A large gray ash cloud covers the majority of visible Earth in a photo taken from the International Space Station.
The volcanic ash plume from the Hunga Tonga–Hunga Haʻapai eruption sent water vapor into the stratosphere. Credit: NASA Earth Observatory

When Hunga Tonga–Hunga Ha‘apai (HTHH) erupted in January 2022, it shot the standard volcanic cocktail of ash, gas, and pulverized rock into the sky. But the eruption included one extra ingredient that’s now causing climate concerns: a significant splash of ocean water. The underwater caldera shot 146 metric megatons of water into the stratosphere like a geyser, potentially contributing to atmospheric warming over the next 5 years, according to a new study published in Nature Climate Change.

Earth’s average temperature is teetering on the edge of surpassing its preindustrial level by 1.5°C—the target set by the United Nations in the Paris Agreement. In May 2022, the World Meteorological Organization announced there was a 50% chance of exceeding the 1.5°C threshold over the next 5 years. The new study showed that slight warming caused by the HTHH eruption increased the likelihood of Earth temporarily tipping past that mark by another 7%.

Straight to the Stratosphere

HTHH ranked a 5.7 on the volcanic explosivity index, alongside other historymakers like Vesuvius (79 CE) and Mount St. Helens (1980). Explosive volcanoes inject ash and sulfur dioxide into the atmosphere, typically leading to global cooling. Mount Tambora’s sulfur dioxide plume in 1815, for example, inhibited sunlight from reaching Earth, triggering the “year without a summer.”

The HTHH eruption was unusual. Unlike other eruptions its size, HTHH had a relatively low sulfur dioxide content. Researchers assumed it led to only 0.004°C of global cooling in 2022.

HTHH also exploded just below the ocean surface, injecting vaporized seawater into the stratosphere like a syringe. Though seemingly innocuous, water vapor is the planet’s most common greenhouse gas. Whereas volcanic sulfates commonly block sunlight from reaching Earth, water vapor keeps it from leaving.

“If we’re only a quarter of a degree from 1.5°C, those four hundredths of a degree do actually make a tangible difference.”

The HTHH blast sent water vapor and other gases to at least 40 kilometers (25 miles) above Earth’s surface and punched through the boundary of the stratosphere. In this atmospheric layer, cool, heavy air rests below less dense warmer air. Because there is little turbulence to stir the system, “you can get a perturbation lasting for, in an atmospheric sense, quite a long time,” said Stuart Jenkins, an atmospheric physicist at the University of Oxford and lead author of the new study. The eruption boosted the water vapor content of the stratosphere by 10%–15%, according to the study.

Using reconstructions of global climate, Jenkins and his colleagues established the monthly baseline conditions for the 7 years prior to the eruption, then simulated the effect of water vapor in the stratosphere for 7 years after the event. The researchers assumed that the injected water vapor would settle out of the stratosphere in that time. Their model parameters are conservative, Jenkins said, and assumed the volcanic plume spread widely between altitudes and latitudes.

The model calculated the monthly change in Earth’s energy balance caused by the eruption and showed that water vapor could increase the average global temperature by up to 0.035°C over the next 5 years. That’s a large anomaly for a single event, but it’s not outside the usual level of noise in the climate system, Jenkins said. But in the context of the Paris Agreement, it’s a big concern.

“If we’re only a quarter of a degree from 1.5°C, those four hundredths of a degree do actually make a tangible difference,” he said. The planet was already 50% likely to warm past 1.5°C in the next 5 years, and the presence of HTHH’s water vapor increased the odds of temporarily exceeding that threshold to 57%, according to the simulation.

What About 1.5°C?

The volcano launched an “unprecedented” amount of water vapor into the stratosphere, said Patrick Sheese, a climate physicist at the University of Toronto who was not involved in the study. But the event’s impact can’t compare with that of human emissions, he said. Even if the eruption increases temperatures as the simulation predicted, that’s only a small, temporary lift toward the 1.5°C threshold. Decades of research have shown that humans are still responsible for most of the warming.

The study “is just another reminder that nature isn’t going to help us out of climate change.”

The first year that world temperatures breach the Paris Agreement will inevitably spur a flurry of headlines. But as the study showed, part of that warming is caused by natural anomalies. Any number of phenomena can sway global temperatures, from El Niño conditions in the Pacific Ocean to wildfires in Siberia. The HTHH eruption may nudge the temperature past 1.5°C of warming, but that doesn’t mean the Paris Agreement has failed yet; the event demonstrated how close the world is to its agreed-upon tipping point.

The study “is just another reminder that nature isn’t going to help us out of climate change,” Sheese said. “This clearly is up to us to fix.”

—J. Besl (@J_Besl), Science Writer

Citation: Besl, J. (2023), Tonga eruption may temporarily push Earth closer to 1.5°C of warming, Eos, 104, https://doi.org/10.1029/2023EO230104. Published on 16 March 2023.
Text © 2023. The authors. CC BY-NC-ND 3.0
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