Earth’s climate isn’t easy to cool down – especially if you want it cold enough for ice caps to form. According to a model of the geological forces shaping the climate over the past 420 million years, reaching such “ice house” temperatures requires a combination of rapid rock erosion removing carbon dioxide from the atmosphere and less CO2-releasing volcanic activity.

Despite record high temperatures on brief human timescales today, Earth’s climate is currently colder than usual. “When you look back in the geological past and you try to…