Giant Eruption at Bali Volcano Could Unleash Global Cooling at Scale Unseen for Decades

In late September, MAGMA Indonesia, the country's volcano monitoring service, spotted a serious uptick in seismic activity at Mt. Agung. And that volcano has a history: When it erupted in 1963, it killed more than 1,000 people and erupted powerfully enough to cool the planet by up to a full degree Fahrenheit. So when the ground around the mountain started shaking again, scientists across disciplines wanted to see if the same sort of Earth-cooling eruption would happen again.

"The problem with Agung is we can't tell if this is actually certainly going to erupt," said volcanologist Janine Krippner, who has been monitoring the situation from the U.S. "This volcano hasn't stopped. Whether that will continue to go and actually produce an eruption, I'm throwing my hands up in the air with that one."

Eruptions simply aren't that predictable. But if Agung does erupt and in a manner similar to its last eruption, climate scientists will also be ready and watching. That's because the impact of volcanic eruptions on climate is one of the biggest weaknesses in current climate models—unsurprisingly, since there haven't been a whole lot of eruptions large enough to affect climate during the era of satellites.

That means that we didn't have very good observations of previous climate-changing eruptions, Flavio Lehner, a climate scientist at the National Center for Atmospheric Research, told Newsweek—the 1991 eruption at Mt. Pinatubo is the most recent such event, and that technology was all more than 26 years old. (For context, the Hubble Space Telescope had launched just the year before.) Today, there's a fleet of satellites monitoring a whole range of atmospheric characteristics.

But most of the time, those satellites don't have anything big to watch. "Every day there is a volcano erupting somewhere, somehow, but a lot of those are not as explosive," Lehner said. For an eruption to affect climate, it has to shoot aerosols—tiny particles that can seed clouds, which reflect sunlight away from Earth—into the highest layer of the atmosphere, where they can migrate away from the volcano that created them. "As soon as it rains, a lot of these [aerosols] get washed out, they are not able to spread globally." No global spread, no climate impact.

Whether or not an eruption will manage to put aerosols in the upper atmosphere depends on characteristics of the volcano itself, and particularly of its magma—thicker magma builds up more pressure, making for a more explosive eruption. But magma is changing all the time, and scientists don't have a way to monitor those changes as they happen, said Krippner. "You need a big enough eruption to get it high enough," she said. "You also need the right kind of gases."

And what impact those aerosols will have on climate once they get to the upper atmosphere varies too, Lehner added. One particularly important factor is what phase of the El Nino/La Nina cycle the planet is in when the volcano erupts. During an El Nino phase, the Pacific Ocean is warmer than usual, and during a La Nina phase, it's cooler than usual.

Right now, climate scientists think we may be entering a La Nina phase, which adds to their interest in Mt. Agung. "All these three eruptions [Agung in 1963, El Chichon in 1982 and Pinatubo in 1991], big ones we've seen in the last 50 or 60 years, they all occurred during an El Nino, so they all cooled a little less than they could have," Lehner said.

Ocean temperature cycles and volcanic eruptions are linked because the upper layers of the ocean are a key place to spot climate change in action: It's where additional heat from climate change is lurking, and if a highly explosive volcano were to erupt, it's where scientists would go looking for its cooling signature.

And a new study in Nature Communications argues that ocean role could be a problem in the future. The paper examined the giant eruption at Mt. Tambora in 1815, which created a "year without a summer" because of its climate impact, and a similarly large hypothetical eruption in 2085. The team found that a future eruption could have a greater climate impact because the layers of the ocean might mix less, making them more vulnerable to large swings based on what's happening around the world.

Predictions and past eruptions aside, because of the uncertainty about how an eruption would play out, the only way to truly know what will happen at Mt. Agung is to keep watching. Even the best science can't solve that right now. "If anyone's gonna know, they are going to know," Krippner said of Indonesia's volcano-monitoring team. "Knowing what the magma is going to do in the future is just an incredibly big ask."