Which came first?

Did you see "The Great Global Warming Swindle" on TV the other day? Very interesting. I remember reading a book by Michael Crichton a couple of years back on the same theme - I think it was called "State of Fear". What is clear is that there is a statistical correlation between the levels of atmospheric CO2 and temperature. The question is - which causes which? We are continually being told that CO2 levels drive temperature whereas the TV programme and the book maintain that it's the other way round - variations in temperature cause variations in CO2. Given that both sides agree that there is a significant lag factor involved, it should surely be easy enough to determine who's right? Let's settle the issue once and for all.