Researchers have pinpointed a biological mechanism that reveals how the natural world dealt with carbon dioxide over the past 24 million years.

The team found that when carbon dioxide (CO2) levels became too low for plant growth, ancient trees would slow their rate of absorption, a European Geosciences Union news release reported.

"As CO2 concentrations in the atmosphere fall, the Earth loses its greenhouse effect, which can lead to glacial conditions," lead-author Joe Quirk from the University of Sheffield said in the news release. "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?"

Before humans contributed to atmospheric CO2 levels by burning fossil fuel, nature worked to regulate itself. Back then volcanic eruptions were one of the larger contributors of CO2, while content weathering ("the breakdown of minerals within rocks and soils" such as silicates) worked to remove it from the atmosphere.

Silicates work to remove carbon from the atmosphere when it comes in contact with carbonic acid (rain and atmospheric CO2). This process creates carbonate rocks, such as limestone, when transported into oceans and rivers. These stones can keep carbon locked away for millions of years, preventing it from entering the atmosphere.

When trees and fungus break down minerals in the soil with their roots they are contributing to this weathering process.  The researchers found that when carbon levels were low (at about 200 parts per million) these plants had a harder time breaking down silicate minerals and removing additional carbon from the atmosphere.

"We recreated past environmental conditions by growing trees at low, present-day and high levels of CO2 in controlled-environment growth chambers," Quirk said. "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 researchers found that low CO2 levels create "carbon starvation" by reducing the plants' ability to perform photosynthesis and contribute to the weathering process.

"The last 24 million years saw significant mountain building in the Andes and Himalayas, which increased the amount of silicate rocks and minerals on the land that could be weathered over time. This increased weathering of silicate rocks in certain parts of the world is likely to have caused global CO2 levels to fall," Quirk said.

"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," he said.