The unprecedented quantity of water vapor hurled into the environment, as detected by
“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 investigation examining the amount of water vapor that the Tonga volcano injected into the stratosphere, the layer of the atmosphere between about 8 and 33 miles (12 and 53 kilometers) above Earth’s surface.
Published in Geophysical Research Letters, the study by Millán and his colleagues estimates that the Tonga eruption sent an incredible 146 teragrams (1 teragram equals a trillion grams) of water vapor into Earth’s stratosphere. That’s an amount equal to 10% of the water already present in that atmospheric layer. That’s nearly four times the amount of water vapor that scientists estimate the 1991 Mount Pinatubo eruption in the Philippines lofted into the stratosphere.
“We’ve never seen anything like it.” — Luis Millán
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 the measurements in the plume to make sure they were trustworthy,” said Millán.
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 event in Alaska and the 2015 Calbuco eruption in Chile – sent appreciable amounts of water vapor to such high altitudes. But those were mere blips compared to the Tonga event, and the water vapor from both previous eruptions dissipated quickly. The excess water vapor injected by the Tonga volcano, on the other hand, could remain in the stratosphere for several years.
This extra water vapor could influence atmospheric chemistry, boosting certain chemical reactions that could temporarily worsen the depletion of the ozone layer. It could also influence surface temperatures. Massive volcanic eruptions like Krakatoa and Mount Pinatubo typically cool Earth’s surface by ejecting gases, dust, and ash that reflect sunlight back into space. In contrast, the Tonga volcano didn’t inject large amounts of aerosols into the stratosphere, and the huge amounts of water vapor from the eruption may have a small, temporary warming effect, since water vapor traps heat. The effect would dissipate when the extra water vapor cycles out of the stratosphere and would not be enough to noticeably exacerbate climate change effects.
The sheer amount of water injected into the stratosphere was likely only possible because the underwater volcano’s caldera – a basin-shaped depression usually formed after magma erupts or drains from a shallow chamber beneath the volcano – was at just the right depth in the ocean: about 490 feet (150 meters) down. Any shallower, and there wouldn’t have been enough seawater superheated by the erupting magma to account for the stratospheric water vapor values Millán and his colleagues saw. Any deeper, and the immense pressures in the ocean’s depths could have muted the eruption.
The MLS instrument was well situated to detect this water vapor plume because it observes natural microwave signals emitted from Earth’s atmosphere. Measuring these signals enables MLS to “see” through obstacles like ash clouds that can blind other instruments measuring water vapor in the stratosphere. “MLS was the only instrument with dense enough coverage to capture the water vapor plume as it happened, and the only one that wasn’t affected by the ash that the volcano released,” said Millán.
Reference: “The Hunga Tonga-Hunga Ha’apai Hydration of the Stratosphere” by L. Millán, M. L. Santee, A. Lambert, N. J. Livesey, F. Werner, M. J. Schwartz, H. C. Pumphrey, G. L. Manney, Y. Wang, H. Su, L. Wu, W. G. Read and L. Froidevaux, 1 July 2022, Geophysical Research Letters.
The MLS instrument was designed and built by