Consider that stomata show greater intrinsic variability than other proxies indicates not that all the other proxies are wrong, but that something is going on with plant stomata that is different than what goes on with other proxies. One consideration is that a leaf grows over a much shorter time than an annual ice ring. Combine that with a plant's sensitivity to water levels, humidity levels, light levels, stress levels caused by grazing or disease... essentially this is the same argument "skeptics" use against tree rings. (Why would they accept one but not the other?) But there is a deeper argument, drawn from both history and logic.
The initial CO2 measurements, many done in Germany, showed readings bouncing all over the place from <50 ppm to >5000 ppm. Some of the variance was from inaccurate equipment, or poor methodology. But some was from another source, if you don't mind the bad pun, since the source of the CO2 was nearby industrial operations. These measurements were taken in the "advanced" cities of Germany, and other places, and this meant they were industrial cities. Labs were often near or part of an industrial complex (or what passed for it at the time) and these were the sources of the high levels of CO2. Foundries of the time were powered by burning coal, and this does produce CO2. Thus the real reason for the great variance in CO2 measurements was poor methodology - even though these researchers might have been using the very best methods of the day, they did not have enough knowledge to realize their siting was bad. Once this was discovered, and people learned how to make and use more accurate equipment, these absurd swings in CO2 were seen for what they were: transient phenomena in particular locations.
The Mauna Loa CO2 observatory was established about 1947, iirc. The observations over the past several decades have not shown any such pattern of extreme ups and downs as were seen in the CO2 readings from a century or so earlier. Instead, a broader look at the problem showed the actual answer. Note that the volcanic eruptions during the time Keeling's observatory has been recording have not caused any such variability. In fact, we do not see any mechanism for such variability on a global scale. No one has ever demonstrated any possible natural phenomena that would give rise to such variations. Given this, and the knowledge that plant stomata are affected by more influences than the ambient air and snow that makes up an ice core, we can be reasonably confident that it is ice cores and not plant stomata that give a better picture; maybe not much better, but overall, better. Combine this with all the other information we have, and it is rather easy to see why most scientists go with all the data, rather than some plant stomata data only. From another perspective, very simply, it is the scientific method we use to choose between all the evidence, and a small subset of it which seems to be an outlier, for obvious reasons.
We've been able to compare ice-core data with directly-sampled atmospheric CO2 (amongst other gasses, particulates, and a few radioneuclides we can directly attribute) since 1958. The ice cores can be accurately dated year by year back to at least 1255 c.e. and correlate tightly with atmospheric data. They can actually be compared with plant stomata data to get more accurate climate and biological data for the area where the stomata samples were taken, if they're within the approximate million-year range of the ice core data. It can tell us a lot about both local weather effects and evolution, among other known effects on stomata.
Ice core data cannot be calibrated to the present. It can't be calibrated to a known time where CO2 was actually measured. There are numerous problems with the accuracy and precision of ice core data.
Plant stomata data is dependent on finding good samples of particular plants at different times. It is difficult to be 100 per cent sure that a plant is going to behave the same way a thousand years ago or in a different place. Species show variation. That said, plant stomata proxies indicate a much greater variation in concentration than does ice core data. I think it is more likely that the interpretion of variation is from real variation. It is fairly simple to figure out reasons why ice core data doesn't show much variability over periods less than about 2 to 5 hundred years.
Oops gotta go. Meeting calls.
I don't know anything about stomates as proxies for CO2 so I don't have an opinion on it.
Ice cores should contain the same concentrations of CO2 present in the atmosphere on local scales at the time the snow was deposited. As someone without a background or any real knowledge in this field, I can't really comment on potential biases from this process. I understand why ice cores can indicate temperature (delta18O ions) and age (annual variations in d18O) so without learning more about it I'd be inclined to think that ice cores are at least somewhat reliable. That doesn't mean stomatal proxies couldn't be more reliable.
Is not really necessary to choose among proxy data, nor is it probably wise. There were several lines of proxy data such as ice cores, geological records, isotope ratios, sedimentation and even plant and tree ring records. There is some uncertainty in all proxy records but the the most reliable results can be found by combining all of the proxy records, historical accounts, and thermometer records.
The reference below has historical proxy records put together by several different researchers, and interestingly, they all form pretty much the same pattern.
Dr Ian Clark is the expert on proxy data. You should read his papers for your answers, but since you know that he is a denier that use to be an alarmist (he taught man-made Global Warming), then I suspect that you won't go there for your answer.
http://video.search.yahoo.com/video/play...
The fact of the matter is that CO2 does not control the climate. Natural Climate Variability Deniers need to stop sipping the cool-aid of The Global Governance crowd at the UN IP CC. Their climate models are totally wrong. How many more billions do we waste on the science when there are true environmental issues to tend to?
Plant stomata or ice cores?
If ice cores, what could effect plant stomata other than carbon dioxide?
If plant stomata, what happens to carbon dioxide ?
1. In ice cores?
2. In the atmosphere?
