Researchers in the UK have discovered a biological mechanism that could hold the key to how ancient forests rationed carbon dioxide (CO2) to stabilize the climate in the last 24 million years.

The study titled "Weathering by tree root-associating fungi diminishes under simulated Cenozoic atmospheric CO2 decline," which was published in Biogeosciences, the open-access journal of the European Geosciences Union (EGU), explained the process.

"As CO2 concentrations in the atmosphere fall, the Earth loses its greenhouse effect, which can lead to glacial conditions," explains lead-author Joe Quirk from the University of Sheffield. "Over the last 24 million years, the geologic conditions were such that atmospheric CO2 could have fallen to very low levels - but it did not drop below a minimum concentration of about 180 to 200 parts per million. Why?"

The answer to the "why" lies in the fact that when the CO2 levels became low for plant growth, the forests kept the climate in control by slowing down the elimination of CO2 from the atmosphere. If there are low levels of CO2 in the atmosphere then trees are required to be chopped down to reduce CO2 consumption. However, in this manner, the Earth's atmosphere and climate were stabilized naturally in the past 24 million years without resorting to the removal of trees.

"We recreated past environmental conditions by growing trees at low, present-day and high levels of CO2 in controlled-environment growth chambers," per Quirk. "We used high-resolution digital imaging techniques to map the surfaces of mineral grains and assess how they were broken down and weathered by the fungi associated with the roots of the trees."

The research team from Sheffield University discovered that when the CO2 concentration was low (about 200 ppm), then trees and fungi were less effective at breaking down the silicate minerals that could contribute to a reduced rate of CO2 removal. When the concentration of CO2 drops from 1500 ppm to 200ppm then the weathering rates drop by a third, per the scientists.

The weathering rates reduces as low CO2 decreases plants' ability to perform photosynthesis, which basically means that less carbon energy supply goes to the roots and the fungi. What this implies is that the nutrient uptake from minerals in the soil is less, which in turn is responsible for slowing down weathering over the years.

"It is important that we understand the processes that affect and regulate climates of the past and our study makes an important step forward in understanding how Earth's complex plant life has regulated and modified the climate we know on Earth today," concludes Quirk.

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