Bali's Mt Agung eruption could send temperatures plunging

The eruption of Bali's Mt Agung could take some of the heat out of climate change, scientists say - but they don't think it'll make a difference in the long run.

The volcano has been spewing ash into the skies above the Indonesia island in recent weeks, and a 10km evacuation zone has been established. Last time Mt Agung erupted in 1963, up to 1500 people lost their lives.

That eruption - comparable in size to the one which buried the Roman city of Pompeii - saw global temperatures drop by around 0.2degC.

Ash and sulphur dioxide particles that reach the stratosphere can linger for a few years, reflecting light back into space, says environmental scientist Zeke Hausfather of Berkeley Earth, a climate research non-profit.

Berkeley Earth looked at 1963 and other recent eruptions, including El Chichon in 1982 and Mt Pinatubo in 1991, to see what direct effects they had on temperatures.

The blue line shows what effect Mt Agung might have on global temperatures, based on previous eruptions.
The blue line shows what effect Mt Agung might have on global temperatures, based on previous eruptions. Photo credit: Carbon Brief/Berkeley Earth

"When eruptions occurred, the average cooling was about 0.2degC, with most of the effect dissipating over five years as sulphate aerosols fell out of the stratosphere," said Mr Hausfather.

Volcanoes also release carbon dioxide, which causes global warming, but not enough to overcome the reflective properties of ash and sulphur.

"To have a notable climate impact, there needs to be an explosive enough eruption (to get material in the stratosphere) and sulphur-rich," NASA climate scientist Chris Colose said on Twitter.

"If these conditions are met, the eruption cools the surface/troposphere and warms the stratosphere, the opposite of both patterns associated with CO2 increases. But both are very short-lived."

However, as both he and Mr Hausfather mentioned, the cooling effect isn't long-lasting. Within a few years, global warming would be expected to be back on track - as demonstrated by the blue line in the graph.

Dr Colose later expressed dismay his comments had been interpreted by one British newspaper as suggesting the Earth was about to be plunged into an "ice age".

"Jesus. My name is ruined forever," he wrote. "The next headline will presumably be, "NASA declares that big eruption will SAVE THE POLAR BEARS."

The cooling effect of the Pinatubo eruption of 1991 was so strong, scientists estimate it caused sea levels to drop 6mm as the ocean cooled and contracted. Over the next few years it warmed up rapidly, making up lost time.

If Mt Agung doesn't spectacularly blow, scientists say it will have little to no effect at all.

"It seems unlikely this Agung eruption will be explosive enough to have large climate impact," said Dr Colose.

On the other hand, the 1815 eruption of the nearby Mt Tambora lowered global temperatures 0.6degC, causing a 'year without a summer' which devastated crops as far away as Europe.

Mt Tambora was the world's biggest eruption since New Zealand's own Lake Taupo blew its top in 180 A.D.

Under the Paris Agreement, almost all of the world's countries have promised to take steps to keep global temperature rise "well below" 2degC above pre-industrial levels".