Australia’s summer was one of the hottest since measurements began, with more than 60 weather stations recording their highest ever daily maximum temperatures during a heatwave in January.
But it wasn’t just the days that were sweltering – Australia experienced the fifth-warmest summertime night temperatures on record.
Looking at Bureau of Meteorology data, we found at least nine weather stations that recorded consecutive days (almost all on 28, 29 and 30 of January) when minimum temperatures remained in the 30s or high 20s through the night.
High night-time temperatures can prolong pressure on our cardiovascular system, kidneys and even mental health. The disturbed sleep can also hamper recovery from the hot days.
This is especially worrying for older Australians, and those with comorbidities (it’s estimated more than 4.5 million Australians live with cardiovascular disease), or who either can’t or can’t afford to ameliorate hot temperatures with fans or air conditioning.
“The vast majority of people that are turning up to hospitals, and that are vulnerable to heat events, are really turning up with other chronic conditions that get worse during these heat events,” says Dr Aaron Bach from Griffith University.
“So for instance, cardiovascular disease, renal conditions or kidney conditions, pulmonary conditions, even mental health illnesses. All these are exacerbated during the heat and the heat really finds the weak link within these comorbidities.”
Heatwaves are the climate hazard that cause the most deaths – more than half of in the most disadvantaged areas .
Extreme heat is also becoming more common – climate change is leading to an increase in the frequency and intensity of maximum and minimum temperatures. A recent CSIRO report found that “very high night-time temperatures” are now seen five times more often than in the period 1960-1988.
We can see this intensity reflected in the minimum temperatures on 27 January – with some stations recording temperatures 10C above the average minimum for the month:
Bach says both the intensity and duration of heatwaves are important factors in the medical risks they pose, because of the “risk that occurs to the body over prolonged heat exposure without the opportunity to recover”.
“That’s where overnight temperatures come into play,” Bach says. “Because without the opportunity to recover throughout the hot days, that duration becomes more problematic even at lower intensities.”
But just looking at raw temperature numbers alone won’t capture the full extent of the risk, as these are often measured outdoors, in the shade. Prof Ollie Jay from the University of Sydney says there are other factors that can compound the temperature, including what you are doing, what you are wearing, your environment and your physiology.
Jay and colleague Dr Federico Tartarini built an app called HeatWatch to capture some of these factors and turn them into a risk score between 1 and 6. Federico modelled heat stress for three different people – someone working outside in the sun, someone working outside in the shade, and someone aged 82 years old with cardiovascular disease:
Their model shows elevated risk for all three profiles from midnight on the 26th to midnight on the 28th for Adelaide – which experienced the hottest night since records began. The heat risk for the 82-year-old was “severe” or “extreme” for a considerable part of the days, and still “high” overnight.
“Even the same combination of all of these [risk] factors, the way it affects an elderly person versus a young person, it’s completely different,” Jay says.
As people get older they become less able to sweat, Jay says. “So for a given set of conditions, an older person is going to get hotter, their core temperature is going to get hotter relative to a young person.”
For a longer heatwave, Jay says there might be some cumulative effect of extended heat, for instance as people become more dehydrated. But another way to think about it, he says, is that longer heatwaves mean more opportunity to be exposed to the heat.
“If it’s one hot day, you can just stay inside the house, have the air con on. If you’ve got air con – we recognise that a lot of people don’t have air con. But if it’s happened for five days then, you know, eventually you’ve got to get out there.”
