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.

A cubic metre is 1000 litres, which is not really that much air:

It’s simple enough to verify this figure. One mole of an ideal gas at standard room temperature and pressure occupies 22.4 litres. A mole, for those not familar with chemistry, is defined as follows:

The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities (e.g., atoms, molecules, ions, electrons) as there are atoms in 12 grams of pure carbon-12 (12C), the isotope of carbon with atomic weight 12. This corresponds to a value of 6.022142 × 1023 elementary entities of that substance [Avagadro's number]. It is one of the base units in the International System of Units, and has the unit symbol mol.

So, 1 cubic metre contains 44.64 moles of gas, of which 0.0389% is CO2 = 0.0174 moles = 10,458,094,447,812,500,000,000 molecules of CO2. We have a match!

Finally, let’s try to get a feel for just how large a number this is. You can write 1022 in words as a ten billion trillion molecules (0r 10 million quadrillion if you prefer).

The number of stars within the 14-billion-light-year radius of the visible universe (Hubble volume) is estimated to be thirty billion trillion (click on image above), i.e., 3 x 1022. Thus, a mere 3 cubic metres of air, which would sit comfortably on most dining tables, contains as many CO2 molecules as there are stars in the vast span of the visible Universe (which includes an estimated 350 billion galaxies the size of our Milky Way spiral, as well as another 7,000 billion dwarf galaxies similar to the Small Magellanic Cloud).

Bearing these mind-boggling numbers in mind, it’s perhaps not quite so hard to understand how trace atmospheric gases in our atmosphere really do a good job at intercepting infrared radiation. Don’t you think?