Climate Health

Emergence of forests on Earth did not cut down CO2 levels in atmosphere, says study

Renewable energy

HQ Team

December 28, 2022: A collaborative study done by scientists of the University of Copenhagen and the University of Nottingham has discovered that the CO2 levels in Earth’s atmosphere, when forests emerged on our planet some 385 million years ago, were much lower than previously thought

This new study, published in Nature Communications, has important implications for understanding how land plants affect the climate.

Before the emergence of trees and forests, the Earth was covered with shallow shrub-like plants with no flowers. Our understanding of the period led us to believe that the atmosphere at that time had far higher CO2 levels than today. The emergence of forests was believed to have led to the reduction in CO2 levels in the atmosphere. This also attributed to the long ice period on Earth, it is believed.

It is difficult to map the geological history of CO2 on Earth. Climate scientists agree that CO2 plays a crucial role in shaping the climate and it has always been a challenge to determine the CO2 levels on Earth from the beginning.

“We calibrated a mechanistic model for the gas-exchange between plant leaves and the ambient air to the oldest lineage of vascular land plants, namely clubmosses. With this approach, we could calculate the CO2 level in the air solely from observations made on the plant material”, said associate professor Tais W. Dahl from the Globe institute at the University of Copenhagen, who led the study in collaboration with an international team of researchers from Germany, Saudi Arabia, UK, and USA.

The team arrived at its conclusion by observing living plants and fossil plant tissue, the ratio of two stable carbon isotopes, carbon-13 and carbon-12, and the size and density of stomata (pore openings) through which the plant takes up CO2. The researchers then used the same method in living clubmosses to test the approach and found it accurately reproduces ambient CO2 levels in the greenhouse.

“The newly calibrated method to study CO2 levels from the geological record is superior to previous approaches that produce estimates with unbound error bars simply because they depend on parameters that cannot be independently constrained in the geological record,” said Barry Lomax Professor at the University of Nottingham and a co-author on the study. 

The research team studied the oldest vascular plant fossils that lived before and after trees evolved on our planet and discovered that the ratio of the two stable carbon isotopes, carbon-13 and carbon-12, is very similar to that of modern plants. Further, the stomata density and size were also very similar to the ones found in plants now.

Fossil Study

To further test their approach, Dahl and colleagues collected data from 66 fossils of three distinct species of club mosses found in 9 different localities worldwide 410 to 380 million years in age. In all cases, the atmospheric CO2 levels were only 30-70% higher (~525 – 715 ppm) than today (~415 ppm). This is far lower than previously thought (2000-8000 ppm). The Earthis believed to have been a temperate planet with mean tropical surface air temperatures of 24.1-24.6°C. 

“We used a fully coupled atmosphere-ocean model to find that Earth had ice-covered poles when forests emerged. Yet, land plants could thrive in the tropical, subtropical and temperate zones,” explains Georg Feulner from the Potsdam Institute for Climate in Germany, who co-authored the study. 

The new study suggests that trees actually play an insignificant role on atmospheric CO2 levels over longer time scales because early trees had deeper root systems and produced more developed soils that are associated with lower nutrient loss. With more efficient nutrient recycling in soils, trees play a lesser role in the weather than the shallow shrub-like vegetation that came before them. This idea goes against previous thinking that trees with deeper root systems promoted CO2 removal through enhanced chemical weathering and dissolution of silicate rocks. 

Dahl and colleagues showed that the primitive shrub-like vascular plants could have caused a massive decline in atmospheric CO2 earlier in history than believed when they first spread on the continents. The model shows that the vascular ecosystem would have simultaneously led to a rise in atmospheric O2 levels.

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