[Gondring]

AndyTheBear>>The late 19th century and early 20th century direct measurements tend to disagree drastically varying from 280 to 550.

cogitator>Are you getting that from Zbigniew?

Sounds like Calendar/Slocum debate.

The varied, elevated, 19th Century measurements were conducted by chemical means, different from the Mauna Loa measurements. If you take a look at the corresponding measurements of oxygen concentrations from back then, they were supposedly lower than today's and fluctuating all over the place. The implication is that perhaps we're a bit better able to measure CO₂ now than we were back then.

[Gondring]

uh...Callendar. Sorry.

[AndyTheBear]

The implication is that perhaps we're a bit better able to measure CO2 now than we were back then.

But without dependable data from back then, how can we calibrate our technique of measuring the past CO2 levels? They are certainly more smooth, so I tend to trust them for finding relative change. But what about getting the actual amplitude right. The ice core folks seem to confidently account for a lot of "known" factors, and I don't doubt they are trying their best. But science is about testing your assertions--preferably by real world verification rather then making more complex assertions with computers.