Shallow-rooted shrubs had big impact on paleoclimate

Published online 23 December 2022

Gas exchange may have reduced atmospheric CO2 levels enough to lead to significant climate cooling, long before forests evolved.

Letizia Diamante

AI-generated picture of clubmoss shrubs
AI-generated picture of clubmoss shrubs

Tais Dahl
Atmospheric CO2 levels may have been much lower some 385 million years ago than previously thought, with the earliest vascular plants substantially reducing levels long before forests evolved. The findings come from paleoclimate and Earth systems modelling data, which suggest that early CO2 levels were only about 1.5 times greater than current levels, compared to 10 times greater as previously assumed.

An international research team, including Kion Norrman from King Fahd University of Petroleum and Minerals in Saudi Arabia, calibrated an existing leaf gas-exchange model with two species of modern clubmosses presumed to be physiologically similar to their ancient relatives. The researchers correlated CO2 uptake by the plants to the CO2 in the environment, using the density and size of the plants’ stomata – small openings on the leaves that allow the exchange of gases and water – and their carbon isotope composition. They then applied the model to 66 fossilised clubmosses dating back to 410-382 million years ago. 

The modelling predicted lower atmospheric CO2 and higher O2 levels before the evolution of deep-rooted trees compared to other studies. Specifically, it predicted a decline in CO2 from around 2,500 parts per million (ppm) CO2 to around 525–715 ppm when shrub-like plants were spreading on our planet. 

The findings suggest that CO2 decline and simultaneous O2 increase, even by the earliest land plants, was enough at the time to have led to significant climatic cooling and partial glaciation, consistent with geological evidence.

“Was it the first floras or the first forests that had the most impact on the Earth? For too long this question has been answered by the results from models built on models, and supported by a few rather anomalous data points based on untested proxies. This study has moved the debate forward by being able to acquire new data from fossil plants supported by robust data from living relatives. We can all look forward to more data from fossil plants to further constrain this new CO2 curve,” says John Marshall of the University of Southampton (UK), who was not involved in this study. 


Dahl T.W. et al. Low atmospheric CO2 levels before the rise of forested ecosystems. Nat. Commun (2022)