Small volcanic eruptions can cool global climate

Even minor volcanic eruptions can release enough aerosols into the atmosphere to affect global temperatures, thanks to a boosting effect from weather systems such as monsoons.

Until now, scientists thought that only a massively energetic eruption could inject aerosols past the troposphere – the turbulent atmospheric layer closest to Earth – up into stratosphere.

“If an aerosol is in the lower atmosphere, it’s affected by the weather and it precipitates back down right away,” says Adam Bourassa, from the University of Saskatchewan Institute of Space and Atmospheric Studies.

“Once it reaches the stratosphere, it can persist for years, and with that kind of a sustained lifetime, it can really have a lasting effect.”

The cooling effect is caused by the scattering of incoming sunlight, and can be pretty significant – for example, the massive eruption of Mount Pinatubo in the Philippines in 1991 temporarily dropped temperatures by half a degree Celsius world -wide.

The research team looked at the June 2011 eruption of the Nabro volcano in Eritrea in northeast Africa, after which winds carried the volcanic gas and aerosol – minute droplets of sulfuric acid – into the path of the annual Asian summer monsoon.

The stratosphere’s always been assumed to be a barrier that storms simply couldn’t pierce – indeed, the distinctive flattened ‘anvil’ shape at the top of large thunderstorms is caused by the storm squashing agaisnt it.

But while dust from the Nabro volcano settled out, the monsoon lifted volcanic gas and lighter liquid droplets right up into the stratosphere, where they were detected by the Canadian Space Agency’s OSIRIS instrument aboard the Swedish satellite Odin.

Indeed, the Nabro volcano caused the largest stratospheric aerosol load ever recorded by OSIRIS in its more than 10 years of flight.

The findings should help improve climate change models.

“There are only a few instruments that can measure stratospheric aerosols, and OSIRIS is one of them,” says Bourassa. “It’s become extremely important for climate studies, because we’ve captured more than a full decade of data. The longer it’s up, the more valuable it becomes.”