Carbon Dioxide is an atmospheric trace gas?

Following on from my post about the molecular composition of the greenhouse gases, I thought I would follow it up with a deconstruction of the 'myth' that Carbon Dioxide is an atmospheric trace gas, at least from the (anthropogenic) global warming perspective.

At first the statement looks quite convincing, because taken literally and without any consideration to the properties of compound gases like Carbon Dioxide, then one can fool many people into believing Carbon Dioxide is insignificant.



As one can see from a pie chart indicating the proportions of gases in the atmosphere, Nitrogen, Oxygen, Argon and Water Vapour constitute the greatest part of the Earth's atmosphere, with Carbon Dioxide, Methane, Nitrous Oxide, Neon and other gases failing to register at all on the chart because the amounts are apparently so tiny.

However, as discussed previously, this hides the issue of the response many compound gases (such as Carbon Dioxide) have to Infra Red radiation. So if you were an Infra Red electromagnetic wave, what components of the atmosphere would you be interested in?



Well it would look a lot different from the one that we humans are aware of. You would mainly see Carbon Dioxide and Water Vapour, with Methane and Nitrous Oxide still hardly registering. In fact Carbon Dioxide becomes 9% of the atmosphere that you would be sensitive to and Water Vapour would be about 90% at least in physical quantity terms.

However there are added complications in that Methane and Nitrous Oxide are more potent warming gases than Carbon Dioxide, so even though they physically represent a smaller proportion of the total, their effects are greater. One also has to remember that water changes state very easily on Earth and hence dynamically responds to the quantity of other warming gases in the atmosphere. Clearly we can see from the pie chart, that doubling Carbon Dioxide would increase the physical quantity from an Infra Red radiation perspective to around 20%, even though the actual amount would be far less.

Well that was a relatively simple analysis, which I hope dispels the idea that Carbon Dioxide is simply a trace gas.

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