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How to Talk to a Climate Skeptic: ‘Climate scientists dodge the subject of water vapor’

Gristmill – December 24, 2006

Gristmill has put a series of articles and for the most part they are excellent.  I suggest you read all of them.  This article is also very good although it does have a few items that, while not untrue, do not completely discuss the issue. I am sure that the author knows better but I wanted to point the incomplete statements.

The article concerns the use of water in the atmosphere and its effects on global warming. The article is correct that, to the best of my knowledge, all climate models include and textbooks discuss the atmosphere’s most prevalent greenhouse gas, water vapor.  However, not all climate models handle water vapor very elegantly.  Most of them assume a fairly constant relative humidity for the zone in question and I am not aware of any of them that effectively models the flow of moisture in the atmosphere that agrees with the latest NASA studies (see this study for new research on this particular issue).  Also, most (but not all) models do not perform the advanced mathematics to model cloud behavior (especially their formation).  Most models simply assume a certain level of cloud opacity and factor this in a percentage of sun absorption, sun reflection, and heat absorption.

The article also says that the amount of H2O varies as a function of temperature.  This is not purely true and the article doesn’t describe where it is not true so this is more an error of omission than commission.  If the atmosphere in a certain locale contains a given amount of water, increasing the temperature will not immediately change the amount of water in the air.  Rather, the relative humidity for that locale will change and the pure amount of water in the atmosphere will remain constant.  This lower (drier) relative humidity will increase the rate of evaporation of water that is on the ground, in lakes and in rivers but that effect will not be immediate and if the atmosphere is moving at the time (windy) than the drier relative humidity will remain in that mass of air.

Finally, it is true that water vapor does have a fairly short “life” in the atmosphere before it returns to earth.  However, it is a slight exaggeration to say that carbon dioxide lasts for centuries.  The word “centuries” implies 200 years or more.  Most theories typically say that carbon dioxide stays in the atmosphere for decade or maybe 100 years but I have never seen a credible resource that says that it takes 200+ years to sink it.

Interestingly, the longer the carbon dioxide takes to sink, the worse the argument is that man made carbon dioxide is causing the recent increase.  We know that CO2 has increased approximately 100PPM in the last 75-100 years, if one adds up all of the CO2 produced by the industrial revolution though and you assume that it takes 200 years to sink out, you find that the CO2 levels are not high enough – they should have been much higher than they are today.  This calls into question our understanding of the carbon cycle but it is easier to simply have the cycle be 20-30 years rather than 200.

Objection: Climate scientists never talk about water vapor — the strongest greenhouse gas — because it undermines their CO2 theory.

Answer: Not a single climate model or climate textbook fails to discuss the role water vapor plays in the greenhouse effect. It is the strongest greenhouse gas, contributing 36% to 66% to the overall effect for vapor alone, 66% to 85% when you include clouds.


…the amount of H2O in the air basically varies as a function of temperature.


CO2 put into the air by burning fossil fuels, on the other hand, stays in the atmosphere for centuries before natural sinks finish absorbing the excess. This is plenty of time to have substantial and long-lasting effects on the climate system.

As I said above.  In general, this series of articles is quite good and I suggest you read them here.  I will continue to comment on them occasionally and I am sure that most of my reviews will be slightly more positive.  In this case, I am sure that the author was trying to simplify the discussion and in doing so, may have slightly misled the reader.

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