Current concentrations of carbon dioxide (CO2) in Earth’s atmosphere are unprecedented in human history. But CO2 levels today, and those that might occur in coming decades, did occur millions of years ago. These studies reveal a radically different Australia to the one we inhabit. The continent was warmer and wetter, and filled with unfamiliar plant and animal species. This concentration last occurred on Earth between 3 million and 5 million years ago, in the Pliocene period.