Scientist challenges metrics of extreme heat
Climate change has already made summer longer in some parts of the country, where heat extremes have been playing out in ways more typical of the Mediterranean, reports Jamie Morton.
AJUSTPUBLISHED analysis that used a new measure to document previously unobserved changes warns we need to urgently rethink how we define ‘‘extreme heat’’ in our fastwarming world.
Along with finding that summer seasons have effectively been growing in certain places, New Zealand climate scientist Luke Harrington’s new paper has also turned up some heatwaves that earlier went unrecognised.
With its relatively cool, temperate climate, New Zealand has been one of many higher latitude countries thought to be less threatened by extreme heat driven by global warming, predicted to lift our temperature by several degrees this century.
Dr Harrington argued this had meant metrics to gauge impacts of extreme heat here had not kept pace with wider improvements in how heatwaves are defined.
Dr Harrington, a research fellow in climate extremes at Oxford University’s Environmental Change Institute, saw a particular flaw in using 25degC as a catchall threshold for extreme heat.
‘‘Yet that is precisely what the vast majority of reports commissioned for the Government and local councils in recent years have focused on.’’
A new measure
In New Zealand, 25degC is formally defined as a ‘‘hot day’’ and purportedly stems from the claim that beef and dairy cattle start experiencing heat stress at temperatures above it, rather than any wider impacts on human health.
Part of the reason we had not looked closer at this metric much, Dr Harrington argued, was that most of us thought we never really experienced temperatures hot enough to produce negative impacts — particularly compared with Australia next door.
Yet ‘‘extreme’’ was a relative concept — and there was ample evidence from overseas that people were adapted to the temperatures they were familiar with.
‘‘For example, people have been hospitalised or even died from extreme heat in cool places like Finland, Stockholm or even Sheffield,’’ he said.
‘‘This is exposure to temperatures which, while relatively hot, might be considered normal summer weather to someone living in Sydney.’’
In his paper, Dr Harrington began looking at past hot years using the 25degC metric and found three years in the last 40 — 199899, 200708 and 201213 — when that threshold was crossed in many regions, over more than three months of the year.
‘‘Of course, when some major farming regions like Waikato experience a full summer of ‘extreme heat’ three times within 15 years, questions are raised as to how well the threshold even represents heat stress in livestock,’’ he said.
‘‘Also, because of the nature of the threshold, it was hard to figure out which of these three years were more extreme.’’
But when he applied a different metric, gauging relative heat, he found that 199899 was by far the most anomalous year on record, followed by 201213 and then 200708.
‘‘My new metric works by digging into what temperatures we might expect in a specific part of New Zealand and for a specific time of year.
‘‘But rather than asking a binary yesno question of whether some chosen threshold is exceeded during a spell of hot weather, this new approach instead quantifies how severe these temperatures were, relative to what we are familiar with in this part of the country at this time of year.’’
By removing the binary nature of the index, he said, we could better compare the relative differences, such as those between three different heatwaves which might all exceed some absolute temperature threshold.
That was crucial, given our notion of ‘‘extreme’’ heat under climate change remained heavily contextspecific.
‘‘If temperatures stayed above 10degC all winter in Te Puke, for example, that would fit the bill as an extreme event for a kiwifruit grower there,’’ he said.
‘‘We also shouldn’t be surprised, then, if the temperatures needed for a dairy cow to suffer heat stress will differ from that of a human, or if someone living in Hawke’s Bay has a higher tolerance for experiencing a week of temperatures above 35degC than someone from Westport.’’
It might also be the case that there were no bad effects for anyone in New Zealand experiencing a few summer days in the mid30s.
‘‘But the fact is we don’t really know, because we haven’t properly examined the chain from unusual temperatures to impacts before,’’ he said.
‘‘My concern is, what happens in the summer of 2030, when we have a weeklong spell of temperatures approaching 40degC in Auckland or Hamilton?
‘‘Sure, this will fit the hot day criteria of ‘above 25degC’. But that doesn’t help us understand what the impacts would be, which is the entire point of these metrics being developed.
‘‘On the basis of overseas work, we could speculate that people will be hospitalised and some will even die because of the heat. But we can’t yet provide any more detail than that, which is troubling.’’
Hidden heatwaves, longer summers
Dr Harrington’s analysis threw up some heatwaves from the past that had never been defined as such.
‘‘I wanted to find events from our recent past which ticked the box using multiple heatwave criteria, and hence will be the best candidates to try and understand what the impacts of past heatwaves looked like.’’
The two most promising events were found in March 1999 and January 2018 — a month that went down as New Zealand’s hottest on record, amid our warmest ever summer.
Dr Harrington also compared how the number of extreme hot days in each month had changed between the 1980s and 2010s, a period during which there was about 0.5degC of global warming.
He found that February showed the most striking changes, with a more than fivefold increase in relative measures of extreme heat across the country.
More worryingly, November and March also show pronounced increases over Waikato and Otago, which meant an effective lengthening of the summer.
‘‘Heat extremes are worsening faster than mean temperatures in these regions. This is the first time such changes have been seen in New Zealand observations,’’ he said.
‘‘This behaviour is actually more typical of the changes being seen in the Mediterranean, and suggests drying soils might be making hot days even hotter.
‘‘But I stress though that these results are preliminary, and a lot more research is needed to get understand the details of what is going on.’’
His paper ultimately put forward several new ways to analyse extreme heat in cool climates such as New Zealand, in addition to the one approach used already.
‘‘But the key point is that no single metric will do a good job of capturing the full range of temperaturerelated impacts as our climate becomes increasingly unfamiliar over the next few decades.
‘‘If we walk into this future without having a range of approaches — each of them tailored to understand how extreme heat will impact our people, our livestock and our horticultural industries separately — then we are walking down the road blindfolded.’’ — The New Zealand Herald