Could a Bali eruption mean a cooler NZ summer?

Could a volcanic eruption in Indonesia mean a slightly cooler spring and summer for New Zealand?

One prominent climate scientist thinks it's a possibility.

There has been unprecedented seismic activity around Bali's Mt Agung volcano, with local authorities raising the alert level to four, and Kiwis travelling there being asked to register with Safe Travel.

The last time the volcano blew, in March 1963, it depressed New Zealand temperatures between 0.5C and 1C in the first year, and slightly less in the second year.

Could it happen again?

"This depends on the explosiveness of Agung's eruption and the amount of volcanic ashes and gases injected into the stratosphere," explained Dr Jim Salinger, an honorary research fellow with Otago University, and deputy editor of scientific journal Climactic Change.

"If it is not stratospheric then the impacts on our climate will be negligible."

But, should significant amounts of volcanic material be discharged into the stratosphere, then cooling would occur with likely more southerly and southwesterly winds, the former Niwa principal scientist said.

"Our climate is warmer now anyway, so the cooling will be from a warmer base."

Scientists could make such predictions by examining what happened to New Zealand's climate during major explosive volcanic eruptions in the past, which injected massive amounts of dust and sulphur dioxide into the air.

"The key feature is that these eruptions must inoculate the volcanic debris into the stratosphere, a height of 12 kilometres over Bali," Salinger said.

Once there, the gases rapidly combined with water vapour to form a sulphuric acid mist, increasing Earth's reflectivity and reducing the sunlight reaching its surface.

The cooling from the Mt Pinatubo eruption in 1991, for instance, lasted up to two years.

To affect our climate, eruptions had to occur either in tropical latitudes or in our hemisphere.

Other major explosive eruptions that influenced our climate since the late 19th century included Indonesia's Krakatau, in 1883, Mt Tarawera in 1886, Guatemala's Santa Maria in 1902 and Mexico's El Chichon in 1982.

The common pattern of behaviour of New Zealand temperatures was one of cooling of between -0.2C and -0.7C - or an average -0.4C - for up to 21 months after the eruption.

The largest cooling occurred during the first six months immediately following the eruption, Salinger said.

"In these examples, during the first six months more winds from the south prevailed over New Zealand - even to 15 months out, strong west to southwesterly winds occurred."

'Business as usual' in the ring of fire

Monaro volcano, on Vanuatu's Ambae Island. Photo / Vanuatu Red Cross

Mt Agung is one of three major volcanoes firing up around the so-called Pacific ring of fire.

More than 11,000 people have been ordered to flee Vanuatu's Ambae Island - captured belching columns of smoke and ash by the New Zealand Defence Force Orion aircraft this week - while the Popocatepetl volcano outside Mexico City has been erupting glowing rock and ash on to nearby towns.

But such activity wasn't unusual around the 40,000sq km, horseshoe-shaped ring of fire, which is home to three quarters of the world's volcanoes.

"Volcanic activity is very common around the Pacific Rim, this is very much business as usual," GNS Science volcanologist Brad Scott said.

A NZDF aerial survey this week found huge columns of smoke, ash and volcanic rocks billowing from the crater of an erupting volcano on Vanuatu's Ambae Island. Photo / NZDF

"There will always be around five to 10 volcanoes active at any one time, and there may be another five or so showing signs of volcanic unrest."

Scott said the unrest at Mt Agung - which hasn't erupted for half a century - meant it was waking up.

"The locals have moved a lot of people as a precaution," he said.

"Up there, the population is so dense they end up living in places that shouldn't be occupied, hence the need for evacuations when there is a problem and big numbers involved.

"The volcano is capable of significant local eruptions, should the unrest develop into an eruption."

Popocatepetl, meanwhile, was frequently active and the level of activity was now increasing.

"One could say that's how this volcano works. Sometimes it's quiet, other times it's very active."

In Vanuatu, there were many active volcanoes, some which were constantly erupting.

The 361m-high Mt Yasur, on Tanna Island, had been erupting since it was first spotted by Captain James Cook 250 years ago.

Mt Ambae was last active between 2005 and 2006, when a volcanic cone grew in one of its summit crater lakes, forcing some residents to be relocated.