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Explained: The bigger picture behind the Port Hills fire

Author
Jamie Morton,
Publish Date
Thu, 15 Feb 2024, 1:08pm
Firefighters battling a fire on Christchurch's Port Hills. Photo / George Heard
Firefighters battling a fire on Christchurch's Port Hills. Photo / George Heard

Explained: The bigger picture behind the Port Hills fire

Author
Jamie Morton,
Publish Date
Thu, 15 Feb 2024, 1:08pm

Despite most of summer now behind us, New Zealand鈥檚 most dangerous fire conditions are just beginning. Why? Science reporter Jamie Morton explains.

Follow our听Port Hills fire live updates听here.

What鈥檚 the background picture to the Port Hills fire?

Whatever sparked the听Port Hills鈥 latest big blaze yesterday, background conditions couldn鈥檛 have been much more risky.

Fire danger in the area was assessed at 鈥淰ery High鈥 to 鈥淓xtreme鈥 at the time the fire broke out, with low humidity, temperatures reaching into the high 20s and flames fanned by warm, gusty, north-to-northwest winds.

Fire and Emergency New Zealand鈥檚 (Fenz)听听had already singled out parts of Canterbury as a hot spot to watch for fire danger 鈥 pointing particularly to those stronger, west to northwest winds.

鈥淭hese often align to produce days with high temperatures and low humidity that escalate fire dangers significantly, especially in eastern areas, contributing to easier ignition and rapid fire spread.鈥

Climate scientist Dr Nathanael Melia said the 鈥渄rought code鈥 in the area registered a value of near 400 yesterday 鈥 well above the threshold for extreme fire danger 鈥 while relative humidity had fallen to around 20 per cent.

A local state of emergency has been declared after a fire broke out in the Port Hills. Photo / RNZ, Charlotte CookA local state of emergency has been declared after a fire broke out in the Port Hills. Photo / RNZ, Charlotte Cook

Added to that was 鈥渆xtremely dry鈥 and gusty winds which drove extreme fire spread rates, he said.

鈥淭hese conditions resulted in immense difficulty in containing the fire as it spread fast up the steep Port Hills and was often able to leapfrog ahead, starting new fires in a phenomenon known as 鈥榮potting鈥.鈥

This extreme fire behaviour made fighting at the main fire front far too dangerous, he said, as ground crews could become rapidly surrounded.

At this point, it wasn鈥檛 clear what, or who, started it.

鈥淩ecent wildfire ignition sources include a camping stove, malfunctioning powerlines, and a metal landscaping blade sparking off rock,鈥 Melia said.

鈥淚 renew my call to ban certain activities that may lead to ignition during extreme wildfire conditions, and I encourage relevant parties to investigate plans and legislation along these lines.鈥

What is New Zealand鈥檚 fire danger for the rest of the season?

Niwa鈥檚听听predicted a shift to more westerly winds between February and April, meaning regions yet to experience drier conditions 鈥渟hould remain prepared for that possibility as this change occurs鈥.

Fenz鈥檚 outlook said the likelihood of El Nino conditions continuing through summer and into autumn could also mean an extended fire season.

鈥淚n some areas, the worst fire danger conditions may not occur until late February, March or even April.鈥

Many parts of New Zealand are already in prohibited or restricted fire seasons.

Fireworks are currently banned in Ahipara township and surrounds, Karikari Peninsula, Ripiro Beach, Mackenzie Basin, Central Otago, Naseby, Upper Waitaki, Lake 艑hau, Otago Lakes and Glendhu Bluffs, Canterbury, Kaik艒ura, Wellington District, K膩piti and Wairarapa.

This week, Auckland, Northland and Bay of Plenty also moved to restricted seasons.

Bay of Plenty district manager Jeff Maunder said there鈥檇 been very little rainfall across most of the Bay of Plenty in the past two weeks which has dried out grass and vegetation significantly and increased the fire risk.

鈥淭here is also little rain forecast for the next few weeks, so vegetation and grass will continue to dry out, further increasing the fire danger,鈥 he said.

鈥淭his means vegetation fires will start more easily, spread more rapidly and be more difficult to control.鈥

鈥淲e have the largest block of commercial forestry in New Zealand (over half a million hectares) in the Bay of Plenty plus a million hectares of Department of Conservation land.

