Yahoo News 1 Jul 09;
PARIS (AFP) – Vegetation helped save Earth from runaway cooling that would have encased the planet in ice, according to a study published on Wednesday.
The paper sheds light on the big natural mechanisms that over hundreds of millions of years have swung the globe like a pendulum between deep chill and intense heat.
Around 50 million years ago, the planet's poles were ice-free and crocodiles roamed the Arctic. But that was followed by a long period of cooling, in which levels of carbon dioxide (CO2), the principal "greenhouse" gas that traps solar heat, progressively declined.
Belching volcanoes provided the main source of this CO2 -- in contrast to today, when the gas comes overwhelmingly from burning fossil fuels and is driving dangerous climate change.
But there was also a force which removed CO2: a chemical reaction that occurs when silica rocks are weathered.
Over the aeons, the gas is dissolved into groundwater, which flows to the sea and eventually the carbon is sequestered on the ocean floor.
Climate scientists have long puzzled about what happened at a key point in this weathering process.
Around 25 million years ago, Earth was wrenched by a period of mountain building that threw up the Himalayas and the Andes.
This created conditions that, in theory, should have sucked nearly all the CO2 out of the atmosphere and plunged the planet into a deep freeze.
Yet it clearly did not happen, and the question is why.
The answer, according to US geophysicists, lies in the buffering power of plants.
Vegetation, especially trees, suck in atmospheric CO2 in the process of photosynthesis and also play a key role in the weathering of rocks.
Their roots secrete acids that dissolve minerals, they hold soils and they increase the amount of CO2 dissolved in groundwater.
As the CO2 levels plummeted, plants were starved of their essential gas for life, according to the team's hypothesis.
This slowed the weathering process down, and led to less burial of the carbon. As a result, there remained enough CO2 in the air to avoid the "Iceball Earth" scenario.
"As the CO2 concentration of Earth's atmosphere decreased to about 200 to 250 parts per million (ppm), CO2 levels stabilised," lead author Mark Pagani, an associate professor of geology and geophysics at Yale University, said in a press release.
The study, published in the British journal Nature, is based on simulations of the global carbon cycle and observations from plant growth experiments.
If plants saved Earth from endless chill, they are unlikely to do the same when it comes to man-made warming, say the authors.
CO2 levels in the atmosphere today are around 385ppm, compared with 280ppm before the Industrial Revolution.
"We are releasing CO2 to the atmosphere about 100 times faster than all the volcanoes in the world put together," said Ken Caldeira of the Carnegie Institution for Science.
"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."
Plant life saved Earth from an icy fate
Anil Ananthaswamy, New Scientist 2 Jul 09;
Besides the obvious benefits they bring, it looks like we owe our very existence to plants, which helped prevent the Earth from freezing over during the past 25 million years.
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 about 1000 to 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.
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: the 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.
Negative feedback
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
