Earth 700 my ago

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How life survived the big freeze

Scientists have put forward a theory that could explain how life survived a time when the Earth nearly became a snowball. There is growing geological evidence that our planet went into a big freeze at least twice during the late Proterozoic era, 600-800 million years ago, with the polar icecaps a kilometre deep extending to the equator.

What has puzzled researchers is how such a cold and desolate environment could have prepared lifeforms for the evolutionary explosion that can be seen about 50 million years later in the fossil record.

But a computer model now suggests there may have been gaps just big enough in the ice coverage to provide a refuge for developing life to flourish.

National Geographic

Neoproterozoic 'snowball Earth' simulations with a coupled climate/ice-sheet model

WILLIAM T. HYDE et al
Ice sheets may have reached the Equator in the late Proterozoic era (600-800 Myr ago), according to geological and palaeomagnetic studies, possibly resulting in a 'snowball Earth'. But this period was a critical time in the evolution of multicellular animals, posing the question of how early life survived under such environmental stress.

The authors present computer simulations of this unusual climate stage with a coupled climate/ice-sheet model. To simulate a snowball Earth, they use only a reduction in the solar constant compared to present-day conditions and keep atmospheric CO2 concentrations near present levels. They find rapid transitions into and out of full glaciation that are consistent with the geological evidence. When they combine these results with a general circulation model, some of the simulations result in an equatorial belt of open water that may have provided a refugium for multicellular animals.

Nature 405, 425 - 429 (2000) (c) Macmillan Publishers Ltd.

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