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'Everyone is under great threat': Call for cyclone-like categories for heatwaves

Peter Hannam

Heatwaves cause more deaths in Australia than all other natural hazards combined, so authorities should consider a cyclone-like rating system to help people prepare for hot spells, a risk researcher says.

While the Bureau of Meteorology has developed its own system to rank heatwaves, there's still scope to complement it with ratings similar to those used to warn about cyclone intensity, said Thomas Loridan, a lead scientist with research centre Risk Frontiers. .

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Weekend heatwave warning

The east coast is in for hot nights and scorching days over the coming weekend.

Dr Loridan said the public generally understood the relative threat posed by a category 1 tropical cyclone compared with a category 4 or 5 severe storm, as well as different levels of fire danger.

"When you hear warnings about tropical cyclones or bushfires, it's scary," he said. "But when you hear a warning for a heatwave over the weekend, you tend to take it a bit more lightly."

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"Only by having a clear definition of what a heatwave is, and the differing degrees of risk, can we start to educate people," he said, adding "everyone is under great threat in the high categories".

Summer's first burst

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Dr Loridan's call came as much of eastern Australia sizzled in the region's first major heatwave of summer.

The delayed arrival of sea breezes meant Sydney's temperature peaked at 35.7 degrees, or almost four degrees higher than the bureau forecast. It was the city's hottest day since mid-January.

Parts of northern NSW climbed into the 40s, while inland Queensland recorded 46.7 degrees in Birdsville. According to the bureau, the heatwave has several days to run, and will stretch further south towards Victoria on Saturday. (See bureau chart below.)

Dr Loridan said historical records reveal a clear link between intensity of hot spells and rising fatalities.

By forecasting the expected peak intensity and accumulated heat load from a heatwave, it should be possible to anticipate the impact on people, he said.

Five severity ratings for heatwaves could inform the public of risks and help health authorities and other services prepare. Such events were also typically easier to forecast than bushfires.

Based on historical analyses, the threat risks from heat rise rapidly from category 3 onwards, tripling for category 4 and again for category 5, Dr Loridan said. (See chart below of predicted fatalities per 100,000.)

Early stages

A spokesman for NSW Health said it had considered a range of heatwave measures to gauge health impacts and "trigger appropriate responses".

The department is currently using the excess heat factor system developed by the bureau for its heatwave service.

John Nairn, developer of the bureau's heatwave service and regional director of its South Australia office, said his team and Dr Loridan's team were "in fierce agreement that the increasing severity of heatwaves results in increasing impacts". 

The Risk Frontiers approach was in its early stages and may be better suited to targeted groups, such as health professionals, Dr Nairn said.

The bureau's three levels of heatwaves – low intensity, severe or extreme – were designed for easy communication to a broad audience. 

"The simpler the message that they get...is easier to respond to, particularly when you want a whole-of-government response," he said. "The last thing you want is for people to be confused."

'Suck it up'

However Sarah Perkins-Kirkpatrick, a heatwave researcher at the ARC Centre of Excellence for Climate System Science, based at the University of NSW, said the two systems could complement each other.

The public now accepted multiple ratings of bushfires up to "catastrophic", and even for tornadoes.

Australians also tended to be complacent to the risks even though heatwaves killed far more people than cyclones, bushfires or other natural hazards, she said. (See chart below from a 2014 study of heatwave deaths in Australia. compared with other natural threats.)

"We have this mentality – 'it's hot, suck it up, get used to it ' – when it's getting hotter and our thresholds of what we can cope with haven't actually changed, but the climate has," Dr Perkins-Kirkpatrick said.

Having refined heatwave warnings will be increasingly important. "As populations increase, and climate change continues to take hold, [impacts] will get worse, undoubtedly," she said.

Bureau's view

The bureau's system combines long-term and short-term temperature anomalies at each locality.

The long-term test checks that the three days needed for a heatwave are in the top 5 per cent of the climate record for the location. The short-term gauge checks to see if those days are hotter than the previous thirty days, which would amplify the impact of the heatwave conditions, Dr Nairn said.

For now, the method does not include humidity, but since the tests are run against local data if the site is normally humid, that condition woud be picked up in the assessment.

The bureau's low-intensity heatwaves would account for 85 per cent of the hot spells. "Severe heatwaves affect the infirm, very young and aged, while extreme heatwaves impact everyone and our supporting infrastructure unless we take protective action," he said.

"The latter, extreme heatwaves were occurring roughly every 40 to 50 years, but have been increasing in frequency more recently," Dr Nairn said.

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