Why Ōhau blaze is a taste of climate change's fire risk

Dunedin man Andy Winneke caught sight of the Ohau blaze from where he was camping about 4.30am on Sunday morning. Photo / Andy Winneke

The stage for Lake Ōhau Village's devastating fires was set by a combination of severe La Nina-influenced winds, unusually high temperatures, and pastures left dry by the warmest winter on record, an expert says.

It falls against the backdrop of a warming climate predicted to raise the risk of severe fire weather by three-fold in some parts of New Zealand.

Up to 50 structures have been destroyed in the blaze at the idyllic lakeside Waitaki District town, which had been fanned by strong, northwest winds described as "absolutely horrendous" by locals.

Thought to have been sparked by an electrical arc from overhead powerlines, the fire was likely fuelled by dried pastures before it spread to nearby forestry, Scion fire scientist Grant Pearce said.

It came at a time a freshly-developing, moderate-strength La Nina climate system had been bringing stronger westerly to north-westerly winds, instead of south-westerly winds usually seen at this time of year.

"What we'll expect to see is a change over the summer, to more north-easterlies, which bring more traditional La Nina conditions - and that means we often have a wetter east coast than normal."

Although the Mackenzie Basin was typically sheltered by this wind and weather from both directions, that hadn't proven so over recent days.

And inland basins tended to be particularly affected by the sort of climate cycles that had delivered New Zealand its record warm winter.

"We've had a dry period over late winter, and into spring, where we haven't had much rain, but cold temperatures and frost curing. That's left a lot of dead grass on the ground at present, and it hasn't started greening up.

"This dead grass ignites easily, and then fires spread very rapidly. With these dry conditions, once the fire gets established, it gets into heavier fuels, like the plantation blocks, or wilding pines, as we saw last month."

"So, what we have here is a combination of seasonal conditions, one of the warmest winter periods on record, and then dry conditions in that part of the country."

Was this what we could expect under climate change?

Pearce saw two well-linked factors.

One was a general warming and drying pattern that's expected to increase some parts of New Zealand - including those "rain shadow" areas immediately to the east of mountain ranges, such as the Mackenzie Basin.

Projections of the annual frequency of Very High and Extreme forest fire danger over the fire season months. Source / Scion

"The other key feature around climate change is greater extremes - that's greater variation in heavy rainfall, right through to these really dry events as well.

"If you stack the extreme events on top of the general increasing trend in a normal natural cycle, you get these really extreme days.

"So we expect more of these extreme days - and for conditions on these extreme days to be more severe."

Climate scientist Professor Jim Salinger saw a clear link to climate change - that local temperatures were more than 10C above normal over the weekend was "very unusual" for early October - but also the handprint of a fast-arriving La Nina.

Federated Farmers has pointed the finger at the Department of Conservation for "mis-managing" fire risk and locking up high country land without "proper care".

But Salinger said that while wilding pines left to grow posed a fire risk, so did grass converted from tussock for farming.

"Tussock does not ignite spontaneously, but dried out pasture grass will."

Rural fires already wreak enough damage to cost the economy around $67 million each year.

But modelling has indicated almost all areas of the country would see an increase in fire danger, with the average fire season length increasing by about 70 per cent up to 2040 - and by about 80 per cent up to 2090.

Big droughts, too, would become an increasingly common occurrence, especially in northern and eastern areas of the North Island, and along the eastern side of the Southern Alps.

By the end of the century, there could be more than 60 extra days over 25C in northern areas each year, and, even as soon as 2040, the average time in drought could double or triple.

Other research has suggested this scenario is somewhat here already: one study indicated that the $1.3 billion drought which hit the country in 2012-13 was 20 per cent more likely to have occurred than just over a century ago.

Although those typically dry regions of Gisborne, Marlborough and Canterbury would remain the most at risk of fire, the relative increase in risk was highest in Wellington and coastal Otago, where it could double and triple to 30 days and 20 days per season, respectively.

And several degrees of temperature rise, along with more and stronger westerly winds, doesn't just mean a greater number of fires, but a different type of fire.

What firefighters call "extreme fire" has dangerous features like spotting - where embers and other particles are hurled ahead of the fire front; fire tornadoes and whirls; and "blow-up" conditions, where the inferno suddenly escalates in size and intensity.

Up until now, there haven't been many cases of these volatile characteristics in New Zealand fires - one was a blaze that scorched 90ha of land near Hanmer Springs in 2016, another was last year's Pigeon Valley blaze.

But research has suggested "extreme fire" risk could double this century.