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Escape from snowball Earth

Early Earth didn’t do things half-way: It may or may not have ever been a solidly frozen “snowball” in the deep geological past, but it was never a half-frozen ball of slush, according to a new study. New models, based on more accurate estimates of how much of the sun’s radiation is reflected off snow and ice on Earth’s surface, holds no answers for the still-controversial snowball Earth hypothesis, but determined that conditions for a middling “slushball Earth” could not have existed.

The snowball Earth hypothesis — the idea that Earth has frozen over completely at least once — has ignited controversy among geologists since geobiologist Joe Kirschvink of Caltech coined the term in 1992. Some scientists have also proposed an intermediate slushball Earth hypothesis, which proposes that the glaciation, while far-reaching, did not reach the equator.

Most studies modeling the snowball process have focused solely on what it would take to send Earth into a deep freeze. Recently, however, the focus has shifted to how Earth could escape such snowball conditions, and how much carbon dioxide it would take to create a greenhouse effect that would transform a frozen Earth into a habitable planet.

Reporting in the Dec. 7, 2006, Geophysical Research Letters, Jeff Lewis, a graduate student in climate science at the University of Victoria’s School of Earth and Ocean Sciences, and his co-authors examined the factors that would allow Earth to thaw. They found that getting Earth into or out of a snowball state depends largely on the parameters used in models, and that these numbers often depend on too many confounding factors.

The main factor Lewis and his colleagues examined was Earth’s albedo — how much solar radiation the planet’s frozen surface reflects back into space. Snow and ice form and decay under many different conditions, and the reflectivity values of the different forms can range quite a bit, Lewis says. For example, old snow has a lower reflectivity than fresh snow, and sea ice has a lower reflectivity value than glacial ice. Consequently, Lewis’ team modeled possible ice, snow and land albedos, and examined the way their models responded to those minute variations.

The team studied the effects of these brightness values on greenhouse “forcing” — the change that greenhouse gases (such as carbon dioxide) make in Earth’s climate system. For each different range of albedos they put into the model, they found that a correspondingly, and massively, different amount of carbon dioxide was required to thaw the snowball. Even a minor albedo change could mean that a five- or ten-fold increase in carbon dioxide was needed to thaw Earth, Lewis says.

The finding means that previous models predicting how much carbon dioxide is necessary to thaw a frozen Earth may have significantly underestimated the situation, Lewis says. Furthermore, he says, the error bars on estimates of the albedo in a snowball Earth “are huge,” he says.

“You’re never going to be able to say what exactly happened with snowball Earth,” says Andrew Weaver of the University of Victoria, Lewis’ adviser and a co-author on the paper. There is no way to model whether most of Earth was covered in a thin layer of white snow or dark, older, sea ice, he says, because “you can’t go back in time and see what the climate was doing.”

Such research hasn’t been published before, says James Kasting, a geoscientist at Pennsylvania State University who specializes in atmospheric evolution. Kasting’s work has also shown the importance of careful interpretation of the variable parameters in snowball Earth models. While he agrees “that you can’t use models to tell you what happened back then,” he says that “you can use models to show what the possibilities are.”

Kirschvink says that climate modelers need more data from the rock record. “Let’s get the geological picture first and then worry about whether you can build a climate model to match it.”

As for slushball Earth, Weaver says that this study has determined that slushball conditions could not have occurred. With their models, “we can have snowball or not,” he says, “but nothing in between.”

Sally Adee
Geotimes contributing writer

"Slushball life," Geotimes, December 2005
"Space dust and snowball Earth," Geotimes, April 2005

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