How the Earth could turn into a snowball

About 717 million years ago, Earth's humid landscapes and swirling blue waters became an icy, barren world. Scientists nicknamed this phase of geological history, and others like it, Snowball Earth.

What exactly made the planet so nearly frozen has been a mystery, as has how it stayed that way for 56 million years. On Wednesday, a team of researchers from the University of Sydney said they had figured it out. According to them, the formation of ice on Earth could be the result of a global decline in carbon dioxide emissions, a result of fewer volcanoes releasing the gas into the atmosphere.

Less carbon dioxide makes it harder for Earth's atmosphere to retain heat. If the depletion was extreme enough, they reasoned, it could have pushed the planet into its longest ice age yet.

The theory, published in the journal Geology, adds insight into how geological processes influenced Earth's past climate. It can also help scientists better understand trends in our current climate.

“Today, of course, humans have a major influence on CO2 emissions into the atmosphere,” said Adriana Dutkiewicz, a sedimentologist at the University of Sydney who led the study. “But there used to be no people, and so everything was basically modulated by geological processes.”

There are many ideas about what turned the Earth into a snowball. One popular theory suggests that minerals released from the weathering of igneous rock sucked enough carbon dioxide from the atmosphere to cause a deep freeze.

Perhaps that helped trigger a global ice age, said Dr. Dutkiewicz, but it could not have kept the Earth frozen for 56 million years on its own.

“So there must be some other mysterious mechanism that would have kept the ice age going for so long,” she said.

Dr. Dutkiewicz and her colleagues turned their attention to volcanoes because a newly available model of the Earth's shifting tectonic plates. As the continents spread apart, they studied the changing length of the mid-ocean ridge – a chain of underwater volcanoes – predicted by the model.

The team then calculated the amount of volcanic gas emissions at the beginning of and during the ice age. Their results showed a drop in the amount of carbon dioxide in the atmosphere sufficient to initiate and sustain an ice age lasting 56 million years.

A reduction in volcanic gas emissions has previously been proposed as an explanation for Snowball Earth. But according to Dr. Dutkiewicz, this is the first time that researchers have proven that the mechanism was viable through modeled calculations.

Dietmar Müller, a geophysicist at the University of Sydney and author of the study, said the work was a way “to distinguish between alternative models for this very old part of Earth's evolution.” If scientists know that an ice age has occurred, Dr. Müller explains, “then we can say that this one reconstruction model may be more likely than the other.”

Of course, a model is still just that: a model. Without real-world data to support it, the researchers can't rule out other possibilities.

“One thing about geology: there are no definitive answers,” said Dr. Dutkiewicz. “But based on a combination of several lines of evidence, we can suggest that this is a very likely process.”

Francis Macdonald, a geologist at the University of California, Santa Barbara, who was not involved in the work, said these types of studies are important for learning why climates fail. But he is hesitant to accept results from models of the ancient seafloor at face value, because there is little data revealing what Earth's oceanic crust looked like at the time.

“How do we actually test that?” Dr. Macdonald asked about the team's model. “I think it is a very big challenge.”

Still, Dr. thinks Müller that it is important to set limits on the amount of volcanic gas emitted in the past, especially when it comes to running climate models for the future. “Usually that is the most uncertain parameter,” he said.

Research like this can help scientists distinguish the influence of geological activity from human-induced climate change. But could a natural decline in volcanic emissions ever save us from the amount of carbon we've pumped into our atmosphere today?

“Unfortunately not,” said Dr. Dutkiewicz. “We can study these ancient disturbances,” she added, “but human-induced change is a different kind of beast.”