About 50 million years ago, Earth was a hothouse – the poles were ice free, and crocodiles lived in the Arctic. Then, the concentration of carbon dioxide in the atmosphere started dropping from around 1000-1500 parts per million (ppm), and the Earth began to cool.
By about 24 million years ago, the uplift of the Himalayan and Andes mountain ranges led to large-scale weathering of rocks, a process that removes massive amounts of CO2 from the atmosphere. This reduced the greenhouse effect and cooled the planet.
Plant life is the reason the Earth has not frozen over (Image: Jim Ryce / Rex Features)
But something kept the CO2 levels from dropping beyond a certain point, preventing Earth from turning into an icehouse. Now, Mark Pagani of Yale University in New Haven, Connecticut, and colleagues have identified our saviours: plants.
Trees play an important role in the sequestration of atmospheric CO2 in magnesium and calcium carbonate rocks. As mountains grow, rocks break down and become transported to the foothills, where trees hold them in place in the soil and break them down into minerals. These then combine with CO2 to form, for example, limestone.
The team used computer models to simulate the sensitivity of vegetation to atmospheric CO2 and climate, and found that as the CO2 concentration dropped to about 200 parts per million, the plants started starving and suffocating. This caused a negative feedback, preventing weathered rocks from turning into carbonates, thus putting a natural brake on the sequestration process and letting CO2 levels rise again.
"The carbon dioxide level came down and banged up against this lower limit, and has more or less been banging up against this lower limit for the last 20 odd million years," says team member Ken Caldeira, of the Carnegie Institution of Washington in Stanford, California. "Plants [played] a critical role in preventing the Earth from going into a deep freeze."
Mathew Huber, a climate scientist at Purdue University in Lafayette, Indiana, says that the work provides a "nice explanation of a negative feedback in the climate system".
Journal reference: Nature, DOI: 10.1038/nature08133