The relative power of an eruption is ranked on an “explosivity index,” a scale from 0 to 8 depending largely on the volume of ejected ash and gas and how high it goes — 100,000 feet or more in some cases. Agung’s 1963 eruption was rated 5 on the scale, as was Pinatubo’s in 1991. But the index does not necessarily correlate to impact on climate: The eruption of Mount St. Helens in Washington in 1980 was of similar explosiveness but had little cooling effect because most of the ash and gas was expelled laterally rather than upward.

Image Mount Agung spewing smoke and ash in Bali in November. Credit... Firdia Lisnawati/Associated Press

NASA researchers are mapping out a plan to monitor a Pinatubo-like event — “the scale of eruption that would lead to both ozone depletion and a lot of surface cooling,” said Paul A. Newman, a senior scientist at the agency who is helping to develop the plan .

Of particular interest would be to measure the amount of sulfur dioxide in the first few weeks, before the gas combines with water vapor to make the reflective aerosols.

It would also be important to monitor the aerosols over time, to see how big they get and how they eventually break down. Bigger aerosols would fall out of the atmosphere sooner, lessening the cooling impact.

Some environmental satellites can monitor volcanic eruptions, but balloon flights would be an important component of any rapid-response program. Balloons are relatively low cost and can be launched from various locations. It would be important to fly them near the same latitude as the erupting volcano, because the gas plume tends to spread east-west first.

Over the longer term, a robust monitoring program would need aircra ft from NASA and other agencies, said Jack Kaye, associate director for research at NASA’s earth science division. That would most likely involve diverting aircraft time from other research projects, he said. “It may mean some people’s plans will have to change,” he said.