It’s been an unsettling few weeks on the Indonesian island of Bali, where the volcano Mount Agung has been erupting in fits and starts, particularly during late November.
Last weekend, more than 100,000 people were asked to evacuate from within 10 kilometers (6 miles) of the volcano. Bali’s main airport, Ngurah Rai International Airport (Denpasar), was closed for three days, reopening on Wednesday local time.
It’s too soon to know if this event will end up affecting global climate in a significant way. However, the bigger question marks are more geological than meteorological. We don’t know if a much larger eruption is in store for Agung over the coming weeks or months—but we do know that major volcanic eruptions can measurably cool the planet for several years, by way of a process that’s fairly well understood. This means it’s possible to make some reasoned speculations on how Agung’s potential behaviors would translate into climate effects.
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What it takes for a volcano to torque climate
To cool the planet, a volcanic eruption needs to push vast amounts of ash and gases—particularly sulfur dioxide—up into the stratosphere, to heights of around 15 – 25 km (9 – 16 mi). If the sulfur dioxide doesn’t make it this high, it tends to be quickly mixed and dispersed through the troposphere, the “weather layer” of the atmosphere. Much of the gas can get scoured out by precipitation within a few days to weeks.
Compared to the troposphere, the stratosphere is a more stable, tranquil home for sulfur dioxide. Sunlight and moisture can gradually convert the gas into airborne droplets of sulfuric acid (sulfate aerosols), which are quite good at reflecting and absorbing solar energy before it can reach Earth. Volcanic ash has similar sun-blocking effects, but it tends to settle out of the stratosphere fairly quickly, whereas it can take one to three years for the lighter sulfate aerosols to do the same. For this and other reasons, the amount of sulfur dioxide pumped into the stratosphere is the main factor in an eruption’s climate impact—a finding that emerged from the eruptions of Mount St. Helens in 1980 and El Chichon in 1982, as discussed by geologist Karen Harpp in Scientific American.
As a rule, equatorial volcanoes are more effective than high-latitude volcanoes at cooling the planet. That’s because the plume of sulfates from a large eruption near the equator has the potential to spread into both northern and southern hemispheres, and there is little disruption from high-latitude jet streams. Moreover, the shielding effects of the plume are concentrated at lower latitudes, where there’s more solar energy to block.
Agung in context
One reason why it’s disconcerting to see Mt. Agung so restless is because of its latitude: at 8°S, it’s not far from the equator. Another reason is its history. A year-long eruption event that peaked with an explosive blast on March 17, 1963, killed some 1600 people and had a measurable effect on global climate (see below). Bigger still were the planet’s two cataclysmic eruptions of the 19th century, both of which took place on nearby Indonesian islands: Tambora (1815) and Krakatoa (1883). The Tambora eruption cooled global temperatures by as much as 0.5°C for several years and played into widespread regional chaos, including monsoon disruptions and famine in India, crop failures as far afield as northern Europe, and New England’s devastating “year without a summer” (1816).
The most recent single eruption to have a major effect on global climate was that of Mt. Pinatubo in the Philippines on June 15, 1991. The eruption pumped roughly 17 million tons of sulfur dioxide into the equatorial stratosphere.
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Global surface temperatures dipped by more than 0.2°C, interrupting the long-term warming trend of the late 20th century for several years.
Compared to Mt. Pinatubo, Agung is a nothingburger—so far, at least. “The current eruption of Mount Agung is not yet energetic enough to affect global temperatures in the way that Mount Pinatubo did after its eruption in 1991,” said David Considine of NASA’s Earth Science Division. “To do that, the Agung eruption would have to deposit a significant amount of sulfate aerosol in the lower stratosphere, which in the tropics means higher than 10 miles in altitude. The current eruption height is about 6 miles, so fairly far below what is necessary to result in an appreciable global effect on surface temperatures.”
The NOAA/NASA Suomi/NPP satellite, which measures sulfur dioxide on a daily basis, has been keeping tabs on emissions from Agung. In a Twitter exchange, volcanologist Simon Carn (Michigan Technological University) told me that Suomi/NPP measurements on each of the four days from Nov. 26 to 29 resulted in a cumulative airborne total of around 40,000 tons of sulfur dioxide (SO2) in the vicinity of Agung, or less than 0.2% of the output from Pinatubo. If anything, this is a high-end estimate, since there could be some carry-over of sulfur dioxide from one day to the next. “We measured very little SO2 on Nov 30, so most of it has already dispersed,” said Carn.
A new observing tool has come on the scene just in time to watch Agung. On Friday, the European Space Agency released the first high-resolution SO2 imagery from its Sentinel-5P satellite, launched in October (see embedded tweet below).
Using models to peer into Agung’s potential
What if Agung were to kick into action and produce a far bigger eruption over the next few weeks or months? In a very helpful analysis at Carbon Brief, climate scientist Zeke Hausfather (Berkeley Earth) looked into how climate might be affected if Agung were to send roughly the same amount of ash and sulfates into the stratosphere as it did in 1963. (Of course, any Agung eruption could be much bigger or much smaller than that one.)
In 1963, Agung produced a multiyear cooling that reduced global temperatures by around 0.2°C at its peak. This was a bit less than the coolings produced by El Chichon in the early 1980s and Mt. Pinatubo in the early 1990s. None of these volcanoes put a long-lasting dent in the warming trend being caused by human-produced greenhouse gases. A repeat of the 1963 Agung eruption could reduce global temperatures by an average of 0.1 – 0.2°C from 2018 to 2020, said Hausfather, but “with temperatures mostly recovering back to where they otherwise would be by 2023.”
There’s more to the story—namely, the El Niño–Southern Oscillation (ENSO).
By coincidence, the major eruptions from Agung (1963), El Chichon (1982) and Pinatubo (1991) all occurred during weak to moderate El Niño events, which tend to raise global surface temperature. (Months after the El Chicon eruption, the 1982-83 El Niño intensified to become one of the strongest on record—and in fact, there’s some evidence that a major eruption can itself favor El Niño conditions in the following 1-2 years, as discussed in a study led by Myriam Khodri (Sorbonne) published last month in Nature Communications.)
Analysis shows that the cooling effects of Agung, El Chicon, and Pinatubo on global climate were partially masked by El Niño–related warming. Similarly, La Niña could accentuate any cooling produced by a major eruption. As it happens, La Niña events often last two to three years, which is similar to the lifespan of sulfate aerosols in the stratosphere.
Climate models do a much better job simulating volcanic impacts when they incorporate ENSO, as shown in a 2016 Geophysical Research Letters paper led by Flavio Lehner (National Center for Atmospheric Research).
Recently, Lehner and NCAR colleague John Fasullo used a large sequence of runs from the Community Earth System Model to explore how a major eruption in 2017-18 might be modulated by ENSO. They found that the subsequent global cooling might be less than 0.1°C if El Niño is occurring, or close to 0.3°C if a La Niña is in place (the more likely scenario at this point). In both cases, they found, global temperature would resume warming within about two years and would be at pre-eruption levels by the early 2020