How Earth became a giant snowball 700 million years ago
Around 717 million years ago, the Earth froze over. The Sturtian glaciation, as this event is known, was no ordinary Ice Age but one so extreme that it caused the Earth to become a giant snowball for at least five million years. How it happened has been a mystery for the ages – until now. In a new study, Harvard scientists suggest that the answer might lie in the way volcanic eruptions caused the Earth's temperatures to plummet.
At the heart of this mystery is the Franklin Large Igneous Province (LIP), which extends across modern-day Alaska, Greenland and northern Canada. An LIP is a vast swath of igneous (magmatic) rock that is associated with a hotspot – regions within the mantle where rocks melt to form magma. The birth of these formations are cataclysmic events resulting in various kinds of mayhem, including mass extinctions and devastating climate change, as enormous clouds of volcanic matter are released into the atmosphere for geological short periods (read: a few million years).
In the case of the Franklin LIP, its formation coincides with the onset of the Sturgian glaciation. But exactly what made these volcanic eruptions different from others? As Robin Wordsworth, an assistant professor of environmental science and engineering points out, such events are by no means unique and if the planet froze over every time there was a volcanic eruption of this nature, we'd all be in trouble.
"These types of eruptions have happened over and over again throughout geological time but they're not always associated with cooling events. So, the question is, what made this event different?" he ponders.
More to the point: what's the link between the two?
"We know that volcanic activity can have a major effect on the environment, so the big question was, how are these two events related," says Francis Macdonald, an associate professor of geology at Harvard University.
The answer, as it turns out, lies not in any one single factor but a confluence of factors.
Macdonald had a hunch that the aerosols emitted from the volcanos might have had something to do with the rapid cooling of the Earth. As it turns out, this is possible under the right conditions.
When the magma that eventually formed the Franklin LIP erupted to the surface, it shot particles of sulphur-rich sediments into the atmosphere as sulfur dioxide. This is a gas that absorbs solar radiation and it does this particularly well when it gets past the tropopause, the boundary between the troposphere and the stratosphere. At this height, there's a greater chance of it remaining in the atmosphere for a longer period of time, extending its presence from about a week to about a year, without being brought back down to earth in precipitation or mixed with other particles.
The question is: would it have made it past the tropopause? An important thing to note about this barrier is that its position is not fixed but is determined by the background climate of the planet – the cooler the planet, the lower its position. And Earth at that time was not exactly going through a heatwave.
"In periods of Earth's history when it was very warm, volcanic cooling would not have been very important because the Earth would have been shielded by this warm, high tropopause," says Wordsworth. "In cooler conditions, Earth becomes uniquely vulnerable to having these kinds of volcanic perturbations to climate."
That said, releasing aerosols into the atmosphere is just one part of the equation. Where the gasses are released is equally important and back in those early days, before the supercontinent Rodinia broke up, the Franklin LIP was situated much closer to the equator. Given this region receives the bulk of the solar radiation that keeps the Earth warm, this means that the sulfur dioxide was in the right place to enter the atmosphere and cause a significant drop in temperatures.
Even so, this was probably not enough to send the planet into a deep freeze. Case in point: despite launching 15 million tons of sulfur dioxide into the atmosphere, the eruption of Mount Pinatubo in the Philippines in 1991 caused just a 1 degree Fahrenheit (0.6 degrees Celcius) drop in global temperatures that lasted 15 months.
What really turned the planet into a snowball was the nature of the volcanic explosions. They weren't one-off events like the Mount Pinatubo eruption but continuous ones that lasted tens of thousands, if not millions, of years. In addition, just imagine the amount of aerosols being released by volcanoes stretching nearly 2,000 miles across Canada and Greenland. According to the researchers, just a decade or so of continual eruptions would have been enough to rapidly destabilize the climate.
"Cooling from aerosols doesn't have to freeze the whole planet; it just has to drive the ice to a critical latitude," says Macdonald. "Then the ice does the rest."
In modern-day geography, that latitude would be California. Once the ice reaches this point, there's no stopping it, say the researchers.
"It's easy to think of climate as this immense system that is very difficult to change and in many ways that's true," says Wordsworth. "But there have been very dramatic changes in the past and there's every possibility that as sudden of a change could happen in the future as well."
In addition to understanding how such Snowball Earth events cause extinctions and how geoengineering proposals could impact climate – some scientists have suggested seeding the atmosphere with sulphur dioxide to combat global warming – the researchers say they could also offer another take on the way we define a habitable zone. Current consensus states that liquid water is a pre-requisite for life but if this snowball event is any indication, a planet's climate is far from static.
"This research shows that we need to get away from a simple paradigm of exoplanets, just thinking about stable equilibrium conditions and habitable zones," says Wordsworth. "We know that Earth is a dynamic and active place that has had sharp transitions. There is every reason to believe that rapid climate transitions of this type are the norm on planets, rather than the exception."
The study was published in Geophysical Research Letters.