Low atmospheric CO2 levels before the rise of forested ecosystems
1 day ago Charles Rotter 29 Comments
Tais W. Dahl,
Magnus A. R. Harding,
Julia Brugger,
Georg Feulner,
Kion Norrman,
Barry H. Lomax &
Christopher K. Junium
Nature Communications volume 13, Article number: 7616 (2022) Cite this article
Abstract
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.
Introduction
Atmospheric CO2 is a greenhouse gas that has affected Earth’s climate throughout geological history2,3. Its variation in the past informs us about the natural long-term sources and sinks. In the absence of anthropogenic fossil fuel combustion, the dominant atmospheric CO2 source is volcanic outgassing, and this source is balanced mainly by the removal that occurs when CO2-bearing fluids chemically react and weather silicate rocks followed by deposition of carbonate in the oceans4. The dissolution of silicate minerals in the weathering zone occurs via interactions between the terrestrial ecosystem and geological processes that make fresh rock available at the surface for reaction. Yet, the role of biology and the CO2-sensitivity of the feedbacks governing global CO2 removal is debated5,6,7,8,9. Enhanced continental weathering is suggested to have caused a decline in atmospheric CO2 pressure (pCO2) from a level ~10 times higher than today’s concentration3,10,11 to near modern levels linked to the Devonian-Carboniferous transition from greenhouse to icehouse conditions in response to the afforestation of the continents. This process is accompanied by burial and preservation of organic matter that also influence atmospheric CO2 levels and, in turn, acts as the main long-term source of atmospheric O2. However, recent geochemical evidence and Earth system models12 suggests atmospheric oxygenation occurred well before trees evolved on the continents ~393–383 Ma1,3. Further, the temporal correlation between plant colonization and the Permo-Carboniferous glaciation has been disputed8. There is compelling evidence that Earth also transitioned into a glaciated state in the Ordovician-Silurian13. Yet, the link between glaciation and atmospheric CO2 is complicated, and palaeoclimate models shows that glaciations could persist even at 12–14 times pre-industrial atmospheric levels (PIAL, 280 ppmv)14. Therefore, a precise reconstruction of atmospheric pCO2 in relation to plant evolution is key to assess the impact of the terrestrial biota on Earth’s climate. Here, we show that atmospheric pCO2 was markedly lower than previously thought when trees and forests appeared on our planet.
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