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, 14
C and 12
C is the most common. 13
C is about 1% of the total. 14
C accounts for only about 1 in 1 trillion carbon atoms.
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 (12
C vs. 13
C); thus they have lower 13
C ratios. Since fossil fuels are ultimately derived from ancient plants, plants and fossil fuels all have roughly the same 13
C ratio – about 2% lower than that of the atmosphere. As CO2
from these materials is released into, and mixes with, the atmosphere, the average 13
C ratio of the atmosphere decreases.
Isotope geochemists have developed time series of variations in the 14
C and 13
C concentrations of atmospheric CO2
. One of the methods used is to measure the 13
C 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 13
C in atmospheric CO2
goes up or down, so does the 13
C 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 13
C ratios. Because the age of each ring is precisely known** we can make a graph of the atmospheric 13
C ratio vs. time. What is found is at no time in the last 10,000 years are the 13
C ratios in the atmosphere as low as they are today. Furthermore, the 13
C 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 13
C 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 13
C is decreasing. Measurements of 13
C 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 13
C ratio in the CO2
trapped in ice cores. The tree ring and ice core data both show that the total change in the 13
C 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 13
C 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