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

Atmospheric CO is thought to play a fundamental role in Earth's climate regulation. Yet, for much of Earth's geological past, atmospheric CO has been poorly constrained, hindering our understanding of transitions between cool and warm climates. Beginning ~370 million years ago in the Late Devonian and ending ~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 ~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 CO. Our record reveals that the main phase of the Late Palaeozoic Ice Age glaciation was maintained by prolonged low CO, unprecedented in Earth's history. About 294 million years ago, atmospheric CO rose abruptly (4-fold), releasing the Earth from its penultimate ice age and transforming the Early Permian into a warmer world.