鈥淢any of these forestry areas are surrounded by ungrazed grass and scrublands which can transfer fires rapidly to forested areas.鈥

In Northland, region manager Ron Devlin said high westerly winds were already causing 鈥渟pike days鈥, where the fire danger hit the highest level, 鈥淓xtreme鈥.

A restricted fire season is in place for Auckland and Northland from February 14, 2024. Photo / RNZA restricted fire season is in place for Auckland and Northland from February 14, 2024. Photo / RNZ

鈥淭here is a major risk in our Te Tai Tokerau Northland district of a fire occurring that will be very difficult to control,鈥 he said.

鈥淔or the greater Auckland area, weather forecasts show there is very little to no rain in the next seven days with a long-range forecast of warmer temperatures.

鈥淚t is also forecast that from February to March, the fire season will have 40 per cent less rain than normal, with a greater wind speed, which means the fire danger will be slightly above normal.鈥

The most recent large wildfire in Northland was in March 2023 near Cape Reinga, and took eight days to extinguish, at a cost of more than $1.5 million.

A restricted season means no open-air fires can be legally lit unless a fire permit has been applied for and approved by Fenz.

Fenz national wildfire manager Tim Mitchell said fire restrictions didn鈥檛 just apply to rubbish fires, bonfires and controlled burnoffs.

鈥淚t is important to note a fire covers a broad range of activities involving a flame or heat such as using gas cookers, barbecues and fish smokers.鈥

Are fires like this something we鈥檒l see more of in the future?

Scion wildfire scientist Shana Gross said fire conditions were gradually worsening, meaning more and more extreme fire days each year.

鈥淭he changing climate is increasing the frequency and severity of wildfires, and escalating the risks, not just in rural areas like we saw last week in Lee Valley, but also where people feel more secure like the edges of cities,鈥 she said.

鈥淭he Port Hills area is another example of this.

鈥淥verseas this trend is even more advanced with fires burning well into established cities across the globe, including fires that burnt more than 15,000 homes in the cities of Valparaiso and Vina Del Mar in Chile.鈥

Rural fires already wreak enough damage to cost the economy tens of millions of dollars each year. 2019鈥瞫 Pigeon Valley blaze and the following year鈥檚 Lake 艑hau disaster were two of our most dramatic examples.

Projections indicated听notable fire seasons on average increase in length by 30 per cent by the end of the century, and the direct economic cost of wildfires could balloon from around $142m a year today to $547m in 2050.

Canterbury, Hawke鈥檚 Bay, Gisborne - but also Waikato, the Central North Island and Northland - each year recorded the highest number of rural fires, while Otago and Nelson and Marlborough had fires that burned across larger areas.

Those trends came down to basic geography: eastern areas were drier as they sat in the rain shadow of mountain ranges, with prevailing westerly wind flows.

They also tended to receive strong fohn winds that delivered strong gusts on top of high temperatures and low humidity.

But the regions most at risk could widen under climate change.

This map shows how the number of "Very High" and "Extreme" fire danger days could grow across New Zealand this season. Source / ScionThis map shows how the number of "Very High" and "Extreme" fire danger days could grow across New Zealand this season. Source / Scion

In already fire-prone places like Canterbury, studies have indicated 鈥渟evere鈥 danger days could balloon from 30 to 40 days to 40 to 50 days in future.

But the more dramatic shifts would be seen in greener Manawat奴, Whanganui and coastal Otago, where severe fire weather could double or even treble, climbing from five to 10 days to 15 to 20 days.

By 2040, the annual average number of fire danger days would jump from around six to more than eight in Kaitaia, eight to 12 in Auckland, nearly 17 to nearly 33 in Wellington, 39 to 45 in Christchurch, six to 14 in Kaikoura, and five to 18 in Dunedin.

Researchers have also found the types of fire crews would confront could also change.

Cases of extreme fire behaviour have so far been seen in big events like Pigeon Valley, but also unexpectedly in smaller ones, including a blaze that devastated 90ha near Hanmer Springs in 2016.

Over time, dangerous characteristics like 鈥渇ire whirls鈥 -听something scientists happened to create听in a world-first experiment near Twizel last year 鈥 could occur in more parts of the country.

One听, co-led by Melia, also found the same extreme conditions that led to Australia鈥檚 鈥淏lack Summer鈥 could form in parts of New Zealand 鈥 and more frequently than first thought.

Jamie Morton is a specialist in science and environmental reporting. He joined the听Herald听in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.

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