About 700 million years ago, during a time nicknamed “Snowball Earth”, runaway glaciers covered most, if not all, of the planet in ice. Scientists have been baffled as to what caused this to happen and have been searching for a logical answer for years. Last week, researchers at Harvard University announced a new hypothesis as to why this occurred. They believe that it was a combination of volcanic activity and expanding ice sheets that caused the event. Their research was published in the Geophysical Research Letters.

It has been shown many times in our planet’s history that periods of intense volcanic activity, or the eruption of a supervolcano, can result in cooling of the planet. For example, the 1991 eruption of Mount Pinatubo in the Philippines, which shot about 10 million metric tons of sulfur into the air, reduced global temperatures about 1 degree Fahrenheit for a year. When a volcano erupts, it releases ash into the atmosphere which can spread across the planet. Particles contained in the ash block incoming solar radiation resulting in a net cooling of the planet that can last years if not decades or centuries. It is believed that a series of volcanic eruptions contributed to the period known as “The Little Ice Age” which ran from roughly 1300-1850. During The Little Ice Age in 1815, a supervolcano named Tambora located in Indonesia erupted and had significant impacts of the weather the following year especially in parts of North America (Particularly New England) and Europe where it was known as “the year without a summer”.

Researchers have pinpointed the start of the Snowball Earth event to about 717 million years ago. At around that time, a chain of volcanoes devastated an area from present day Alaska to Greenland. Harvard professors Francis MacDonald and Robin Wordsworth believe that the occurrence of these events is no coincidence. “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 MacDonald, the John L. Loeb Associate Professor of the Natural Sciences in Harvard’s Department of Earth and Planetary Science. Macdonald turned to Wordsworth, who models climates of non-Earth planets, and asked if aerosols emitted from volcanos could rapidly cool Earth. Wordsworth confirmed that under the right conditions this would be plausible. “It is not unique to have large volcanic (regions) erupting,” said Wordsworth, assistant professor of Environmental Science and Engineering at the Harvard John A. Paulson School of Engineering and Applied Science. “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?”

Sulfur 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’s 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. As mentioned earlier, the effects of Mount Pinatubo and Tambora only lasted around a year. The height of the tropopause barrier all depends on the overall climate of the planet; the cooler the planet, the lower the tropopause. “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.”

It is believed that the combination of an already cooler earth and the location of the erupting volcanoes is what led to Snowball Earth. Due to continental drift, the area from Alaska to Greenland where the volcanoes erupted was situated near the equator, the entry point for most of the solar radiation that keeps the Earth warm. The researchers demonstrated that a decade or so of eruptions from these volcanoes could have poured enough aerosols into the atmosphere to rapidly destabilize the climate. “Cooling from aerosols doesn’t have to freeze the whole planet; it just has to drive the ice (sheets) to a critical latitude.” said Macdonald. Ice reflects more of the Sun’s radiation back out to space so the more ice the planet has, the cooler the planet becomes allowing more ice to develop and reflect radiation. Once the ice sheets reache latitudes around present-day California, the positive feedback loop takes over and the runaway snowball effect is pretty much unstoppable.