It has long been gospel among proponents of dangerous anthropogenic global warming (AGW) that atmospheric content of carbon dioxide has never, in the last 400,000 years, risen above about 275 ppmv (parts per million by volume) prior to the industrial revolution and its consequent burning of fossil fuels. Current concentration of atmospheric carbon dioxide hovers around 400 ppmv. The AGW contention is based on analysis of bubbles entrained in Antarctic glacial ice and the unproven assumption that the composition of the bubbles accurately reflect the composition of earth’s atmosphere at the time the bubbles were formed. There is much uncertainty about that assumption. There are also the troubling facts that values of pre-industrial carbon dioxide derived from Antarctic ice core estimates are inconsistent with those derived from most other proxy methods and that carbon dioxide values derived from Antarctic cores often don’t agree with the much higher values derived from Greenland ice cores which show values between 330 and 350 ppmv through most of the Holocene.
How ice core bubbles form:
As snow falls and accumulates, the weight of the snow causes it to eventually recrystallize into ice granules called “firn.” At this point the spaces between firn granules are open, allowing migration of gases. With sufficient burial, the firn becomes glacial ice with closed off bubbles. This process can take tens to thousands of years with the result that what eventually becomes trapped in bubbles may have an uncertain relation with the age of the ice and actual atmospheric composition at that time.
After the bubbles are encased in solid ice, it was assumed that the relative amount of gases in the bubble could not change, but that is an unwarranted assumption. It has been shown that super-cooled water exists in the bubbles even at very low temperatures. This super-cooled water preferentially dissolves carbon dioxide compared to nitrogen, oxygen, and argon, leaving the gas component depleted in carbon dioxide. Also, at high pressure and low temperature, carbon dioxide is further sequestered in gas hydrates. The very act of drilling to collect ice cores decompresses the ice, and some of the gas hydrates decompose allowing carbon dioxide to escape into the drilling fluid thereby giving uncertain readings. For instance, Neftel et al. (1982) found gas bubbles with carbon dioxide ranging from 237ppmv to 436ppmv from a Greenland core and values of 257ppmv to 417ppmv from an Antarctica core. (They arbitrarily choose the lower values to represent the “true” reading.)
Enter fossil plants:
Another proxy, plant stomata can be used to estimate pre-industrial carbon dioxide content. Stomata are the microscopic pores in leaves and stems of plants that are used for gas exchange. The density of stomata varies inversely with carbon dioxide concentration. These stomata can be empirically calibrated by comparing plant stomata density to known carbon dioxide concentrations. The stomata of fossil plants can be used to estimate past carbon dioxide concentrations. Estimates of carbon dioxide from stomata show much higher and more variable values compared to ice core estimates. The stomata proxy shows that pre-industrial carbon dioxide levels where similar to those today: 360- to 390ppmv.
In the end, the contention that “pre-industrial” carbon dioxide was 275 ppmv and remained at that level for hundreds of thousands of years until humans began burning large quantities of fossil fuels, is not supported by all the empirical evidence.
It is true that humans have been pumping more carbon dioxide into the atmosphere of late, but natural warming since the “Little Ice Age” (ca. 1300-1850) has also caused more carbon dioxide to exsolve from the oceans. Human contribution to the total carbon dioxide flux is only about 3% (source) and it seems that the contention that we alone have raised atmospheric carbon dioxide more than 100ppmv in the last 100 years is also fraught with much uncertainty.
Geologist David Middleton provides a much more detailed discussion and many graphs here.
References:
Jaworowski, Z., Segalstad, T.V. and Ono, N., 1992b. “Do glaciers tell a true atmospheric CO2 story?” The Science of the Total Environment, Vol. 114, pp. 227-284. Elsevier Science Publications.
Kouwenberg et al., 2005. Atmospheric CO2 fluctuations during the last millennium reconstructed by stomatal frequency analysis of Tsuga heterophylla needles. GEOLOGY, January 2005.
Neftel et al., 1982, Ice core measurements give atmospheric CO2 content during the past 40,000 years. Nature, 295, 220-223.
Wagner et al., 2004. Reproducibility of Holocene atmospheric CO2 records based on stomatal frequency. Quaternary Science Reviews 23 (2004) 1947–1954.