Cooling effect in 2km-thick ash plumes

Travel board at Auckland International Airport after cancelling some New Zealand Flights due to ash from the Chilean volcanic eruption. 12 June 2011 Photo / Greg Bowker

Plumes of volcanic ash blanketing New Zealand's airspace are more than 2km thick and could cool the country's climate.

Only the tip of the Far North was not covered yesterday by the ash cloud from the Puyehue-Cordon Caulle volcano.

The Civil Aviation Authority warned the particles would interfere with the airspace for at least the rest of the week, and ash would ebb and flow over the country for six days after the volcano ceases its largest explosions.

National Institute of Water and Atmospheric Research (Niwa) physicist Ben Liley found the ash cloud first appeared 8.5km above New Zealand and in a 2km-deep layer. That has since risen to 10km, thickened, and been joined by lower emissions of ash.

Climate scientist Jim Salinger said if sulphur dioxide in the plumes mixed with water it could cause some climate cooling in the next two months.

"It is like putting a curtain around the hemisphere, which reflects the sunlight, and cools the air [below the ash]."

Dr Salinger said the cooling effect would be barely noticeable - the tens of thousands of tonnes of sulphur units could lead to a cooling of 0.2C over two months.

The Pinatubo eruption in the Philippines in 1991 spewed one million sulphur units into the air, leading to a 2C to 3C drop over three years.

The Chilean ash cloud reached New Zealand because it was thrown 15km into the air, where it piggy-backed on fast-moving trade winds - which are the same height as jet aircraft cruising levels.

It is being propelled by two different jet streams over New Zealand, which are travelling between 30m and 60m a second.

University of Auckland volcanologist Jan Lindsay said the vast size of the plumes was the result of its highly volatile source.

"Eruptions of rhyolite magma [in Chile] are among the most explosive on earth, because the magma's high silica content makes it hard for gases to escape passively.

"Instead they tend to blast the magma apart into billions of tiny fragments, generating huge, high ash columns."

Those vast columns have forced contrasting responses from trans-tasman airlines.

Jetstar and Qantas said it was "not worth the risk" to fly, but Air New Zealand has not cancelled any flights.

Aviation commentator Peter Clark said the differing decisions depended on airlines' safety policies, but were probably also influenced by cost.

Flying an aircraft below about 6000m uses around 10 per cent more fuel because of greater air resistance.

Mr Clark said Air New Zealand would have easily recouped this cost through passengers from cancelled flights who changed carriers.

He also suggested that Jetstar and Qantas may have been reluctant to send too many planes to New Zealand during an Australian national holiday for fears they would become trapped in overseas airports by heavier ash.

Volcanic ash acts like coarse sandpaper on an aircraft's exterior, but can also clog engines.

But because it was launched into the stratosphere (roughly 6100m upwards), Air New Zealand faced little risk in flying its planes below it.

When the ash dropped into the troposphere, it was broken up by normal weather patterns.

Ash cloud details

Distance travelled: 9,000km
Speed over NZ: 30-60 metres/second
Height above NZ: 8.5km - 10km
Depth: at least 2km
Potential effect on climate: -0.2C
Concentration of SO2: 10,000s of tonnes
Size of volcanic particles: 50 microns (2 hair widths)