Can supervolcanic eruptions lead to an ice age?

Sunlight-blocking particles generated by a massive volcanic eruption will not cool Earth's surface temperatures as much as previously estimated, according to new research.

About 74,000 years ago, the Toba volcano in Indonesia exploded with a force equivalent to a thousand times the force of the eruption of Mount St. Helens in 1980. The eruption sent a huge column of ash and gas into the atmosphere, covering a large part of the globe with a thick layer of debris. But how this affected Earth's climate after that remains a mystery.

And when it comes to the most powerful volcanoes, scientists have long speculated about how global cooling after an eruption — sometimes called a volcanic winter — could pose a threat to humanity.

Previous studies agreed that some cooling occurred at the planetary level, but they differed about its amount. Estimates ranged from 2°C to 8°C (3.6°F to 14°F).

In the new study, a team from NASA's Goddard Institute for Space Studies (GISS) and Columbia University in New York used advanced computer models to simulate massive explosions like the Toba event. They found that the massive eruption would likely cause a drop in global temperature of no more than 1.5 degrees Celsius (2.7 degrees Fahrenheit) even in the strongest eruptions. This is a far cry from the catastrophe of the end of civilization.

“The relatively modest temperature changes we found that are most consistent with the evidence could explain why no major volcanic eruption has produced conclusive evidence of a global-scale catastrophe for humans or ecosystems,” said lead author Zachary McGraw, a researcher at the Goddard Institute for Space Studies and Columbia University. . 

For an eruption to be described as massive, it must release more than 1,000 cubic kilometers (240 cubic miles) of magma. These explosions are very powerful and rare.

The last massive explosion occurred more than 22 thousand years ago in New Zealand. Perhaps the most famous example is the eruption that blew out the Yellowstone crater in Wyoming about two million years ago.

The role of sulfur molecules

McGraw and his colleagues set out to understand why model temperature estimates differ because climate shifts that occurred so long ago do not leave clear records. They settled on a variable that may be difficult to quantify: the size of microscopic sulfur particles being sent into the atmosphere.

In the stratosphere (at an altitude of about 10,000 to 50 km), sulfur dioxide gas emitted by volcanoes undergoes chemical reactions to condense into liquid sulfate molecules.

These particles can affect the Earth's surface temperature in two ways: by reflecting incoming sunlight (causing cooling) or by trapping outgoing thermal energy (a type of greenhouse effect).

Scientists discovered that the size of particles from volcanic eruptions determines the extent of cooling. The smaller the particles, the greater their ability to block sunlight.

Unfortunately, determining the particle size of volcanic eruptions dating back thousands of years is extremely difficult, leading to widely varying estimates.

In the atmosphere, the size of particles changes. Even when they return to Earth and are preserved in ice, they leave no clear physical record.

By simulating massive explosions over a range of different particle sizes, scientists found that massive explosions may not be able to significantly change global temperatures.