Ancient forests may have helped stabilize the Earth's carbon dioxide levels and, therefore, its climate, a new study published in Biogeosciences suggests.

"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 a statement. "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?"

To answer this question, the researchers turned their attention to a process known as weathering.

Weathering refers to the breakdown of rocks' and soils' minerals, of which silicates are often a main ingredient. When silicate minerals come in contact with rain and atmospheric CO2, also known as carbonic acid, they weather, removing CO2 in the process. The greenhouse gas is then transported via the products of these reactions into the oceans and rivers, where they remain locked away in the form of carbonate rocks such as limestone.

Forests play an important role in the weathering process, speeding it up as the trees and fungi around their roots break the rocks and minerals in the soil down for food.

"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."

They estimated that when CO2 concentrations were roughly half than they are today, the trees and fungi became far less effective at breaking down silicate minerals. This is because low levels of CO2 hurt a plants' ability to photosynthesize, leading to less carbon-energy for the roots and fungi and, as a result, diminished nutrient uptake. In the end, the researchers hypothesized, this would have likely put a brake on weathering rates for millions of 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," Quirk said.