@Canuck RE: "please don't quote any ice core data."
Can you cite a good reason as to why they should not use such data? Don't the samples yield a direct measure of the atmospheric components at the time the ice formed (limited to available ice, of course)? Much like now: To study the very recent composition and change, 50-yr-old still-inflated tires are sampled. Isn't the ice better than silt and tree ring observations which must be interpreted to 'infer' what the levels were?
Granted: Silt {with its pollen an other organics) and tree ring records give a better measure of the *response* of living things to conditions during a given time and some 'radio-isotopes' can be pulled out of there for further clarification.
Two things that probably have not changed in the time-spans that are relevant to any of these different records:
(1) mean atmospheric pressure -- (essentially, it's the partial pressures of individual gasses that are recorded in the ice)
(2) the freezing temp (triple point, actually--very important)of water.
Based on these two points, I *suspect* that the ice-core data is valid.
