Big freeze turned Earth into a snowball
By Roger Highfield
A COMPUTER analysis of how glaciers formed in the tropics 600 million years ago supports the idea that the Earth was once a giant snowball, a conference was told yesterday.
Dr David Pollard, of Pennsylvania State University, told the American Geophysical Union in Boston, Massachusetts: "There is strong geologic evidence of tropical glaciation at sea level during those times. We wanted to determine how low-level tropical glaciers could have formed."
The analysis suggests the deposits could only have been formed after the oceans were covered by thick sea ice, when the Earth was cloaked in a white blanket around a kilometre thick. Tropical glaciers exist today only on high mountain peaks such as the Andes and Mount Kilimanjaro, and do not reach anywhere near sea level.
Dr Pollard and Prof James Kasting first studied whether tropical ice sheets formed before the oceans completely froze into a snowball Earth, when equatorial oceans were still ice-free and could supply enough moisture for substantial snowfall. They concluded that it was unlikely.
Dr Pollard said: "Ice sheets did form on the tops of these mountains. However, the ice sheets never flowed down to sea level, where we find glacial deposits. Tropical temperatures were still too warm and melted the ice before it could flow down from the mountains." But having the sheets form after the oceans froze also seemed problematic because once the oceans were frozen, the rates of snowfall would fall drastically.
Dr Pollard added: "However, in further simulations with the global climate model for full snowball conditions, snowfall did exceed evaporation of snow and ice in some land areas, allowing a slow build up of tropical ice sheets that would eventually flow to the sea."
Some scientists question whether life could have survived on a snowball Earth. But others argue that ocean life could have survived below gaps in the ice around volcanic islands, or in tropical oceans where sunlight may have limited sea ice thickness to only a few metres. The first hints of snowball Earth came in 1964, when Brian Harland, a Cambridge University geologist, began puzzling over glacial debris close to the equator which suggested glaciers must have reached the tropics.
Around the same time, one of the first climate modellers, Mikhail Budyko of the Leningrad Geophysical Observatory, calculated that if the polar ice caps had spread past a crucial point, a runaway chill would have followed, eventually freezing over the whole of the planet.
The idea fascinated scientists but foundered because once the Earth was frozen there seemed to be no way out - the Earth would remain frozen. Then came a series of insights from Joe Kirschvink of Caltech, who came up with a solution. Volcanoes, protruding above the frozen landscape, would have carried on pumping out carbon dioxide, the greenhouse gas.
On Snowball Earth there was no rain to wash this carbon dioxide out of the atmosphere. Instead it would have built up to higher and higher concentrations - until eventually it sparked off not just global warming but global meltdown. The Earth was roused from its cryogenic slumber. Paul Hoffman, a Harvard geologist, came up with further clues from carbon isotopes in rocks. Living things tend to boost levels of the carbon-13 isotope.
At the time of Kirschvink's proposed snowball, Hoffman measured a gradual drop in carbon-13 relative to carbon-12 and reasoned that algae and photosynthesising bacteria on the planet had struggled to cope with falling temperatures. A colleague, geochemist Dan Schrag, confirmed and extended his work.
Scientists are now starting to believe that this striking climate reversal happened as many as four times between 750 million and 580 million years ago. Each time, the Earth froze over completely for 10 million years then warmed up rapidly when the ice melted because of the accumulated carbon dioxide. Life experienced a series of "bottlenecks and flushes". After the last of these climate shocks came the rise of the Ediacara - odd organisms that may have been the first large multi-celled animals.
Hoffman and Schrag believe the snowballs created conditions that would encourage new species to thrive, notably the first complex creatures on Earth.
-- Anonymous, May 29, 2001