Rapid rise in atmospheric CO2 marked the end of the Late Palaeozoic Ice Age

Atmospheric CO2 is thought to play a fundamental role in Earth's climate regulation. Yet, for much of Earth's geological past, atmospheric CO2 has been poorly constrained, hindering our understanding of transitions between cool and warm climates. Beginning similar to 370 million years ago in the Late Devonian and ending similar to 260 million years ago in the Permian, the Late Palaeozoic Ice Age was the last major glaciation preceding the current Late Cenozoic Ice Age and possibly the most intense glaciation witnessed by complex lifeforms. From the onset of the main phase of the Late Palaeozoic Ice Age in the mid-Mississippian similar to 330 million years ago, the Earth is thought to have sustained glacial conditions, with continental ice accumulating in high to mid-latitudes. Here we present an 80-million-year-long boron isotope record within a proxy framework for robust quantification of CO2. Our record reveals that the main phase of the Late Palaeozoic Ice Age glaciation was maintained by prolonged low CO2, unprecedented in Earth's history. About 294 million years ago, atmospheric CO2 rose abruptly (4-fold), releasing the Earth from its penultimate ice age and transforming the Early Permian into a warmer world.