As we struggle to cope with the ongoing buildup of CO2 produced by burning fossil fuels, can we acquire guidance from the geologic record? Although our ability to reconstruct past atmospheric CO2 content reliably is currently confined to the last 800 thousand years, we do have compelling evidence that this green-house gas played a key role throughout the Earth’s history. It certainly compensated for the young Sun’s lower luminosity. There is no question that it bailed us out of two snowball episodes or that it led to a brief 5 °C warming at the onset of the Eocene. Less certain is that diminishing atmospheric CO2 content was responsible for the global cooling that began 50 million years ago when the Indian subcontinent collided with Asia. Finally, it colluded with changing seasonality, ocean circulation re-organisation and iron fertilisation to generate the 100 thousand year glacial cycles that dominated the last half-million years.