 00:08 AM

Comments

  1. Philippe Chantreau at 00:16 AM on 2 October 2007
    Incidentally, it is fun to point that burning hydrocarbons from fossil fuels is also the only net addition of water vapor to the system, except for a comet impact.
    The simplest case, methane (natural gas) goes like this if I'm not mistaken (fully developed, with the "real" molecule numbers): 3CH4+6O2 gives 3CO2+6H2O. For every molecule of methane burned, 3 molecules of water are added to the atmosphere. It obviously does not change anything to the John's argumentation above and the existing research results, but, even in the absence of that evidence, arguing that water vapor is a more potent GHG would not undermine the argument that we should veer away from fossil fuels.
  2. Philippe, your chemistry is a little askew. Natural oxidation of methane is a chemical process which I don't believe would fall under your definition of burning, yet has the same chemical equation. Why isn't that adding water vapor to the system? Or respiration, or a gazillion other processes occuring all over the globe on, under and above the surface, continually removing and adding water to the atmosphere in a wonderfully dynamic process, varying over hours and millenia, over elevation and geographic location, in ways that we continue to struggle to accurately model and understand.

    Despite what John would have you think in the analysis above, more than one climatologist is continuing to study the effect of water vapor on global climate. To blithely state that the global atmospheric water vapor content is essentially a simple function of global temperature because it is governed by the Clausius-Clapeyron relation (John, is that an unfair paraphrase?), should make a climatologist wince. For example, this relationship is approximately true only near the ground-- and only higher altitude (free troposphere) water vapor effects earth's cooling (precisely because it is colder than the earth), and up here, water content is governed not by C-C relation, but by transport processes such as the rise/fall of warming/cooling air. I can give references. In fact, a quick google just pointed me to IPCC TAR Ch7.2.1 Physics of Water Vapor and Clouds.

    John, although I find your site a delight and in general think you have the right mind-set, you shouldn't over-simplify just to show up the global-warming skeptics. Or you may fall into a similar trap as Philippe and over-extend your reach attempting to bolter to eagerly your already-held belief. Specifically, here I think you are attempting to stifle a scientifically-valid continuing attempt to understand a complicated process, by prematurely asserting the issue is resolved, with a simplistic and/or flawed line of reasoning. A trait I greatly fault Al Gore for sharing.
    Regards,
    Kenton
    [ Response: Good feedback, kenton. I'm not saying we should stop studying water vapor systems - if I give that impression, I'll revisit my words. In fact, I think climate will never be completely understood - there'll always be nooks and crannies to further understand. But I do believe we know enough to act on reducing CO2 emissions.

    Re over-simplification, that's a framing issue I'm still working out. I'd like to make the science accessible without compromising scientific accuracy. The point of this page is not to say water vapor is completely understood but that rather than contradict anthropogenic global warming, it is actually consistent with it. But I will have another look at my treatment of the C-C relation so thanks for the feedback ]
  3. Wondering Aloud at 06:43 AM on 13 December 2007
    Continued study. Very recent but I didn't write down all the details so you may have to dig a bit.

    Spencer etal in Geophysical Research letters Vol 34 have a paper on Rainfall events in the tropics. It appears to clearly show empirical evidence on Lindzen's proposed "iris effect" of water vapor. In addition to being very important to how storms develop over time, it appears to reduce any positive effect of increased water vapor on temperature by about 75%
  4. "Water vapour is also the dominant positive feedback in our climate system and a major reason why temperature is so sensitive to changes in CO2."

    Here you've gone right over the edge into circular thinking. We need some actual evidence.
  5. Philippe Chantreau at 12:56 PM on 31 December 2007
    Good point Kenton. However, for all practical purpose, the carbon/hydrogen compounds locked deep in the crust are not normally available to natural processes. I may not have expressed it very well, but it was in that sense that I said WV from FF combustion constituted a net addition.

    I assume that you were refering to methane from digestive processes or fermentation. Those and other natural processes have to somehow combine carbon/hydrogen that is available. Geologic events can make a bunch of it so from time to time, but not exactly on the scale allowed by the wholesale extraction and combustion of fossil carbon that we have practiced.

