One interesting thing I learned when reading Tyler Volk's CO2 Rising deserves a blog post of its own: It's called the Suess effect, after the Austrian chemist Hans Suess, a fellow who I've quoted here before. The basic idea here is that by burning fossil fuels (oxidizing fossil carbon), we are diluting the amount of ¹⁴C in the atmosphere of our planet. As you may be aware, ¹⁴C is produced continuously in the upper reaches of our atmosphere as nitrogen atoms get bombarded by solar particles (specifically, thermal neutrons). Hydrogen is a byproduct of the reaction. It goes something like this:
This ¹⁴C isn't stable over the geologic long-term: it spontaneously breaks down, via radioactive decay, with a half-life of about 5730 years. This property means that ¹⁴C is really useful for dating organic matter of the relatively recent geologic past, a time of particular interest to us, since that's when our species developed its distinctive cultures. But the short half-life means that by the time 60,000 years or so have gone by, there's so little left that it's no longer useful for radiometric dating.

Of course, most of the fossil fuels we use are far older than 60,000 years [A lot of the coal we use formed during the Carboniferous, about 360-299 million years ago], so their store of ¹⁴C long ago reverted to ¹⁴N. When we burn this carbon, we combine it with oxygen and send it into the atmosphere. Isotopically, this fossil carbon looks different from the rest of the carbon in the biosphere.

So overtime, as we burn low-¹⁴C fossil fuels, we would expect to see the total atmospheric ratio of ¹⁴C to other isotopes of carbon decrease. The carbon in the atomsphere becomes more and more enriched in ¹³C and ¹²C as low-¹⁴C coal, oil, and natural gas get oxidized.

In other words, the abundance ratios of these different isotopes of carbon provide a fingerprint for where all that extra carbon dioxide is coming from: it has to be from ¹⁴C-depleted sources, like old carbonaceous sedimentary deposits. For a nice graph illustrating this, click here.

Last thing: The Suess effect holds up only until the early 1950s because after that extra ¹⁴C produced during nuclear bomb testing starts to build up again, skewing the overall trend.

See also this image. (A high-res slide explaining the phenomenon, and detailing different natural repositories of carbon isotope data.)

References:

P.P. Tans, A.F.M. de Jong, and W.G. Mook. "Natural atmospheric ¹⁴C variation and the Suess effect," Nature 280, 826 - 828 (30 August 1979); doi:10.1038/280826a0

Labels: books, CO2, coal, isotopes, oil