The Hunga Tonga volcanic eruption that sent shockwaves through the Earth in January 2022 produced the highest volcanic plume since at least the 1883 Krakatoa eruption, a new study reports. And the amount of water that the volcano injected into Earth’s atmosphere could have warmed the planet’s climate.
The volcanic eruption that ripped apart the Polynesian island of Hunga Tonga-Hunga Ha’apai on Jan. 15 was a local disaster, but it has also proven to be a scientific gift that keeps going. The previously nondescript underwater volcano in a remote region of the southern Pacific erupted with unprecedented force in the full view of three weather satellites. These satellites allowed scientists around the world to watch the startling explosion in real time and study its aftermath in unprecedented detail.
Among the researchers fascinated by the fungus of water vapor and mineral ash that shot up into the sky that fateful Sunday evening was Simon Proud, an Earth observation scientist at the STFC Rutherford Appleton Laboratory and the University of Oxford in the UK
Related: Tonga’s massive eruption produced record-breaking winds at the edge of space
Proud, the lead author of a new study analyzing the plume, was quick to realize that Hunga Tonga’s eruption was quite unusual. Temperature measurements from Earth observation satellites, for example, indicated that the volcanic cloud must have reached an unprecedented height.
“When [the cloud] evolved, we looked at the temperatures based on the satellite data,” Proud told Space.com.
Scientists typically estimate the magnitude of atmospheric phenomena by measuring their temperature with satellite-based infrared sensors and comparing those measurements to the temperature of the surrounding atmosphere. In the troposphere, the layer of Earth’s atmosphere closest to the planet’s surface, temperature decreases with altitude according to well-known physical rules. But those rules no longer apply in the stratosphere, the atmospheric layer that extends from 9 miles to 30 miles (15 to 50 kilometers) altitude, where the ozone layer absorbs the sun’s ultraviolet radiation and pushes temperatures higher. As the temperature of the Hunga-Tonga cloud continued to drop, Proud realized that accurately estimating the height would require a more sophisticated approach.
A colour-coded map of the eruption elevation of Mount Hunga Tonga, created by merging data from three weather satellites. (Image credit: Simeon Schmauss / JMA / NOAA / KMA)
Because three different weather satellites observed the burst from three different positions in geostationary orbit, an orbit at an altitude of about 22,000 miles (36,000 km) where objects appear to be floating above a fixed point on Earth, Proud was able to determine the altitude of the cloud top using a method known as parallax.
Parallax allows researchers to calculate the distance to an object using the apparent distances observed from at least two different locations. It’s commonly used to calculate the distances of stars, and Proud previously used it to calculate the height of the Chelyabinsk meteor that exploded over Russia in 2013.
“We were really lucky that we could cover the area with three satellites,” said Proud. “[The calculation] produced really nice results; it worked very well for such a tall volcano. We’ve never seen anything that high.”
The calculation showed that the Hunga-Tonga cloud not only broke through the troposphere, but also rose through the entire stratosphere, reaching a plateau only at an altitude of 35 miles (57 km), well into the frozen and dry layer that known as the mesosphere. This makes the Hunga Tonga volcanic plume the tallest ever observed and quite possibly the tallest in over a century.
“The last [volcanic eruption] that could have reached that height was Krakatoa in 1883,” said Stolz. “There were some younger ones [eruptions] such as [the 1991 eruption of] Mount Pinatubo, and we think we underestimated the height of this one, but not to the same extent.”
Ash from Mount Pinatubo’s eruption has been detected at altitudes nearly 40 km (opens in new tab). Stolz believes that if better satellites had been in orbit at the time, traces of the cloud could have been seen even 6 miles (10 km) higher, but still less than the 35 miles from Hunga Tonga.
The Pinatubo ash in the stratosphere cooled the Earth’s climate by 1 degree Fahrenheit (opens in new tab) (0.6 degrees Celsius) due to the presence of sulfur dioxide in the volcanic material. Sulfur dioxide reflects radiation and is often studied as part of potential geoengineering interventions aimed at slowing climate change.
When Hunga Tonga exploded, scientists thought that the eruption might trigger a similar effect (which was observed for two years in the case of Pinatubo). However, subsequent measurements revealed that the Hunga-Tonga cloud contained only about 2% of the sulfur dioxide produced by Pinatubo, not enough to measurably affect the climate. However, Stolz says the amount of water sprayed into the stratosphere by the blast may actually warm the climate.
“This volcano put a lot of water into the stratosphere and also some into the mesosphere,” Proud said. “Water in the stratosphere generally warms the Earth’s surface. So this could actually contribute to some warming of the earth’s surface in the next few years. We have very good temperature data for the lower atmosphere, so we should be able to figure that out pretty quickly.”
A study published earlier this year found that Hunga Tonga injected the equivalent of 58,000 Olympic-size swimming pools into Earth’s atmosphere, potentially increasing the amount of water vapor in the stratosphere by 5%.
The effects on the mesosphere, Proud said, could be more subtle and come with some pretty intriguing side effects.
“The mesosphere is typically the driest layer of the atmosphere, and extra water up there could mean we get more polar mesospheric clouds,” Proud said. “These clouds are getting more frequent anyway, probably due to climate change, so I want to look at satellite data and see if I can see an increase after this outbreak.”
Polar mesospheric clouds, also known as noctilucent clouds for their ability to glow at night, form over the polar regions at altitudes of 76 to 85 km (47 miles to 53 miles) during the summer months. These clouds often appeared after launches from space shuttles, which emitted large amounts of water vapor in their rocket exhaust.
The mesosphere, Proud said, is fairly unexplored, being too low for satellites to fly through but too high for balloons to reach. The eruption of Hunga Tonga could therefore provide a unique incentive for scientists to study the chemical processes taking place in this region.
Researchers are far from done with the eruption of Hunga Tonga. Many questions remain unanswered, including its exact effect on Earth’s climate and why it exploded with such force after centuries of lukewarm activity.
The new study (opens in new tab) was published online today (Nov. 3) in the journal Science.
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