The unprecedented amount of water vapor released into the atmosphere, as detected by NASA’s Microwave Limb Sounder, could end up temporarily warming the Earth’s surface. On January 15, 2022, the Hunga Tonga-Hunga Ha’apai volcano erupted, setting off a sonic boom that circled the globe twice and unleashed a tsunami around the world. The underwater explosion in the South Pacific Ocean also launched a huge plume of water vapor into the Earth’s stratosphere. In fact, the unprecedented amount of water vapor was so huge, it was enough to fill more than 58,000 Olympic-sized swimming pools. The sheer volume of water vapor could be enough to temporarily affect Earth’s global average temperature. “We’ve never seen anything like it,” said Luis Millán, an atmospheric scientist at NASA’s Jet Propulsion Laboratory in Southern California. He led a recent study that looked at the amount of water vapor the Tonga volcano introduced into the stratosphere, the layer of the atmosphere between about 8 and 33 miles (12 and 53 kilometers) above the Earth’s surface. This satellite image shows an intact Hunga Tonga-Hunga Ha’apai in April 2015, years before an explosive underwater volcanic eruption wiped out most of the Polynesian island in January 2022. Credit: NASA Earth Observatory image by Jesse Allen, using Landsat data from the US Geological Survey Published in Geophysical Research Letters, the study by Millán and colleagues estimates that the Tonga eruption sent an incredible 146 teragrams (1 teragram equals one trillion grams) of water vapor into the Earth’s stratosphere. This is an amount equal to 10% of the water already present in this atmospheric layer. That’s nearly four times the amount of water vapor scientists estimate from the 1991 eruption of Mount Pinatubo in the Philippines entered the stratosphere. “We’ve never seen anything like it.” — Luis Milan Millán analyzed data from the Microwave Limb Sounder (MLS) instrument on NASA’s Aura satellite, which measures atmospheric gases, including water vapor and ozone. After the Tonga volcano erupted, the MLS team started seeing water vapor readings that were off the charts. “We had to carefully inspect all measurements in the plume to make sure they were reliable,” Millán said.
A Lasting Impression
Volcanic eruptions rarely inject much water into the stratosphere. In the 18 years that NASA has been taking measurements, only two other eruptions — the 2008 Kasatochi eruption in Alaska and the 2015 Calbuco eruption in Chile — have sent appreciable amounts of water vapor to such high altitudes. But these were mere drops compared to the Tonga event, and the water vapor from both previous eruptions quickly dissipated. The excess water vapor vented by the Tongan volcano, on the other hand, could remain in the stratosphere for several years. An image from January 16, 2022, shows the ash plume from the Hunga Tonga-Hunga Ha’apai volcanic eruption that occurred the previous day. An astronaut took a photo of the plume from the International Space Station. Credit: NASA This extra water vapor could affect atmospheric chemistry, enhancing certain chemical reactions that could temporarily exacerbate the depletion of the ozone layer. It could also affect surface temperatures. Huge volcanic eruptions like Krakatoa and Mount Pinatubo commonly cool the Earth’s surface by spewing gases, dust and ash that reflect sunlight back into space. In contrast, the Tonga volcano did not eject large amounts of aerosols into the stratosphere, and the huge amounts of water vapor from the eruption may have a small, temporary warming effect, as the water vapor traps heat. The effect would disappear when the extra water vapor leaves the stratosphere and would not be enough to appreciably worsen the effects of climate change. The huge amount of water injected into the stratosphere was probably only possible because the submarine volcano’s caldera—a basin-shaped cavity that usually forms after magma erupts or drains from a shallow chamber beneath the volcano—was at the right depth in the ocean: ca. 490 feet (150 meters) down. Any shallower and there wouldn’t be enough seawater superheated by erupting magma to explain the stratospheric water vapor values Millán and his colleagues saw. Any deeper and the enormous pressures in the ocean depths could have silenced the explosion. The MLS instrument was well-placed to detect this plume of water vapor because it observes natural microwave signals emitted by Earth’s atmosphere. Measuring these signals allows MLS to “see” through obstructions such as ash clouds that can blind other instruments that measure water vapor in the stratosphere. “MLS was the only instrument with a dense enough coverage to capture the plume as it happened and the only one unaffected by the ash released by the volcano,” Millán said. Reference: “The Hunga Tonga-Hunga Ha’apai Hydration of the Stratosphere” by L. Millán, ML Santee, A. Lambert, NJ Livesey, F. Werner, MJ Schwartz, HC Pumphrey, GL Manney, Y Wang, HS Geophysical Research Letters.DOI: 10.1029/2022GL099381 Su, L. Wu, WG Read and L. Froidevaux The MLS instrument was designed and built by JPL, which is managed for NASA by Caltech in Pasadena. NASA’s Goddard Space Flight Center manages the Aura mission.
