Read the future in the flames
The risk of forest fires is expected to spread much more widely as the climate warms. Cities such as Dunedin and Wellington are among those facing the biggest shock.
When the all-consuming fire south of Nelson first flickered to life, and before it had become the largest forest fire in New Zealand for six decades, some were already reflecting on a similar blaze two years earlier.
The 2017 Port Hills fire in Christchurch and the Nelson fire occurred at the same time of the year, during each area’s respective fire danger season, when the fire risk was considered ‘‘extreme’’ under multiple measures.
Aside from those clear similarities, several differences can give us a glimpse of what forest fires are expected to look like in a warmer climate.
Two differences in particular stand out. Christchurch has the longest fire danger season in the country, spending about 40 days each year under a ‘‘very high’’ or ‘‘extreme’’ fire risk; the Nelson area’s fire danger season is just nine days, similar to not particularly fire-prone cities such as Auckland or Tauranga.
It means the length of time in which conditions are primed for a large fire are, on average, four times longer in Christchurch than in Nelson.
The second difference concerns what was being burned. The Port Hills fire sustained itself by gobbling up scrubby plants such as gorse, which burn easily, accelerating the fire’s spread but limiting its power; the Nelson fire had thick plantation forests and dense organic matter to chew through, which don’t burn as easily, but produce big, powerful flames.
Based on these factors it would seem the Port Hills fire was more likely to happen when it did. But it wasn’t.
One way to measure fire risk is through what is called the Build-Up Index (BUI), which calculates how much fuel is available for a fire to burn in a given area, based on weather conditions.
Before the Port Hills fires ignited, the BUI in Christchurch was 100 – above average and into the ‘‘extreme’’ category, but not unusually high for Christchurch.
The average BUI in Nelson at this time of year is around 40.
But due to a severe, prolonged dry period, the BUI soared above 100 in Nelson shortly before the fire started, higher than at any point in the area’s historical record and higher than Christchurch before the Port Hills fire.
While the longer-term risk for fires is higher in Christchurch, due to harsh and specific local conditions, the risk this season was higher in the Nelson area. It shows how year-to-year weather variability is a major influence on the likelihood of large fires.
So what happens when you put that variability on top of a warmer, drier climate?
‘‘The short answer is that most parts of the country are likely to see increases [in fire risk] in the future,’’ says Grant Pearce, a fire scientist at Scion Crown research institute.
‘‘It means more of the hot, dry, windy days, particularly in the eastern areas of the country, but also in those lesser fire-prone areas.’’
One way to think about the fire risk in the future is that places we don’t think of as fireprone now are expected to become so in the coming decades.
Just how much so was calculated in 2010, and reanalysed last year, by modelling the length of the fire danger season around the country based on a hypothetical, middle-of-theroad warming scenario.
For the country as a whole, the fire danger season was projected to expand 71 per cent by the 2040s, and 83 per cent by the 2090s. All parts of the east coast showed an increasing fire danger season, while a couple of areas – such as Hokitika and Invercargill – showed little to no meaningful change.
Because the results were averages, they hid some of the most significant projected changes.
Wellington and coastal Otago were expected to have the biggest relative change. Wellington’s current fire danger season of 17 days would rise to more than 30 days by 2040 – similar to the fire danger in Gisborne now – and coastal Otago’s six-day season would more than treble to about 20 days over the same period.
The areas most at risk now, Christchurch and Gisborne, would also have longer fire danger seasons, but only modestly so, the research showed.
It amounts to both an increase and a levelling-out of the total fire danger risk. A place like Dunedin, which typically doesn’t deal with wildfires,
would in the coming decades be much more likely to face them than Nelson.
‘‘These places don’t automatically spring up as being the most fire-prone parts of the country – they might currently get six or seven, maybe eight or nine days a year – but that could easily double to 15 to 20 days or more,’’ Pearce says. ‘‘They’re quite dramatic increases.’’
While it’s likely that climate change will make wildfires more common, it’s unclear whether they have already become more frequent.
Research in 2008 looked at New Zealand’s wildfire frequency between 1991 and 2007 and found the number of wildfires had increased four-fold over that period, which it largely attributed to better reporting from the public.
It noted, however, the year-to-year variability was ‘‘most likely significantly influenced by weather and fire season severity’’. Only about 6 per cent of the fires in that period, by area, were forest fires.
Since then, there has been a notable increase in large and noteworthy forest fires, Pearce says – in 2015, there were three large forest fires in Marlborough, followed by the Port Hills, and now Nelson.
‘‘We’re increasingly seeing these fires close to urban areas, to people and property,’’ Pearce says.
The likelihood of fires becoming more common due to climate change is simple physics. The conditions that make such fires more likely are heat, a lack of soil moisture, and higher wind, all of which are projected to increase in eastern parts of New Zealand, multiple analyses show.
While it’s impossible to measure the exact influence climate change had on the Nelson fire without an attribution study, it is an example of these forces conspiring to create an environment that made such a fire more likely to emerge than in a typical summer – an environment that is expected to become normal, rather than unusual.
Although its average fire danger season is nine days, the Nelson area’s dry period has made that season twice as long this year, creating vastly more opportunity for a fire to spread. And because of the dense, difficult-to-burn fuel that is plantation forestry, the fire would not have spread as it did under most conditions, Pearce says.
‘‘It’s those larger, heavier fuels that become available to burn. Under low to moderate conditions, they’re not available to burn – but under these drier conditions, they are, and all of it becomes available to fuel the fire, so it leads to more intense fires, with bigger flames.’’
That powerful fuel is largely why the Nelson fire became the largest forest fire in at least 60 years, when a fire in an old mill in North Canterbury’s Balmoral Forest spread widely, burning about 3300 hectares of forest.
While the risks of further fires in a warming climate are growing – both due to changes in the climate, as well as the Government’s ambition to plant one billion trees – those involved in their management are keenly aware of the growing risk, Pearce says.
‘‘The results of the research we’ve been doing we’ve shared with agencies like Fire and Emergency New Zealand, so they’re already starting to consider that in their long-term planning for climate change.’’