On the Indonesian island of Bali, a volcano called Mount Agung is spewing ash 5.5 miles into the sky, causing flight cancellations and trapping thousands of tourists and locals on the island, even as the potential for a bigger eruption looms.

Authorities reopened Bali’s international airport Thursday after closing it for three days due to fear of ash damage to aircraft engines. The closures stranded close to 60,000 passengers and more than 100,000 people near the volcano were told to evacuate as explosions were heard more than 7 miles away.

You can watch the eruption of Mount Agung in this live stream:

Indonesia’s National Disaster Management Authority (BNPB) warned Monday that the eruptions are increasing and declared the highest alert level for the volcano. An Indonesian government scientist said a larger eruption is possible, but also that the current levels of lava, ash, sulfur, and carbon dioxide emissions could continue for weeks. (You can see ongoing volcanic activity in Indonesia on this live map.)

As Denison University volcanologist Erik Klemetti noted on Twitter, no one knows how the current eruption will pan out.

The eruption at #Agung in Indonesia has been relatively small so far, but with lava at the summit crater, it is anyone’s guess how it proceeds from here: https://t.co/1rOS1u9UAW — Erik Klemetti (@eruptionsblog) November 27, 2017

Bali has a tragic history with Mount Agung, as you can see in this old-timey newsreel (complete with offensive tropes about the indigenous population):

In 1963, Mount Agung’s eruption killed more than 1,600 people on the island over several months — and that was merely a “moderate” eruption, Diana Roman, a geologist at the Carnegie Institution of Washington, told the Washington Post. So locals are understandably unnerved.

But beyond the threat of local devastation, the current eruption of Mount Agung has the potential to impact the entire world.

A single volcano can mess with the whole planet’s climate

Scientists have long understood that volcanic eruptions can nudge the planet’s thermostat for months, as millions of tons of gases and particles spread through the atmosphere.

How much the needle moves, however, depends on what’s being erupted, according to NASA climate scientist Chris Colose.

“Most eruptions do not have a meaningful climate impact, and so the risks associated with the eruption are limited to the nearby population,” he wrote in an email. “For climate, the big thing to pay attention isn't the ash but the sulfur emissions.”

Gases like sulfur dioxide spew from volcanic craters during an eruption, hidden among billowing ash. These sulfur compounds react in the sky to form substances that scatter sunlight, thereby cooling the planet.

Data from @NASA Aura satellite shows high SO2 concentrations from Mount #Agung yesterday in #Bali, #Indonesia. People on the ground witnessing the #volcano emitting thick ash clouds and glowing lava. More satellite imagery: https://t.co/oVYhIsjXNx pic.twitter.com/U3wPxQtfHc — NOAA Satellites (@NOAASatellites) November 28, 2017

How much a volcanic eruption cools the planet depends on the amount of material it erupts, how high it reaches, and the composition of that material. Eruptions can also change global rainfall patterns.

Scientists are also toying with the controversial idea of imitating volcanoes to keep global warming in check, a strategy known as geoengineering. This includes deliberately seeding clouds or spraying sulfuric acid into the stratosphere to offset some of humanity’s impacts on the world’s climate.

We don’t know yet whether the ongoing eruption in Bali will pump out enough gas and ash to have a measurable impact on the climate, but we do know that the 1963 eruption of Mount Agung knocked down global temperatures between 0.1 and 0.2 degrees Celsius for a year.

The most recent volcanic eruption that pushed down the planet’s temperature was Mount Pinatubo in the Philippines in 1991. That eruption injected roughly 10 million metric tons of sulfur into the sky. The 1963 Agung eruption emitted about 6 million metric tons of sulfur.

Over at Carbon Brief, climate researcher Zeke Hausfather graphed what would happen to global temperatures if the current Agung eruption were to reach the same scale as the last one amid the current global warming trend.

“This projection, which is based on the historical relationship between volcanic eruptions and temperature, suggests that an Agung eruption would reduce global temperatures between 0.1C to 0.2C in period from 2018 to 2020, with temperatures mostly recovering back to where they otherwise would be by 2023,” Hausfather wrote.

The chart shows that volcanoes leave distinct fingerprints in the global temperature record. This is because volcanoes emit cooling sulfur compounds straight into the stratosphere, though the quantity is smaller than emissions from human activities, which have longer-lasting effects on the climate.

However, locals are more concerned about the next few days, keeping an uneasy eye on their backyard volcano and wearing surgical masks to ward off the ash. Despite evacuation orders, they are wary of leaving their homes without alternative jobs and housing in place, so if the eruption suddenly gets worse, many will likely remain in harm’s way.