    I think it was a little bit of a jump from you to conclude that I was merely reinforcing already held beliefs. I am well aware of a number uncertainties in climate science or other sciences. Believe me, it always makes me somewhat uneasy when I have to administer medications to patients and the mechanism of their action is unknown (happens a lot). You should have a difficult time finding a thread where I describe and assert such beliefs. You may not have noticed that I never attempt to forward any doom/gloom message or anything of the sort. However, just like the water vapor oversimplifications make you cringe, some things like "Global Warming on Pluto" as an attempt to explain things on Earth make me cringe too. I do try to keep myself aware of the existing state of the science and I do try to see where the weight of the evidence is leading, which is not an easy task for a layperson. The mess of overpolitization (inflamed by many skeptic "extremist" organizations) and innumerable clumsy or overhyped press releases certainly does not facilitate that task.
  6. I think the water vapor issue is viable, as humans force 37 times more water vapor into the air than CO2 in the US. This ratio could be much higher in other parts of the world. Forced evaporation does have an effect because you are doing it daily, in the most arid parts of the US. Here’s my two part video on it.

    I’m not sure if imbedded videos will work at this site, but the links will also be at the bottom of this post.






    Part 1
    http://www.youtube.com/watch?v=e8Hdixpk-TQ

    Part 2
    http://www.youtube.com/watch?v=ePLw6DyTYmI

  7. My video did not include contrails, but the moisture from airplane contrails also causes the daily high to low temperature range to decrease. The daily temperature swing from high to low increased in the few days after 9/11 when all planes were grounded and the upper atmosphere had less forced moisture.

    http://facstaff.uww.edu/travisd/pdf/jetcontrailsrecentresearch.pdf
  8. “When looking at water vapor, the amount humans have added to the atmosphere today is the sum of the past few weeks (at MOST), since water has its own equilibrium and just rains out.”
    ------------------
    I agree that the water vapor will rain out, but we are adding the water vapor in a daily process, making the land areas artificially more humid (every day) than they would have been. This man made humidity reduces the heat that is radiated back into space.

    Seventy percent of the earth is covered with water. Let’s look at a “Water World” type earth with no land. The humidity created by being 100% ocean would cause the planet’s temperature to increase. We would have very small daily temperature swings, at any location and I doubt that we would even have ice caps at the poles.

    What if the earth was 100% land with no open water? The result would be a planet with no humidity, with big daily temperature swings, but with the net effect of having a much colder planet.

    The effect of our forcing water into the atmosphere is similar to changing the surface water from 70% to say 75%. It will have and effect on the earths temperature. CO2 is not a factor in these examples and it’s not a major factor in global warming.
  9. During the Cretaceous Period the earth was about 80% covered with water and tropical sea surface temperatures may have briefly been as warm as 42 °C (107 °F), 17 °C (31 °F) warmer than at present and deep ocean temperatures were as much as 15 to 20 °C (27 to 36 °F) higher than today's. (Per Wikipedia)

    http://en.wikipedia.org/wiki/Cretaceous

    -----------------

    “CO2 levels are usually invoked to explain Cretaceous warmth and the flat Cretaceous temperature gradient. This makes sense, since the very active mid-ocean spreading ridges might well have bee associated with out-gassing of CO2 from deep within the Earth. Unfortunately, the geology of the period and stable carbon isotope records, don't really support the idea as well as they might.”

    “Even the most sophisticated quantitative models can't reconstruct the flatness of the Cretaceous temperature gradient. Either our temperature estimates are off, or some important factor is missing from the models. Since dinosaurs and semi-tropical vegetation are known from within 10° of the Cretaceous poles, the problem is likely to be with the theory.”

    http://www.palaeos.com/Mesozoic/Mesozoic.htm

    Take a look at the temperature vs latitude chart. With the earth being cover 80% with water, the “Water World” type moisture effect was coming into play. IMHO
  10. John
    The argument made by BestTimesNow above here is what I was referring to when I remarked about the Clean Air Act of 1975 in the U.S. a while back. Not only does every car with a cat push out more CO2, it pushes out more water vapoer (both by design) and additionally makes the water vapor acidic from Sulphur Dioxide.
    This issue becomes more interesting all the time.

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