Some 74,000 years ago, the Toba volcano in Indonesia exploded with a force 1,000 times more powerful than the 1980 eruption of Mount St. Helens. The mystery is what happened after that.
When it comes to the most powerful volcanoes, researchers have long speculated how post-eruption global cooling—sometimes called volcanic winter—could potentially pose a threat to humanity after a so-called super eruption. Previous studies have agreed that some planet-wide cooling would occur, but they have diverged on how much. Estimates have ranged from 3.6 to 14 degrees F (2 to 8 degrees C).
In a new study published in the Journal of Climate, a team from NASA’s Goddard Institute for Space Studies, an affiliate of the Columbia Climate School, used advanced computer modeling to simulate super eruptions like the Toba event. They found that post-eruption cooling would probably not exceed 2.7 degrees F (1.5 C) for even the most powerful blasts.
“The relatively modest temperature changes we found most compatible with the evidence could explain why no single super eruption has produced firm evidence of global-scale catastrophe for humans or ecosystems,” said lead author Zachary McGraw, a postdoctoral researcher at Goddard and Columbia.
To qualify as a super eruption, a volcano must release more than 240 cubic miles (1,000 cubic kilometers) of magma. These eruptions are extremely powerful, and rare. The most recent super eruption occurred more than 22,000 years ago in New Zealand. The best-known example may be the eruption that blasted Yellowstone Crater in Wyoming about 2 million years ago.
McGraw and colleagues set out to understand what was driving the big divergence in model temperature estimates, because models are the main tool for understanding climate shifts that happened too long ago to leave clear records of their severity. They settled on a variable that can be difficult to pin down: the size of the microscopic sulfur particles that eruptions inject miles high into the atmosphere.
In the stratosphere (about 6 to 30 miles up), sulfur dioxide gas from volcanoes undergoes chemical reactions to condense into liquid sulfate particles. These particles can influence surface temperature on the Earth in two counteracting ways: by reflecting incoming sunlight (causing cooling) or by trapping outgoing heat energy (a kind of greenhouse warming effect).
Over the years, the known cooling effect has spurred questions about how humans might turn back global warming by intentionally injecting aerosol particles into the stratosphere.
The researchers in the new study showed to what extent the diameter of volcanic aerosol particles might influence post-eruption temperatures. The smaller and denser the particles, the greater their ability to block sunlight. But estimating the size of particles is challenging because previous super eruptions have not left reliable physical evidence. In the atmosphere, the size of the particles change as they coagulate and condense. Even when particles fall back to Earth and are preserved in ice cores, they don’t leave a clear-cut physical record, because of mixing and compaction.
By simulating super eruptions over a range of particle sizes, the researchers found such eruptions may not be capable of altering global temperatures much more than the largest eruptions of modern times. For instance, the 1991 eruption of Mount Pinatubo in the Philippines caused a drop in global temperatures of about 1 degree F over two years.
Luis Millán, an atmospheric scientist at NASA’s Jet Propulsion Laboratory, who was not involved in the study, said that the mysteries of super-eruption cooling invite more research. He said the way forward is to conduct a comprehensive comparison of models, as well as more laboratory and model studies on the factors determining volcanic aerosol particle sizes.
Given the ongoing uncertainties, Millán added, “To me, this is another example of why geoengineering via stratospheric aerosol injection is a long, long way from being a viable option.”
Adapted from a press release by NASA.
