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.