How the earth turned into a snowball

About 717 million years ago, Earth's humid landscapes and flowing blue waters transformed into a cold, barren world. Scientists called this stage of geological history and others like it “Snowball Earth.”

What caused the planet to nearly freeze over was a mystery, as was how it stayed that way for 56 million years. A team of researchers at the University of Sydney said on Wednesday they had discovered this. They say that the glaciation of the Earth may have been the result of a global decline in carbon dioxide emissions, as a result of a decrease in the number of volcanoes that expel the gas into the atmosphere.

Low carbon dioxide makes it harder for Earth's atmosphere to trap heat. They reasoned that if the depletion had been severe enough, it could have pushed the planet into its longest ice age to date.

the theory, Published in Geology MagazineIt adds insight into the way geological processes have affected Earth's climate in the past. It may also help scientists better understand trends in our current climate.

“These days, of course, humans have a significant impact on carbon dioxide in the atmosphere,” said Adriana Dutkiewicz, a sedimentologist at the University of Sydney who led the study. “But in the past, there were no humans, so everything was basically modified 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 rocks absorbed enough carbon dioxide from the atmosphere to cause a deep freeze.

That may have helped trigger global glaciation, Dr. Dutkiewicz said, but it could not have kept the Earth frozen for 56 million years on its own.

“So there must be another mysterious mechanism that would maintain glaciation for a long time,” she said.

Dr. Dutkiewicz and her colleagues turned their eyes to volcanoes because A Newly available model Of Earth's shifting tectonic plates. As the continents moved apart, they studied the changing length of the mid-ocean ridge — a series of underwater volcanoes — predicted by the model.

The team then calculated the amount of volcanic gas emissions at the beginning of the ice age and throughout it. Their results showed a reduction in atmospheric carbon dioxide sufficient to initiate and sustain an ice age for 56 million years.

A decrease in volcanic gas emissions has been proposed as an explanation for the snowball Earth before. But according to Dr. Dutkiewicz, this is the first time researchers have proven that the mechanism was workable through model calculations.

Dietmar Müller, a geophysicist at the University of Sydney and author of the study, said the work was one way to “distinguish between alternative models of this very ancient part of Earth's evolution.” If scientists know there was an ice age, “we can say that this reconstruction model is probably more likely than the other model,” Dr. Müller explained.

Of course, a model is still just a model. Without real-world data to support this idea, researchers cannot rule out other possibilities.

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

Francis McDonald, a geologist at the University of California, Santa Barbara, who was not involved in the work, said such studies are important for understanding why climate failure is occurring. But he is reluctant to accept the results of ancient seafloor models easily, because there is little data revealing what the Earth's oceanic crust was like at that time.

“How do we actually test this?” Dr. MacDonald asked about the team model. “I think it's a really big challenge.”

However, Dr Müller believes it is important to try to put limits on the amount of volcanic gas emitted in the past, especially when it comes to running climate models for the future. “Normally, this is the most ambiguous standard,” he said.

Research like this can help scientists distinguish between the impact of geological activity and human-caused climate change. But could a natural decline in volcanic emissions save us from the amount of carbon we pump into our atmosphere today?

“Unfortunately no,” Dr. Dutkiewicz said. “We can study these ancient disturbances, but human-induced change is a different kind of beast,” she added.