It is time to stop quivering in our boots in pointless fear of the future and just roll up our sleeves and build it.
- Ray Pierrehumbert

Friday, April 13, 2007

Isotope evidence

Of course, the fact that modern humans are the source of new carbon is stunningly obvious if you look at the shape of the concentration curve against time.

I'd always heard that carbon isotopes were the clincher, but when asked for evidence, all I'd been able to find was an unsupported assertion in the IPCC TAR, which always irked me. Of course, IPCC has a rule of not referring to anything that isn't published in peer review, and this bit of data is so widely known and accepted among anyone who might conceivably care in a professional capacity that perhaps it appears nowhere in the primary literature.

A sort of catch-22!

Fortunately, Tamino has found a source for enough raw data to make the point.

Anyone who claims the CO2 (never mind the warming) is natural variability hasn't had a leg to stand on for a long time. This is just the icing on the cake.

So (to get meta again for a second) I am not arguing against logical argument. It's necessary, just not sufficient, is all I'm saying. It's nice to finally have solid evidence to point to on this matter.

3 comments:

Heiko said...

I think a simple comparison between human emissions and the annual CO2 increase in atmospheric inventory is the clincher.

This is so easy to check and calculate. I've just done it again for fun, and I get 2 billion tonnes of C per ppm increase of CO2 in the atmosphere just from a basic back of the envelope (surface area of the Earth, mass of the atmosphere from 10 tonnes per square metre, molar mass of air compared to moalar mass of CO2 and C).

So, if human emissions are some 7 billion tonnes of carbon per annum, and the atmospheric increase is 3-4 billion tonnes of carbon per annum, what further evidence do we need?

Michael Tobis said...

You would be amazed at how often this is called into question outside scientific circles. It is true that there is little doubt from the CO2 trajectory and mass conservation arguments.

See the opening paragraphs of This article in Fermi Paradox which is what pointed me to the data, and to Tamino's blog.

People who stubbornly get the mass conservation records wrong are unlikely to be convinced by isotopes, either.

However, this "isotopes clinch the matter" has been common. The TAR makes the point without backing it up, which I consider a minor flaw, but surely a flaw.

It's nice to have something to point to to back up the assertion.

Heiko said...

What I don't understand is why the isotope graph can seriously be called a "clincher".

I find it quite poor evidence on its own. For example, look at the height of the valleys and peaks due to the annual cycle and the thirty year trends. The trend is much more important for CO2 concentrations than for the isotope ratio compared to the annual troughs and peaks. Roughly speaking the annual change is only a third the size of the 30 year trend for concentrations, but it's the same magnitude for the isotope ratio.

And, whatever reason is blamed for the increase in CO2, might be responsible for the change in isotope ratio (eg the supposed undersea volcanoes blamed for the increase get their CO2 from a reservoir rich in one isotope).

Or, the reason for the isotope trend might be completely unrelated, and it's pure co-incidence (say the ratio trend is due to a change in ocean absorption properties and the CO2 trend due to volcanoes).

The isotope data seem to me a little piece in a jigsaw puzzle, and by far and away the clincher is the simple mass balance, for two reasons. Firstly, it's easy to work out based on first principles. It requires essentially no reliance on other people's expertise/scientific authority. That's a point I greatly relish. Even for the simple effect of CO2 as a greenhouse gas, I've got to rely on other people's judgment. It should be straightforward to estimate, I am credibly told by people who should know it is. But, I can't do it myself. I know little about absorption bands, saturation of absorption bands etc.... I can't in fact think of an easy way to get from first principles to a reliable estimate of the simple greenhouse effect of CO2 (leaving out water vapour feedback etc.., which if included would be even more complex)

And secondly, the mass balance argument means we'd have to find a reason for why CO2 shouldn't have risen in spite of the emissions being more than 100% of the increase, and that's rather tough.