Boston: A perfect combination of erupting volcanoes along with a rapid cooling effect may have caused the largest glaciation event in history – known as ‘snowball Earth’ – that covered our planet pole-to-pole in ice more than 700 million years ago, Harvard scientists say. Understanding how these dramatic changes occur may help better understand how extinctions occurred and how climates change on other planets.
The root cause of the runaway glaciation has remained elusive for years. Scientists have pinpointed the start of the Sturtian snowball Earth event to about 717 million years ago. At around that time, a huge volcanic event devastated an area from present-day Alaska to Greenland.
“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 Francis Macdonald, Associate Professor at Harvard University in the US. Researchers wondered if aerosols emitted from these volcanoes have rapidly cooled Earth.
Studies of this region, known as the Franklin large igneous province, showed that volcanic rocks erupted through sulphur-rich sediments, which would have been pushed into the atmosphere during eruption as sulphur dioxide.
When sulphur dioxide gets into the upper layers of the atmosphere, it is very good at blocking solar radiation. Sulphur dioxide is most effective at blocking solar radiation if it gets past the tropopause, the boundary separating the troposphere and stratosphere.
If it reaches this height, it is less likely to be brought back down to earth in precipitation or mixed with other particles, extending its presence in the atmosphere from about a week to about a year. Another important aspect is where the sulphur dioxide plumes reach the stratosphere.
Due to continental drift, 717 million years ago, the Franklin large igneous province where these eruptions took place was situated near the equator, the entry point for most of the solar radiation that keeps the Earth warm.
An effective light-reflecting gas entered the atmosphere at just the right location and height to cause cooling. The eruptions throwing sulphur into the air 717 million years ago were not one-off explosions. The volcanoes spanned almost 2,000 miles across Canada and Greenland. Instead of singularly explosive eruptions, they can erupt continuously.
Researchers showed that a decade of continual eruptions from this type of volcanoes could have poured enough aerosols into the atmosphere to rapidly destabilise the 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.
The more ice, the more sunlight is reflected and the cooler the planet becomes. Once the ice reaches latitudes around present-day California, the positive feedback loop takes over and the runaway snowball effect is pretty much unstoppable.