Last Saturday I took the kids up to the Mesa Lab to see the “Climate Discovery” exhibit at the National Center for Atmospheric Research (NCAR). NCAR has a lot of graphs and other illustrations about climate change. I asked the tour guide how the climatologists sort out which is the cause and which is the effect, between carbon dioxide and temperature?

The guide brought out Caspar Ammann and Carrie Morrill, both of whom I knew from past presentations (they both have PhDs). This was a public conversation at a public event, so I can report it here. Caspar pointed out the correlated graphs of CO2 and temperature proxies, taken from the Vostok ice core (~420,000 years). He remarked that it is difficult to sort out cause and effect from the ice cores alone. As the air bubbles become trapped in the ice during compression over the first 100 years or so, some CO2 migrates by diffusion between the annual layers. The effect is that the annual CO2 and temperature signals are not as precise over very short time scales, and the lead/lag relationship between the peaks can be obscured. I didn’t pursue this line of inquiry further because I plan to investigate the ice core data myself as a project for Climatology class during the upcoming fall semester.

However, Carrie told me that the increase in CO2 displayed by the Keeling Curve can indeed be attributed to human burning of fossils fuels, and here’s how: The air is getting older. Carbon-14 has a half-life of 5,730 years, and it can be used to date objects up to about 50,000 years old. Carbon-14 decays into Nitrogen-14 through beta decay:

Objects older than 50k years have only the N-14 isotope. By measuring the ratio of Carbon isotopes in organic material, one can determine how many years have passed since that organism was last exposed to the air. We can measure the age of all this carbon dioxide that’s building up in the atmosphere. The following article at states:

How do we know that recent CO2 increases are due to human activities?

“Sequences of annual tree rings going back thousands of years have now been analyzed for their 13C/12C ratios. Because the age of each ring is precisely known** we can make a graph of the atmospheric 13C/12C ratio vs. time. What is found is at no time in the last 10,000 years are the 13C/12C ratios in the atmosphere as low as they are today. Furthermore, the 13C/12C ratios begin to decline dramatically just as the CO2 starts to increase — around 1850 AD. This is exactly what we expect if the increased CO2 is in fact due to fossil fuel burning.”

I spent some time looking for a graph of Carbon isotope ratio vs. time that would show the “dramatic” change at 1850, but couldn’t find one. If you have a link, please post it below.

If temperature rise were currently forcing CO2 rise “naturally”, we would expect newer CO2 to get flushed from the earth’s surface. But it’s old CO2 that’s getting flushed instead, and the most obvious cause is human burning of fossil fuels.

The astute reader may wonder, How do we know that the older CO2 isn’t merely coming out of the Arctic peat bogs? The bogs will flush more CO2 as the Arctic climate gets warmer, and peat bogs are really old.

Carl’s answer: The trend of older atmospheric CO2 has been going on since about 1850, which is the start of the Industrial Revolution:

Global temperature rise didn’t really get going in earnest until about 1975. We’ve only been flushing out the peat bogs for about 30 years, not 150 years: