Low atmospheric CO 2 levels before the rise of forested ecosystems

The emergence of forests on Earth (~385 million years ago, Ma) 1 has been linked to an order-of-magnitude decline in atmospheric CO 2 levels and global climatic cooling by altering continental weathering processes, but observational constraints on atmospheric CO 2 before the rise of forests carry large, often unbound, uncertainties. Here, we calibrate a mechanistic model for gas exchange in modern lycophytes and constrain atmospheric CO 2 levels 410–380 Ma from related fossilized plants with bound uncertainties of approximately ±100 ppm (1 sd). We find that the atmosphere contained ~525–715 ppm CO 2 before continents were afforested, and that Earth was partially glaciated according to a palaeoclimate model. A process-driven biogeochemical model (COPSE) shows the appearance of trees with deep roots did not dramatically enhance atmospheric CO 2 removal. Rather, shallow-rooted vascular ecosystems could have simultaneously caused abrupt atmospheric oxygenation and climatic cooling long before the rise of forests, although earlier CO 2 levels are still unknown.