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The climatic effects of water vapour

physicsweb – May 2003

This article is a couple of years old but it is excellent. I have been meaning to write about it for some time but there have been recent discussions on the web regarding the thermodynamic tendencies of the entire planet, as it relates to space and the sun with comparisons to the Moon, Venus and Mars.  While these discussions are scientifically interesting, they do not really address the issue at hand. 

There are articles and bloggers out there that are saying that we are in steady state due to the thermodynamics of the two bodies and that we cannot get appreciably warmer or colder. I have yet to see a mathematical proof of these claims that proves the points and don’t buy into their logic. There is no question that the planet can get warmer or colder – look at the differences in temperature between Boston and Miami!  Since most of the people that contributed to the IPCC report are calling for an increase of 3-6 deg C, this is similar to the difference in temperature that you could experience by driving south a couple hundred miles.  Also remember that when we talk about thermodynamics, we really need to talk in absolute degrees using the Kelvin scale and at that point we are discussing a 1% temperature change predicted by IPCC.

The real issue to understand is how much energy can the existing greenhouse gases absorb and how much would they absorb if CO2, methane, and others changed their concentrations. There are a variety of articles to read on this subject but since we need to have some understanding of chemistry and quantum physics to really dig into this subject, this article is a good overview. I am trying to simplify the conversation by my quotes below.  Click through and read the article to get a better understanding.  If you really want to dig, start googling.

Of significant note (and probably my other reason for choosing this article) is that the author repeatedly refers to the inaccuracies of the climate models and that they need to do a better job. If you have read much of my site, you will know that this is a favorite topic of mine. We need better models!

Extreme variations in local weather and the seasons make it easy for people to mutter “greenhouse effect”, and blame everything on carbon dioxide. Along with other man-made gases, such as methane, carbon dioxide has received a bad press for many years and is uniformly cited as the major cause of the greenhouse effect. This is simply not correct. While increases in carbon dioxide may be the source of an enhanced greenhouse effect, and therefore global warming, the role of the most vital molecule in our atmosphere – water – is rarely discussed.

Many aspects of the seemingly simple water molecule conspire to make it difficult to model its effect on our climate. Unlike most other atmospheric gases, the distribution of water in the atmosphere varies strongly with time, location and altitude. Water is also unique among atmospheric molecules because it changes phase at terrestrial temperatures.

The atmosphere plays a crucial role in the Earth’s radiation budget because it absorbs both the incoming radiation from the Sun and the outgoing radiation that is reflected from the planet’s surface. However, the radiation in each of these processes has very different wavelengths. The Sun radiates approximately as a black body with a temperature of 5800 K, which peaks in the optical region at a wavelength of about 0.6 m. The reflected radiation profile, on the other hand, is much closer to a black body at a temperature of 275 K, and has a peak at much longer infrared wavelengths (about 11 m).

The greenhouse effect is precisely the difference between the long-wave radiation that is emitted by the Earth’s surface and the upward thermal radiation that leaves the tropopause – the upper boundary of the turbulent portion of the atmosphere that we all inhabit. The greenhouse effect is about 146 W m-2 in clear skies and some 30 W m-2 higher under cloud cover.

For clear skies, the models predict that the atmosphere absorbs much less sunlight than is measured by a variety of satellite and aircraft. The difference between the predictions and the measurements can be as large as 30 W m-2. (see “Radiation budget is called to account” by A Maurellis Physics World November 2001 pp22-23). This problem has become known as the absorption anomaly. And there are even worse problems in understanding absorption models when the sky is cloudy.

Not all models underestimate the amount of atmospheric absorption because some physicists choose to add extra absorption to their models to mop up the surplus radiation.

Molecules absorb radiation at characteristic wavelengths that excite one or more of their rotational, vibrational or electronic degrees of freedom.

More importantly for climatic issues, the vibrational degrees of freedom in water, ozone and carbon-dioxide molecules can absorb light in the infrared region. In the case of carbon dioxide it is these vibrations that break the symmetry of the molecule and enable it to become excited by atmospheric radiation.

Furthermore, the vibrational motions of water have a large amplitude because hydrogen atoms are very light. As a result, water does not vibrate as a simple harmonic oscillator – as most molecules do – and its vibrational transitions do not obey the general harmonic-selection rule. The only transitions allowed by this rule are those in which a vibrational quantum number changes by a single quantum. For water, transitions that involve changes of up to eight vibrational quanta are atmospherically important, which means that the water-vapour spectrum covers a large range of wavelengths and line intensities, and is generally very complex.

Experiments that were performed by Roland Schermaul and the late Richard Learner at Imperial College in London in 2001 have cast previous measurements of the absorption spectrum of water into considerable doubt. … They found that the strong spectral lines absorbed significantly more light – between 5% and 25% – than previous laboratory measurements had suggested.

…water turns out to be responsible for about 60% of the greenhouse effect, while the much-reviled carbon-dioxide molecule accounts for just 26%. Ozone accounts for 8%, and methane and nitrous oxide – the atmospheric concentrations of which have been increased by human activity – contribute a further 8% to the greenhouse effect.

It turns out that typical abundances of carbon dioxide are sufficient to make most of its absorption bands relatively opaque (see figure 3). Because the strong absorption bands are saturated, adding more carbon dioxide to the atmosphere increases its absorptions logarithmically rather than linearly….

We have tried to outline some of the unresolved issues concerning water in the atmosphere. But there are others.

Another problem is that there are few data that tell us about the amount of water vapour in the atmosphere over history, which makes it difficult to determine the climatic effects from long-term changes in the atmosphere’s water-vapour content.

A complete solution to the various problems that are associated with water absorption can only be obtained by constructing an accurate and comprehensive theoretical model of the spectrum of water.

It is clear that the absorption of radiation by water vapour determines many characteristics of our atmosphere. While we would not try to provoke any worldwide movement that was aimed at suppressing water emissions, it would seem that the climatic role of water does not receive the general attention it deserves.

I probably have not done total justice by my above edits. You will learn a great deal more by clicking through to the original article here.

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3 Responses to “The climatic effects of water vapour”

  1. […] is some related reading for you to enjoy:The climatic effects of water vapourIs global warming solar induced?Mars is warming up too!Green Myth-Busting: Martian […]

  2. Water vapor is the major greenhouse gas. However, carbon dioxide stays in the atmosphere much longer. Water vapor will in the absence of a disequilibrium due to changes in the amount of carbon dioxide which is in the atmosphere, the amount of water vapor will reach equilibrium due to evaporation and condensation resulting in precipitation.

    Therefore it is largely carbon dioxide which determines the level of water vapor which is in the atmosphere – by raising the temperature, increasing the evaporation – where the level of water vapor in the atmosphere rises while the relative humidity remains roughly constant.

    As such, a doubling of the level of carbon dioxide directly results in a rise in temperature of 1.2 degrees Celsius, but due to further amplification by water raises the temperature by roughly 2.8 degrees. This is something which is well-known at this point as the result of the past 400,000 years of paleoclimate records. is the place to learn more – but I believe you are already familiar with it.

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