I was thinking about the fact that "they" measure the CO2 at Mauna Loa. an active volcano.

What does an active volcano emit???

Do I see any hands up?

Then I found this:

Climate Change: Incorrect information on pre-industrial CO2

Statement written for the Hearing before the US Senate Committee on Commerce, Science, and Transportation:

....In a crushing criticism, two independent groups of scientists from disciplines other than climatology (i.e. not supported from the annual pool of many billion "climatic" dollars), convincingly blamed the Mann et al. paper for the improper manipulation and arbitrary rejections of data. The question arises, how such methodically poor paper, contradicting hundreds of excellent studies that demonstrated existence of global range Medieval Warming and Little Ice Age, could pass peer review for NATURE? And how could it pass the reviewing process at the IPCC? The apparent scientific weaknesses of IPCC and its lack of impartiality, was diagnosed and criticized in the early 1990s in NATURE editorials. The disease, seems to be persistent.

Conclusion

The basis of most of the IPCC conclusions on anthropogenic causes and on projections of climatic change is the assumption of low level of CO2 in the pre-industrial atmosphere. This assumption, based on glaciological studies, is false. Therefore IPCC projections should not be used for national and global economic planning.....