New proxy estimates reveal low atmospheric CO2 levels before the emergence of forested ecosystems

Traditionally, the evolution of trees and the establishment of the first forests during the Devonian (419–359 Ma) have been linked to an enhancement of terrestrial weathering processes and a subsequent reduction of atmospheric carbon dioxide (CO₂) levels by one order of magnitude. However, empirical estimates of early-Devonian CO₂ concentrations are sparse and carry large error bars. Here we use leaf carbon isotopes, stomata density, and stomata pore length from fossilized lycophytes to estimate atmospheric CO₂ levels 410–380Ma based on a mechanistic model for gas exchange calibrated using their closest modern lycophyte relatives. We find that Earth's atmosphere contained about 525–715 ppm of CO₂ before the emergence of forested ecosystems, much less than previously thought. Using a coupled climate model, we show that Earth was partially glaciated at these moderate CO₂ levels and that this cool climate state is in principle agreement with available climate proxies and fossil evidence for the distribution terrestrial vegetation. Finally, we use a process-based biogeochemical model to demonstrate that our results are consistent with a scenario in which enhanced weathering, climate cooling, and atmospheric oxygenation are associated with the earlier emergence of shallow-rooted vascular ecosystems rather than the appearance of the first forests.