Ashley Ballantyne at the University of Colorado, Boulder, and colleagues analysed 4-million-year-old Pliocene peat samples from Ellesmere Island in the Arctic archipelago to find out what the climate was like when the peat formed. At that time, CO2 levels are thought to have been close to current levels – around 390 parts per million – but global temperatures were around 2 to 3 °C warmer than today. It was the last warm period before the onset of the Pleistocene glaciation, and is used by climate researchers as a model for our future climate. Previous studies using computer models have suggested that...