The geologic record makes a strong case that CO2 has been instrumental in driving past climate changes. But as made clear by the record for the last 150 kyr, it has not acted alone. Insolation cycles, ocean circulation re-organisations, latitudinal temperature gradients also contribute. On far longer time scales, tectonics, volcanism and the Sun’s luminosity have as well.

Progress in sorting out the extent to which CO2 has contributed is currently impeded by our inability to reconstruct the atmosphere’s CO2 content reliably for times greater than a million years. All of the proxies involving living organisms are...

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