I was looking up something else, and stumbled on some interesting data about CO2 and the history of the measurement. (There may be a h/t due to someone here, but it was long enough ago that I let the thread lay there, that now I’ve forgotten what, or who, triggered it…)
This one does a great job of showing how Ice Core CO2 data does not track with Plant Stomata (that have a near instant, i.e. one year, record):
This PDF does a great job of showing all the actual measurement data that has been ignored in setting the “baseline” at 280 PPM or so. It was written in 1954, so has not been subject to “revision”…
Reference to the three charts in figure 3 does not reveal
any significant trend in CO2 content, such as is so clearly
shown in figure 1. Indeed, after excluding values which
the observers themselves have designated as non-representative,
but not any of the others, then the mean value
for the nineteenth century is 335, and for the first third
of the twentieth century 334 parts per million. Such a
close approach to identity of values for the two periods is,
of course, an accident. Referring to the texts of the
papers from which Effenberger made his tabulations, it
appears that there has been wide variability in the means
found for differing geographical regions, on land and on
sea, and from one synoptic weather condition to another.
The data-gathering programs were conducted by mutually
independent observers, using differing techniques.
Notice the numbers are up in the 3xx range…
The three values for the twentieth century, however,
which Callendar rejected average lower than those he
accepted. This does not demonstrate that his choice
was bad, but the fact that he considers so many nineteenth
century values to be overestimates and two twentieth
century values to be underestimates raises a question
about his method of selection.
Since techniques have been improving, the latest
observations should be the most accurate. Duerst 
and Ereutz  found values of 400 and 438.5 parts per
million, respectively, from observations made in 1936
and 1939. Duerst bases his mean on 500 observations,
a reasonably large number, if his techniques are correct.
Kreutz made about 25,000 observations. This is more than
were made in all other herein listed observing programs
combined. He expresses confidence in the accuracy of
his measurements and of his computed mean values.
Admittedly Duerst’s and Ereutz’ values may be more
representative of the atmospheric concentration of CO2 at
the time and place of observation than of the earth, or
even the Northern Hemisphere, as a whole. By the same
token, however, might not some of the high nineteenth
century values and low twentieth century values be as
representative as those Callendar accepted?
This site has a lot of CO2 related information and looks to have focused on the CO2 “issues”
A rather well written look at underestimating past CO2 from Google, of all places:
Since I don’t know if Google is prone to the same revisionist history as Wiki, I’m going to quote blocks of it here:
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:
Note the use of the common “Lying with statistics and graphs” technique of putting the zero line near the 250 point. The graph would be much less frightening if the zero were at zero or the scale was a log scale reflecting the presumed EFFECT of CO2.
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.
The actual article has hot links in the text and I need to go back and pick them up at some point.
I’m too far behind on things to do my usual polemic exposition. You will need to hit the links and read them yourself.
My evaluation leaves me with the conclusion that the land instrumental record shows frequently higher CO2 values than the “accepted” baseline value. There looks to have been some cherry picking going on in the early evaluations of the composite data.
Plant stomata say CO2 has had wide ranging spikes (that do not show up in the Ice Cores). Yet we use ice core data and ignore the plants (which leaves me wondering why plant Tree Rings are “acceptable” but the same plant stomata are not?…)
Ice cores are shown to “have issues” with short term variation in CO2.
The “low baseline” is at best an oversimplification error, at worse a deliberate deception. The choice of graphing techniques argues for an intent to deception.
The “trick” looks like the usual: Cherry pick a low baseline of proxy data, then splice on a higher more volatile instrumental record. Instant Hockey Stick. Start the instrumental record as you come from a cold point so the natural variation looks man made.
OK, sorry I don’t have time for “the usual” but at least this gets the links out there for exploration. I’ll try to ‘spruce this one up’ if time permits.