Note:This is an update to an earlier post, which many found to be too technical. The original, and a series of comments on it, can be found here. See also a more recent post here for an even less technical discussion.
Over the last 150 years, carbon dioxide (CO2) concentrations have risen from 280 to nearly 380 parts per million (ppm). The fact that this is due virtually entirely to human activities is so well established that one rarely sees it questioned. Yet it is quite reasonable to ask how we know this.
One way that we know that human activities are responsible for the increased CO2 is simply by looking at historical records of human activities. Since the industrial revolution, we have been burning fossil fuels and clearing and burning forested land at an unprecedented rate, and these processes convert organic carbon into CO2. Careful accounting of the amount of fossil fuel that has been extracted and combusted, and how much land clearing has occurred, shows that we have produced far more CO2 than now remains in the atmosphere. The roughly 500 billion metric tons of carbon we have produced is enough to have raised the atmospheric concentration of CO2 to nearly 500 ppm. The concentrations have not reached that level because the ocean and the terrestrial biosphere have the capacity to absorb some of the CO2 we produce.* However, it is the fact that we produce CO2 faster than the ocean and biosphere can absorb it that explains the observed increase.
Another, quite independent way that we know that fossil fuel burning and land clearing specifically are responsible for the increase in CO2 in the last 150 years is through the measurement of carbon isotopes. Isotopes are simply different atoms with the same chemical behavior (isotope means “same type”) but with different masses. Carbon is composed of three different isotopes, 14C, 13C and 12C. 12C is the most common. 13C is about 1% of the total. 14C accounts for only about 1 in 1 trillion carbon atoms.
CO2 produced from burning fossil fuels or burning forests has quite a different isotopic composition from CO2 in the atmosphere. This is because plants have a preference for the lighter isotopes (12C vs. 13C); thus they have lower 13C/12C ratios. Since fossil fuels are ultimately derived from ancient plants, plants and fossil fuels all have roughly the same 13C/12C ratio – about 2% lower than that of the atmosphere. As CO2 from these materials is released into, and mixes with, the atmosphere, the average 13C/12C ratio of the atmosphere decreases.
Isotope geochemists have developed time series of variations in the 14C and 13C concentrations of atmospheric CO2. One of the methods used is to measure the 13C/12C in tree rings, and use this to infer those same ratios in atmospheric CO2. This works because during photosynthesis, trees take up carbon from the atmosphere and lay this carbon down as plant organic material in the form of rings, providing a snapshot of the atmospheric composition of that time. If the ratio of 13C/12C in atmospheric CO2 goes up or down, so does the 13C/12C of the tree rings. This isn’t to say that the tree rings have the same isotopic composition as the atmosphere – as noted above, plants have a preference for the lighter isotopes, but as long as that preference doesn’t change much, the tree-ring changes wiil track the atmospheric changes.
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. Furthermore, we can trace the absorption of CO2 into the ocean by measuring the 13C/12C ratio of surface ocean waters. While the data are not as complete as the tree ring data (we have only been making these measurements for a few decades) we observe what is expected: the surface ocean 13C/12C is decreasing. Measurements of 13C/12C on corals and sponges — whose carbonate shells reflect the ocean chemistry just as tree rings record the atmospheric chemistry — show that this decline began about the same time as in the atmosphere; that is, when human CO2 production began to accelerate in earnest.***
In addition to the data from tree rings, there are also of measurements of the 13C/12C ratio in the CO2 trapped in ice cores. The tree ring and ice core data both show that the total change in the 13C/12C ratio of the atmosphere since 1850 is about 0.15%. This sounds very small but is actually very large relative to natural variability. The results show that the full glacial-to-interglacial change in 13C/12C of the atmosphere — which took many thousand years — was about 0.03%, or about 5 times less than that observed in the last 150 years.
For those who are interested in the details, some relevant references are:
Stuiver, M., Burk, R. L. and Quay, P. D. 1984. 13C/12C ratios and the transfer of biospheric carbon to the atmosphere. J. Geophys. Res. 89, 11,731-11,748.
Francey, R.J., Allison, C.E., Etheridge, D.M., Trudinger, C.M., Enting, I.G., Leuenberger, M., Langenfelds, R.L., Michel, E., Steele, L.P., 1999. A 1000-year high precision record of d13Cin atmospheric CO2. Tellus 51B, 170–193.
Quay, P.D., B. Tilbrook, C.S. Wong. Oceanic uptake of fossil fuel CO2: carbon-13 evidence. Science 256 (1992), 74-79
*How much they can be expected to absorb in the long run is an interesting and important scientific question, discussed in some detail in Chapter 3 of the IPCC report. Clearly, though, it is our ability to produce CO2 faster than the ocean and biosphere can absorb that it is the fundamental cause of the observed increase since pre-industrial times.
**The development of continuous series of tree rings going back thousands of years by using trees of overlapping age, is known as dendrochronology (see the Arizona Tree Ring lab web pages for more information on this).
***There is a graph illustrating the sponge data posted here. Thanks to F. Boehm for providing this link.