EcoTone

As plants become starved for CO2, rock weathering diminishes. Credit: study coauthor David Beerling

Earth is currently in an ice age. (People, especially climate change naysayers, sometimes forget that.) The growth of the Antarctic ice sheet began about 25 million years ago, and by about 3 million years ago we had a full-blown ice age.  What has remained a mystery to climate scientists and geologists alike is that the geological conditions that ensued should have led to runaway glaciation, with most of the world covered by ice. But that didn’t happen, and scientists have puzzled over why. In yesterday’s Nature, Mark Pagani of Yale University and his colleagues show that the buffering factor that effectively saved the Earth from getting covered with ice was plants.

The backstory goes like this: Volcanic activity over geologic time causes large amounts of CO₂ to be released into the atmosphere, and the weathering of rock through the mountain building, in turn, sequesters atmospheric CO₂ and buries it in the ground. The building of several huge mountain ranges — including the Andes, the Himalayas, the Tibetan Plateau, and ranges in western North America — over the past 25 million years would have been expected to suck so much CO₂ out of the atmosphere that the world should have drastically cooled, creating prime conditions for a world-covering glaciation.

Coauthor Ken Caldeira of The Carnegie Institution says that this is where plants come in. He says in a press release:

“The rates of weathering reactions are largely controlled by plants. Their roots secrete acids that dissolve minerals, they hold soils, and they increase the amount of carbon dissolved in groundwater. But when levels of carbon dioxide get too low, the plants basically suffocate and the weathering slows down. That means less sediment is eroded from the uplands and less carbon can be buried. It’s a negative feedback on the system that has kept carbon dioxide levels from dropping too low.”

Caldeira also adds that we shouldn’t get any bright ideas about these weathering processes sucking CO₂ out of the air and saving us from global warming. We’re releasing CO₂ into the air about 100 times faster than all the volcanoes in the world combined, he says. So unless we want to wait around hundreds of thousands of years for the Earth to clean up our mess, those plants and mountains won’t save us.

Pagani, M., Caldeira, K., Berner, R., & Beerling, D. (2009). The role of terrestrial plants in limiting atmospheric CO2 decline over the past 24 million years Nature, 460 (7251), 85-88 DOI: 10.1038/nature08133