By Chris Skeldon 
About 717 million years ago, Earth's humid landscapes and choppy blue waters transformed into a cold, barren world. Scientists have called this stage of geological history, and others like it, “Snowball Earth.”
What caused the planet to freeze almost completely was a mystery, as was why it remained that way for 56 million years.
Last Wednesday (7), a team of researchers from the Australian University of Sydney said they had discovered it. They say the glaciation of the Earth may have occurred due to a global decline in carbon dioxide emissions, as a result of a decrease in the number of volcanoes spewing the gas into the atmosphere.
Low carbon dioxide makes it harder for Earth's atmosphere to retain heat. If the decline was severe enough, they said, it could have pushed the planet into its longest ice age to date.
The theory published in Geology JournalAdds knowledge about how geological processes affected Earth's climate in the past. It could also help scientists better understand trends in our current climate.
“These days, of course, humans have a huge impact on carbon dioxidetwo “In the atmosphere. But in the past, there were no humans, so everything was basically modified by geological processes,” said Adriana Dutkiewicz, a sedimentologist at the University of Sydney who led the study.
There are many ideas about what snowball Earth is. One popular theory suggests that minerals released from the weathering of igneous rocks pull enough carbon dioxide from the atmosphere to cause a deep freeze.
This may have helped initiate global glaciation, Dutkiewicz said, but it would not have kept the Earth frozen for long alone.
“So there must be some other mysterious mechanism that would maintain glaciation for a long time,” she said.
Dutkiewicz and his colleagues turned their attention to volcanoes because of a newly available model of Earth's moving 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 and throughout the ice age. 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 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 a way to “distinguish between alternative models of this very ancient part of Earth's evolution.” Mueller explained that if scientists knew there was an ice age, “we could say that this reconstruction model is probably more likely than the other.”
Of course, a model is still just a model. Without real-world data to support this idea, researchers cannot rule out other possibilities.
“There are no definitive answers,” 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 the cause of climate failure. But he is reluctant to readily accept the results of ancient seafloor models, because there is little data revealing what Earth's oceanic crust was like at that time.
“How can we actually test this?” MacDonald asked about the team's model. “I think it's a really big challenge.”
However, Mueller believes it is important to try to put limits on the amount of volcanic gases emitted in the past, especially when it comes to running climate models for the future. “Usually, this is the most ambiguous parameter,” 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 are pumping into our atmosphere today?
“Unfortunately no,” Dutkiewicz said. “We can study these ancient disturbances, but human-induced change is a different kind of beast,” he added.