Minimum levels of atmospheric oxygen from fossil tree roots imply new plant-oxygen feedback

Low atmospheric CO2 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 CO₂ levels and global climatic cooling by altering continental weathering processes, but observational constraints on atmospheric CO₂ 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 CO₂ 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 CO₂ 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 CO₂ removal. Rather, shallow-rooted vascular ecosystems could have simultaneously caused abrupt atmospheric oxygenation and climatic cooling long before the rise of forests, although earlier CO₂ levels are still unknown.