The climate is a sensitive balancing act. There are a lot of knobs turning, making the future difficult to model.

But trees have done a surprisingly good job adapting quickly to keep atmospheric carbon dioxide in a 'sweet' spot for plant life.

As most people know, the cycles of the past have shown that about 90,000 of every 100,000 years have been ice ages. And it's been 12,000 years since the last one. But a biological mechanism could explain how the Earth's atmospheric carbon dioxide and climate were stabilized over the past 24 million years, so things never got too drastic.

When CO₂ levels became too low for plants to grow properly, forests appear to have kept the climate in check by slowing down the removal of carbon dioxide from the atmosphere.

Digital images of trenches in a mineral made by networks of fungi. The circular feature in the picture on the right is a depression made by the formation of a terminal spore by a mycorrhizal fungus, which was linked to the roots of a maple tree under high CO₂. Researcher Joe Quirk says: "These spores are characteristic of the ancient type of fungus that has associated with plant roots since plants first emerged onto the land over 400 million years ago. This is why the image is so exciting – it's good evidence this ancient fungus weathers minerals." The width of the trenches is approximately 5 micrometers and the diameter of the circular spore is about 55 micrometers. Credit: Joe Quirk

"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?"

Natural processes kept atmospheric carbon dioxide in check. Volcanic eruptions, for example, release CO₂, while weathering,
the breakdown of minerals within rocks and soils, many of which include silicates
on the continents removes it from the atmosphere. Silicate minerals weather in contact with carbonic acid (rain and atmospheric CO₂) in a process that removes carbon dioxide from the atmosphere.

Further, the products of these reactions are transported to the oceans in rivers where they ultimately form carbonate rocks like limestone that lock away carbon on the seafloor for millions of years, preventing it from forming carbon dioxide in the atmosphere.

Forests increase weathering rates because trees, and the fungi associated with their roots, break down rocks and minerals in the soil to get nutrients for growth. The Sheffield team found that when the CO₂ concentration was low – at about 200 parts per million (ppm) – trees and fungi were far less effective at breaking down silicate minerals, which could have reduced the rate of CO₂ removal 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," says 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." 

They found that low atmospheric CO₂ acts as a 'carbon starvation' brake. When the concentration of carbon dioxide falls from 1500 ppm to 200 ppm, weathering rates drop by a third, diminishing the capacity of forests to remove CO₂ from the atmosphere.

The weathering rates by trees and fungi drop because low CO₂ reduces plants' ability to perform photosynthesis, meaning less carbon-energy is supplied to the roots and their fungi. This, in turn, means there is less nutrient uptake from minerals in the soil, which slows down weathering rates over millions of years.

"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 explains. But the concentration of CO₂ never fell below 180-200 ppm because trees and fungi broke down minerals at low rates at those concentrations of atmospheric carbon dioxide.

"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.