The AR4 SPM claims on page 2 that
"The global atmospheric concentration of carbon dioxide has increased from a pre-industrial value of about 280 ppm
to 379 ppm3 in 2005. The atmospheric concentration of carbon dioxide in 2005 exceeds by far the natural range over the last 650,000 years (180 to 300 ppm) as determined from ice cores."
This is followed by a scary-looking "hockey stick" diagram (Fig SPM1) of carbon dioxide levels over the last 10000 years:
In the main AR4 WG1 report, Chapter 7, p.512 states that:
The concentration of CO2 is now 379 parts per million (ppm) and methane is greater than 1,774 parts per billion (ppb), both very likely much higher than any time in at least 650 kyr (during which CO2 remained between 180 and 300 ppm and methane between 320 and 790 ppb). The recent rate of change is dramatic and unprecedented; increases in CO2 never exceeded 30 ppm in 1 kyr – yet now CO2 has risen by 30 ppm in just the last 17 years.
On p.511 there is a claim that
Prior to 1750, the atmospheric concentration of CO2 had been relatively stable between 260 and 280 ppm for 10 kyr.
However, Kouwenberg et al. (Geology vol. 33, p.33-36, 2005) states that:
A stomatal frequency record based on buried Tsuga heterophylla needles reveals significant centennial-scale atmospheric CO2 fluctuations during the last millennium. The record includes four CO2 minima of 260–275 ppmv (ca. A.D. 860 and A.D. 1150, and less prominently, ca. A.D. 1600 and 1800). Alternating CO2 maxima of 300–320 ppmv are present at A.D. 1000, A.D. 1300, and ca. A.D. 1700. These CO2 fluctuations parallel global terrestrial air temperature changes, as well as oceanic surface temperature fluctuations in the North Atlantic. The results obtained in this study corroborate the notion of a continuous coupling of the preindustrial atmospheric CO2 regime and climate.
The 260-320 ppm range measured by Kouwenberg et al. is twice the range cited by IPCC, moreover, these numbers are averaged over many decades, since individual measurements show a possible range of 230-350 ppm. The variations from minimum to maximum in Kouwenberg et al. occur on a less than 150 year time scale. These measurements are in direct contradiction with the IPCC statements.
Van Hoof et al (Tellus 57B, 351-355, 2005) found CO2 concentration variations of over 30 ppm in the 13th century. This again contradicts the IPCC's claim that it has not varied by more than 30ppm in 1000 years.
Wagner et al (Quaternary Science Reviews 23 1947–1954, 2004) state that "The majority of the stomatal frequency-based estimates of CO2 for the Holocene do not support the widely accepted concept of comparably stable CO2 concentrations throughout the past 11,500 years." This paper shows variation in CO2 of the order of 50 ppm over a few hundred years, and shows that these results are robust and not localized.
The Kouwenberg, Van Hoof and Wagner papers are not cited by
the IPCC report, even though these papers are readily available and published in highly regarded journals. Nor are these scientists amongst the list of IPCC authors, or
reviewers. So these authors were not even given a chance to comment on
the omission of their results.
A post-AR4 paper by Van Hoof et al (PNAS 105, 15815-15818, 2008) is quite critical of IPCC AR4: "Inferred changes in CO2 radiative forcing are of a magnitude similar to variations ascribed to other mechanisms, particularly solar irradiance and volcanic activity, and may therefore call into question the concept of the Intergovernmental Panel on Climate Change, which assumes an insignificant role of CO2 as a preindustrial climate forcing factor." They find pre-industrial levels up to 319 ppm (higher than claimed by the IPCC) and state that diffusion causes a smoothing of the CO2 record in ice cores.
Once again we see that the IPCC creates a misleading picture by gluing together two sources (ice cores and recent direct measurements) that are not directly comparable. Short-term changes are smoothed out in the ice core data, giving a false impression of stability. And again we see the IPCC ignoring scientific research that does not conform to the message it wants to convey.
Further detailed criticism of the IPCC position on past carbon dioxide levels can be found here. Ice core data on carbon dioxide data in the past has been adjusted downwards, and direct measurements of CO2 levels in the 19th century that do not fit the IPCC picture have been ignored (see also this paper from 1955 reviewing studies of CO2 levels going back to the 19th century, showing a wide range of values with a mean of 335 ppm).
See also this interesting and detailed article comparing ice core records and plant stomata.