CO2 is a trace gas, but what does that mean?

Carbon dioxide, methane, nitrous oxide and most other long-lived greenhouse gases (i.e., barring short-lived water vapour), are considered ‘trace gases’ because their concentration in the atmosphere is so low. For instance, at a current level of 389 parts per million, CO2 represents just 0.0389% of the air, by volume. Tiny isn’t it? How could such a small amount of gas possibly be important?

This issue is often raised by media commentators like Alan Jones, Howard Sattler, Gary Hardgrave and others, when arguing that fossil fuel emissions are irrelevant for climate change. For instance, check out the Media Watch ABC TV story (11 minute video and transcript) called “Balancing a hot debate“.

I’ve seen lots of analogies drawn, in an attempt to explain the importance of trace greenhouse gases. One common one is to point out that a tiny amount of cynanide, if ingested, will kill you. Sometimes a little of a substance can have a big impact.  But actually, there’s a better way to get people to understand, and that’s to follow one of the guiding principles of this blog: “Show me the numbers!“.

In response to a recent post by John Cook on George Pell, religion and climate change, commenter Glenn Tamblyn pointed out an interesting fact: Every cubic metre of air contains roughly 10,000,000,000,000,000,000,000 molecules of CO2. In scientific notation, this is 1022 — a rather large number.

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