About 717 million years ago, Earth’s humid landscapes and choppy blue waters transformed into a cold, barren world. Scientists have nicknamed this phase of geological history, and others like it, Snowball Earth.
What exactly froze the planet almost completely has been a mystery, as has what caused it to remain that way for 56 million years.
Last Wednesday (7), a team of researchers from the University of Sydney, Australia, said they discovered it. Earth’s glaciation, they say, may have occurred due to a global drop in carbon dioxide emissions, the result of fewer volcanoes spewing the gas into the atmosphere.
Less carbon dioxide makes it harder for Earth’s atmosphere to retain heat. If the reduction was extreme enough, they argued, it could have thrown the planet into its longest ice age yet.
The theory, published in the journal Geology, adds knowledge about how geological processes influenced Earth’s past climate. It could also help scientists better understand trends in our current climate.
“These days, of course, humans are having a huge impact on COtwo 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 modulated by geological processes.”
There are many ideas about what snowballed the Earth. One popular theory suggests that minerals released by the weathering of igneous rocks drew enough carbon dioxide from the atmosphere to trigger a deep freeze.
Perhaps this helped start a global glaciation, Dutkiewicz said, but it could not alone have kept the Earth frozen for .
“So there must be another mysterious mechanism that would have sustained glaciation for so long,” 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 chain 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 drop in atmospheric carbon dioxide sufficient to initiate and sustain a 56-million-year glaciation.
A reduction in volcanic gas emissions has been proposed as an explanation for Snowball Earth before. But according to Dutkiewicz, this is the first time researchers have proven 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 ancient part of Earth’s evolution.” If scientists know there was an ice age, Müller explained, “then we can say that this reconstruction model is perhaps more likely than the other.”
Of course, a model is still just that: a model. Without real-world data to back it up, researchers can’t rule out other possibilities.
“There are no definitive answers,” Dutkiewicz said. “But based on a combination of different 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 studies like this were important for learning why climates fail. But he is hesitant to readily accept results from models of the ancient sea floor 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.”
Still, Müller thinks it’s important to try to set limits on the amount of volcanic gases emitted in the past, especially when it comes to running climate models for the future. “Typically, this 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 reduction in volcanic emissions save us from the amount of carbon we have been pumping into our atmosphere today?
“Unfortunately not,” Dutkiewicz said. “We can study these ancient disruptions,” he added, “but human-induced change is a different kind of beast.”