What caused ‘snowball Earth?’

Volcanoes seen as likely trigger for global glaciation

In the right conditions, global ice can spread rapidly from pole to pole. @bberwyn photo.

Staff Report

A 10-year string of steady volcanic eruptions may have been the trigger for a massive global cooling event that left much of the Earth encased in glaciers and ice sheets about 771 million years ago.

The eruptions could have spewed so much sulphur dioxide into the atmosphere that the planet’s climate reached a tipping point, resulting in what scientists call ‘snowball Earth,” according to a new study published this month in Geophysical Research Letters. Understanding the natural variability of climate is important to understanding current climate change driven by emission of greenhouse gases.

The research led by Harvard scientists linked the Sturtian glaciation with a volcanic event that occurred in an area spanning present-Greenland to Alaska, and the study theorizes that specific geographical and atmospheric conditions combined with the volcanic activity to spread a layer of heat-reflecting aerosol particles around the globe.

“We know that volcanic activity can have a major effect on the environment, so the big question was, how are these two events related,” said Harvard geology researcher Francis Macdonald in a press release describing the study.

For one thing, not all volcanoes are equal in terms of the chemicals they emit when erupting. But rock samples from the volcanic region in question show that the igneous material was pushed through sulfur-rich sediments, releasing into the atmosphere as sulfur dioxide, which efficiently blocks solar radiation it it’s high in the atmosphere. The 1991 eruption of Mount Pinatubo is a recent example. It cooled global climate by about 1 degree Celsius for about a year after emitting about 10 million metric tons of sulfur into the air.

But not every large volcanic outbreak has triggered global cooling. There have been repeated episodes of large volcanic provinces erupting, according to Robin  Wordsworth, assistant professor of Environmental Science and Engineering at the Harvard John A. Paulson School of Engineering and Applied Science.

So the researchers tried to identify what made this event different, concluding that the Earth’s “background” climate may be the key factor.

“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,” said Wordsworth. “In cooler conditions, Earth becomes uniquely vulnerable to having these kinds of volcanic perturbations to climate … “Cooling from aerosols doesn’t have to freeze the whole planet; it just has to drive the ice to a critical latitude. Then the ice does the rest,” said Macdonald.

When the planet is in a relatively cool state, a critical atmospheric layer known as the troposphere is higher, which enables the aerosols to rise above the zone where they are brought back down to the ground with precipitation. Once the sulfur dioxide gets to that elevation, it spreads out and the cooling effects persist for longer, and glaciers start to expand.

Once the ice reaches latitudes around present-day California, the positive feedback loop takes over and the runaway snowball effect is pretty much unstoppable. The more ice, the more sunlight is reflected and the cooler the planet becomes, the researchers explained.

The location of the eruptions is also important and because of continental drift, the big volcanic event that triggered the Sturtian was situated near the equator. That’s where most of the sun’s warming rays enter the atmosphere, so the high-elevation fog of aerosols may have been in the most effective geographic area in terms of cooling effect.

It’s also important to note that the eruptions may have lasted for years. With modeling, the researchers shows that a decade or so of continual eruptions from this type of volcanoes could have poured enough aerosols into the atmosphere to rapidly destabilize the climate.

“It’s easy to think of climate as this immense system that is very difficult to change and in many ways that’s true. 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,” said Wordsworth.

The scientists also said their work could help explain past extinction events, and, importantly in the current context of climate and energy policy,  how proposed geoengineering approaches may impact climate and how climates change on other planets. Some people have controversially proposed that artificial large-scale sulfur dioxide emissions could slow the current climate warming.

The researchers also warned that their findings suggest Earth is vulnerable to sharp climate transitions. Shifts that upset stable equilibrium conditions and habitable zones “norm on planets, rather than the exception,” Wordsworth said.

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