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How the Earth became a snowball
by Richard Ingham
Agençe France-Presse
March 18, 2004 |
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Did the break-up of ancient land masses plunge the Earth into a frozen hell?
(Image: NASA) |
The break-up of ancient land masses plunged the Earth into a freezing white hell that lasted millions of years, U.S. and French researchers suggest.
This created 'snowball Earth', where ice sheets covered continents and seas froze almost down to the equator, an event that occurred at least twice between 800 million and 550 million years ago.
How these brutally protracted Ice Ages unfolded had always been a puzzle.
Researchers from the French National Centre for Scientific Research (CNRS) and the University of Florida think they have the answer and publish their theory in today's issue of the journal Nature.
Some scientists speculate that the Sun abruptly cooled for a while or that the Earth tilted on its axis or experienced an orbital blip that dramatically reduced solar warmth.
But this latest research throws light on a little-explored theory: how tectonic wrenches that ripped apart the Earth's land surface provoked a runaway icehouse effect.
At the time, the Earth's future continents formed a super-continent dubbed Rodinia, an entity so vast that rainfall, brought by winds from the oceans, failed to travel far inland.
When Rodinia pulled apart, breaking up into smaller pieces that eventually formed today's continents, rainfall patterns changed dramatically.
Rain tumbled over basalt rocks, freshly spewed from vast volcanic eruptions.
That initiated a well-known reaction between water and calcium silicate, in which carbon dioxide (CO2) molecules were taken from the air and sequestered in calcium carbonate, which was then washed down to the seas.
But the computer model published today suggested that the sucking of the greenhouse gas CO2 from the air led to a catastrophic cooling.
This is the opposite of the greenhouse effect, where rising CO2 levels have been blamed for global warming.
According to the simulation, before Rodinia broke up about 800 million years ago, CO2 concentrations were about 1830 parts per million; and the mean global temperature was 10.8°C.
Fast-forward to Rodinia's break-up, 50 million years later, and the picture was greatly different.
CO2 levels are at 510 parts per million and the planet's mean temperature was a frigid 2°C.
"Tectonic changes could have triggered a progressive transition from a 'greenhouse' to an 'icehouse' climate during the neo-Proterozoic Era," the authors said.
Combine this with the rock and rainfall reaction, and the simulation "results in a snowball glaciation".
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