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How did ancient plants affect atmospheric CO2 levels?

A team of scientists led by the University of Copenhagen and the University of Nottingham has found that the Earth’s atmosphere contained far less cardon dioxide than previously thought when forests first emerged on our planet about 385 million years ago. These findings have major implications for understanding how land plants affect the climate.

Before the Earth’s continents were colonized by tall trees and forests, shallow shrub-like plants with vascular tissue, stems, shallow roots, and no flowers had already invaded the land. Scientists have long thought that the atmosphere at that time had far higher CO2 levels than today – with an intense greenhouse effect leading to a warmer climate – and that only with the emergence of forests which assumedly promoted the removal of CO2 from the atmosphere, the Earth tipped into a colder state, with ice covers forming at the poles.

However, by using a cutting-edge method to calculate CO2 levels based on observations made from fossil plant material, the researchers discovered that, between 410 and 380 million years ago, the atmospheric CO2 levels were only 30 to 70 percent higher than today. Moreover, a paleoclimate model showed that the Earth was a temperate planet with average tropical surface air temperatures ranging from 24.1 to 24.6 degrees Celsius.

“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,” said study co-author Georg Feulner, an expert in Climate Modelling at the Potsdam Institute for Climate Impact Research in Germany.  

These findings suggest that trees may actually play an insignificant role on atmospheric CO2 levels over longer time scales, since early trees had deeper root systems and produced more developed soils that were linked to lower nutrient loss. Due to this more efficient nutrient recycling system, trees have a smaller weathering demand than the shallow shrub-like vegetation that came before them. This idea contradicts previous theories arguing that trees with deeper root systems promoted CO2 removal through enhanced chemical weathering and dissolution of silicate rocks.

The researchers concluded that primitive shrub-like vascular plants may have caused a massive decline in atmospheric CO2 levels earlier in history, when they first spread across the continents, while simultaneously leading to a rise in atmospheric oxygen levels.

The study is published in the journal Nature Communications.

By Andrei Ionescu, Staff Writer

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