Trees Buffered Earth From Iceball Fate

July 1, 2009 -- 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.

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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 stabilized," lead author Mark Pagani, an associate professor of geology and geophysics at Yale University, said in a press release.