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

<p>Traditionally, the evolution of trees and the establishment of the first forests during the Devonian (419&#8211;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&#8211;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&#8211;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.</p>