The historic link between CO2 and temperature

Alex asked me to comment on this graph:

1. Correlation is not causation.
2. The causation, such as it is, is almost certainly the other way around, ie temperature is the cause of the change in CO2 (historically the rises in CO2 follow the rise in temperatures). So at most this graph might indicate a supplementary role for CO2 in climate change.
3. The image only shows the last 400,000 years. In the Ordovician period CO2 levels were generally over 4000 ppm, with an average temperature (only) around 4 degrees higher than today (main difference was polar temperature); that period also culminated in an ice age. Try fitting that on to the graph!
4. Peer reviewed research shows that the climate over the last 500 million years is much more strongly correlated with the solar flux than with CO2.

I think most climate scientists would agree that CO2 is not the only driver of temperature (which is what I take you to be suggesting with that graph). I am happy to accept that it plays a role, and, indeed, that it might amplify other trends. However, I also think that the science is more complex than it is sometimes (IPCC summaries) made out to be.

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