Bali volcano: Here's why the Earth will get a little cooler after Mount Agung erupts

Bali volcano: Here's why the Earth will get a little cooler after Mount Agung erupts

Bali volcano: Here's why the Earth will get a little cooler after Mount Agung erupts

Updated 27 September 2017, 7:10 AEST

When Mount Agung finally erupts in Bali — and experts say it could be any moment — global atmospheric temperatures will drop.

Something very interesting will happen when Bali's Mount Agung finally erupts: the Earth will become a little bit cooler.

Yep. It's not exactly what you'd expect after a volcanic eruption, which will see molten lava spewed into the air.

But don't get too excited, it will far from reverse the effects of global warming.

Here's why:

Global temperatures dropped last time Agung erupted

Mount Agung last erupted in 1963 after lying dormant for decades.

When it erupted, experts said global atmospheric temperatures dropped by 0.1-0.4 degrees Celsius.

That might not sound like much, but it's quite a significant drop when you consider the last ice age occurred when global temperatures were only 5C cooler than they are now.

So why the drop?

Because ash and toxic gas were injected into the air

According to Richard Arculus, an Emeritus Professor in geology at the Australian National University, when Mount Agung erupted 54 years ago it spewed vast amounts of ash and sulphur dioxide into the atmosphere.

That sulphur dioxide then reacted with the water vapour in the air and formed droplets of sulphuric acid.

About 10 million tonnes of those droplets accumulated in the Earth's stratosphere the layer above the troposphere within which we live — and formed a haze.

That haze then acted as a barrier and reduced the amount of ultraviolet (UV) rays that made it from the Sun to the Earth's surface, resulting in a cooling effect.

But the effect didn't last for long

According to Professor Arculus, that sulphuric acid haze can persist in the stratosphere for a few years, but eventually the droplets will drop back to Earth.

"They're small enough that they can stay up there for a while … but eventually they get rained out," he said.

And that's why the temperature drop will far from cure global warming.

"These are short-term effects, not like the enduring, year after year injection of carbon dioxide by burning fossil fuels — which keeps accumulating," Professor Arculus said.

So will you notice the difference?

No. If Mount Agung behaves as it did in 1963 — which it's widely expected to — you won't feel a thing.

"[The change in temperature] doesn't last long enough for us to notice. It's more likely to be an instrumental effect that scientists notice," Professor Arculus said.

But volcanoes have caused noticeable changes to the Earth's climate in the past.

In 1815, the eruption of Mount Tambora in Indonesia, had a huge effect on global temperatures.

"After its eruption, it was known in Northern Europe and northeast America as 'The Year Without Summer'," Professor Arculus said.

"It caused a big enough temperature drop that there was frost in the New England region of the United States in August and that's unheard of. And [there were] widespread crop failures.

"Global temperatures were affected enough for the people who were trying to grow things and feed animals to notice the effect.

"There's no notion yet that Agung will have an eruption as large as Tambora."