Hot and cold Earth through time

What was Earth’s temperature tens to hundreds of millions of years ago? The planet has gone through different periods, some with extensive polar ice caps and others being completely ice-free. Estimating past global temperature is important for understanding the history of life on Earth, for predicting future climate, and more broadly, to inform the search for other habitable planets. However, there have been major disagreements about whether there has been an overall decline in Earth’s temperature over time. On page 1316 of this issue, Judd et al. (1) report a new reconstruction of Earth’s temperature over the last 485 million years by combining climate models with geological data. In contrast to some estimates, they conclude that global warm periods were maintained in similar temperature ranges. This corroborates recent predictions by Isson and Rauzi (2) from a large data compilation of different geological samples, establishing a wider agreement.