July 1, 2009

Plants Kept Earth From Becoming Ice Ball

Over the past 24 million years, plants on the Earth have helped reduce the amount of CO2 in the atmosphere and saved the planet from becoming completely frozen, according to a report on Wednesday.

Writing in the journal Nature, researchers at Yale University, the Carnegie Institution of Washington and the University of Sheffield used simulations to show that vegetation actually stabilized the Earth's climate and kept it from becoming locked within an icy shell.

"Ultimately, we owe another large debt to plants" said Ken Caldeira from the Carnegie Institution of Washington at Stanford University's Department of Global Ecology, who co-authored the report.

"Aside from providing zesty dishes like eggplant parmesan, plants have also stabilized Earth's climate by inhibiting critically low levels of CO2 that would have thrown Earth spinning into space like a frozen ice ball."

Despite large emissions of CO2 into the atmosphere by volcanoes, natural processes of sedimentation have occurred to capture carbon-containing minerals within the Earth's crust.

Scientists point to a period of mountain building on Earth that should have absorbed nearly all of the carbon from the atmosphere. This would have resulted in a period of massive freezing over the entire planet.

"But as the CO2 concentration of Earth's atmosphere decreased to about 200 to 250 parts per million, CO2 levels stabilized," said senior author Mark Pagani, associate professor of geology and geophysics and a member of the Yale Climate and Energy Institute's executive committee.

"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," said Caldeira. "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."

"Atmospheric CO2 concentrations have been remarkably stable over the last 20 or 25 million years despite other changes in the environment," he added.

"We can look to land plants as the primary buffering agent that's held CO2 in such a narrow range during this time."

Researchers added that while plants do their part to absorb CO2 from the atmosphere, emissions from human sources are going at such a quick rate that vegetation will be unlikely to catch up with.

"We are releasing CO2 to the atmosphere about 100 times faster than all the volcanoes in the world put together," said Caldeira.

"While these weathering processes will eventually remove the CO2 we are adding to the atmosphere, they act too slowly to help us avoid dangerous climate change. It will take hundreds of thousands of years for these rock-weathering processes to remove our fossil-fuel emissions from the atmosphere."


Image Caption: As plants become starved for CO2, rock weathering diminishes. Credit: David Beerling


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