(Part of the How to Talk to a Global Warming Skeptic guide)
Objection: Over the last 600 million years, there hasn’t been much correlation between temperatures and CO2 levels. Clearly CO2 is not a climate driver.
Answer: While there are poorly understood ancient climates and controversial climate changes in earth’s long geological history, there are no clear contradictions to greenhouse theory to be found.
What we do have is an unfortunate lack of comprehensive and well-resolved data. There is always the chance that new data will turn up shortcomings in the models and unforeseen new aspects to climate theory. Scientists in the field are working hard to uncover such things — every scientist relishes the thought of uncovering new data that overturns current understanding. But it makes no sense to reject CO2 as a primary driver of climate change today because it looks, through the foggy glasses of time, like CO2 has not always completely controlled climate changes in the past.
The climate system is complicated — even the configuration of the continents has a big effect — so one can not expect complete correlation on all timescales between temperatures and any single factor.