There are few forces on Earth more powerful than a large volcanic eruption. At their most potent, volcanoes inject millions of tons of Sun-blocking particles high into the atmosphere that can cool Earth for nearly 5 years, endangering crops and leading to “years without summer.” The most recent, the Philippines’s Mount Pinatubo eruption in 1991, caused a temporary 0.5°C drop in global temperatures.
Yet it’s become increasingly clear that even these monumental forces are being altered by human-driven climate change. Declining ice cover can trigger more frequent eruptions near the poles, in Iceland and elsewhere. And an increasingly layered ocean will allow more volcano-induced cooling to linger at Earth’s surface. Now, a new study suggests increased greenhouse gases will help the plumes from large eruptions reach higher, spread faster, and reflect more sunlight, causing more abrupt and extreme cooling.
Before humanity started in on its planet-altering course, volcanoes were one of the biggest climate players. Over the long term, they belched carbon dioxide from Earth’s interior, causing warming. But in the short term, their sulfur gases often react with water to form highly reflective particles called sulfates, triggering spells of global cooling. Dark smudges of ash littering ice cores—our best evidence of these early eruptions—are a dim reflection of the wild weather left in their wake.
But the opposite is also true, it turns out: Climate can have a big impact on volcanoes. In the new study, Thomas Aubry, a geophysicist at the University of Cambridge, and colleagues combined computer simulations of idealized volcanic eruptions with a global climate model. They simulated the response to plumes released from midsize and large volcanoes both in historical conditions and by 2100, in a scenario when Earth is predicted to warm very rapidly.
The researchers found two countervailing trends. Normally just one or two midsize volcanic eruptions shoot through the troposphere each year, bypassing this cradle of Earth’s weather to reach the stratosphere, the calm, dry zone above. As reflective particles spread through the stratosphere, they cause a small spurt of global cooling. But when the troposphere warms, it expands in height, eventually putting the stratosphere out of reach for these eruptions.
“It’s as if regulation basketball hoops around the world were suddenly raised a few inches, making it that much harder to score,” says Benjamin Black, a volcanologist at Rutgers University, New Brunswick, who is not affiliated with the study.
The story changes with Pinatubo-scale eruptions, however. In a world that warmed 6°C by 2100—an increase that matches only the most dire, and unlikely, projections of the latest Intergovernmental Panel on Climate Change report—the troposphere would grow 1.5 kilometers in height. But ultramassive eruptions would still be able to punch through to the stratosphere; what’s more, their gases would actually reach higher and travel faster than in the present climate, amplifying their cooling effect by 15%, the researchers report this month in Nature Communications. The reasons why come down to the bizarro world that is the stratosphere, Aubry says.
As greenhouse gases trap heat near Earth’s surface, the stratosphere is cooling, especially in its upper layers. That lets air mix more easily up and down in this layer of the atmosphere. By 2100, this mixing should help volcanic plumes travel about 1.5 kilometers higher than before, according to the team’s model. In addition, warming will accelerate the stratosphere’s primary wind pattern, causing the reflective volcanic particles to spread more quickly throughout the upper atmosphere to the poles, before they have time to coalesce into larger particles. And the smaller the particle, the more light it reflects.
The fact that midsize eruptions may no longer reach the stratosphere is “interesting and important,” says Michael Mills, an atmospheric chemist at the National Center for Atmospheric Research who was not involved with the study. And many of the trends identified in the new model—the cooling stratosphere, rising troposphere, and accelerating circulation—have already been seen in the real world. But it’s still uncertain whether the limited particle growth simulated by the new model reflects what would happen in the real world, Mills adds.
Indeed, the study raises more questions than it answers, Aubry says. “It’s more like opening a can of worms.” For one, it studies only tropical eruptions, not those closer to the poles, where the stratosphere is closer. And it is hard to say whether the increased cooling from large volcanoes or decreased cooling from smaller ones will win out as the bigger climate influence. “My gut feeling is that the large eruption effect will dominate,” he adds, simply given those eruptions’ sheer power as a climate lever.
The next step will be testing how these trends work under more realistic future warming levels—and in additional climate models. Researchers also hope to integrate other trends, including the increased eruptions expected to take place as glaciers melt off some polar volcanoes and the increasing stratification of the ocean, which allows more volcanic cooling to linger at the water’s surface, cooling the atmosphere. “My hope is we will never warm the climate enough to influence volcanoes,” Aubry says. “But it’s becoming a narrow, narrow pathway.”
sciencemag.org, 19 August 2021