An updated version of this post is now available.
The fact that CO2 increases in the past 150 years are due virtually entirely to human activities is so well established that one rarely sees it questioned. Yet is is quite reasonable to ask how we know this.
There are actually multiple, largely independent lines of reasoning, discussed in some detail in the IPCC TAR report, Chapter 3. One of the best illustrations of this point, however, is not given in IPCC. Indeed, it seems not all that well appreciated in the scientific community, and is worth making more widely known.
Carbon is composed of three different isotopes 14C, 13C and 12C of which 12C is the most common and 14C (used for dating purposes) is only about 1 in 1 trillion atoms. 13C is about 1% of the total.
Over the last few decades, isotope geochemists have worked together with tree rings experts to construct a time series of atmospheric 14C variations over the last 10,000 years. This work is motivated by a variety of questions, most having to do with increasing the accuracy of the radiocarbon dating method. A byproduct of this work is that we also have a very nice record of atmospheric 13C variations through time, and what we find is that 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 no surprise because fossil fuels have lower 13C/12C ratios than the atmosphere.
The total change is about 0.15%, which sounds very small but is actually very large relative to natural variability. Although it has proved quite challenging to do the analyses, there are a limited number of measurements of the 13C/12C ratio in ice cores. 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% 00 or about 5 times less than that observed in the last 150 years. The ice core data also agree quite well with the tree ring data where these data sets overlap.
I will put a couple of plots up when I get a chance. For those who are interested, 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, 1731�1748. for tree rings, and
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 d 13Cin atmospheric CO. Tellus 51B, 170�193.