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EUR: Green power saved Earth from iceball fate

Thu Jul 2 03:17:32 EST 2009
EDS: EMBARGOED for release at 0300 AEST Thursday July 2

PARIS, July 1 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."