How climate change is driving extreme heatwaves
As average temperatures creep up, climate scientists spy trouble at the far end of the bell curve. Early results from the nascent science of extreme heatwaves are startling, reports Michael Lucy.
BUILT ON THE RICH BLACK SOIL of the northern NSW plains, halfway between the dry red dunes of the Strzelecki Desert and the lush subtropical hills of the east coast, the farming town of Moree is used to extreme weather. Summers are hot, and floods – when the Mehi River can rise by 10 metres or more – are not uncommon.
“You know, ‘droughts and flooding rains’,” says Katrina Humphries, mayor and fish-and-chip-shop proprietor to the 10,000-strong community, quoting a famous century-old description of the Australian climate by the poet Dorothea Mackellar.
Even so, the heatwave that overtook the area at the beginning of 2017 was extraordinary. For an astonishing 54-day stretch from December to February, the mercury topped 35ºC every day, with hardly a drop of rain. That included one run of five days over 40ºC, two three-day runs over 40ºC, and the equal hottest day ever recorded, a blistering 47.3ºC. The previous record for such a heatwave in the area, set in 1912, was a mere 21 days.
“When it’s been dry for a while and windy, the sky gets pink with dust,” notes Humphries. “They’re the days I hate.”
There will be plenty of those days to come, according to climate scientists. More pressingly, there will be plenty of long stretches of hot days without respite. The record-breaking summer heat across south-east Australia may be unexceptional within decades. It is tempting to call it “the new normal” but experts are wary of the phrase because it gives a false impression of stability. Though the temperatures of the 2017 summer could have occurred in a pre-industrial climate, they were made 50 times more likely by the global warming that has already occurred.
Discussions of climate change often use averages as a measure, and they can sound relatively innocuous: global temperatures are already at an average of 1ºC above pre-industrial levels, and the 2015 Paris climate accord aims to keep that average increase below 2ºC. A degree here or there doesn’t seem like much.
According to Andy Pitman, who heads up the Australian Research Council’s new Centre of Excellence for Climate Extremes based at the University of NSW, “it has become clear that averages are not what really matters”.
“How much the world warms [on average] due to CO2 is not where the rubber hits the road,” Pitman says.
“The rubber hits the road with events that break systems.”
The unrelenting heat of the 2017 summer was made 50 times more likely by the single degree of global warming that has already occurred.
Economists agree that the greatest costs of climate change are likely to result from events that push beyond normal limits: bushfires and blackouts and floods, bleached coral and ecosystem collapses, and deaths among the very young, the elderly and the vulnerable. To take a single killer example, the hot European summer of 2003 is believed to have caused more than 70,000 deaths across the continent.
As climate scientists have turned their eyes to the question of extreme events, what they have found has been startling.
A study by Sophie Lewis of the Australian National University in Canberra and Andrew King of the University of Melbourne has raised the prospect of temperatures exceeding 50ºC in Melbourne and Sydney if (or, according to current projections, when) the average global temperature rises 2ºC.
“That’s the headline-grabbing aspect,” King says. Lewis emphasises that “individual hot days aren’t the primary concern – it’s when you have a string of them”.
Another troubling finding, published in a July 2016 paper in Climatic Change, is that a great increase in strings of summer hot days could make large swathes of the Middle East and north Africa uninhabitable within decades.
Increases in the frequency and length of those strings of hot days, like record-breaking peak temperatures, can be driven by deceptively small changes in average temperatures.
The reason is statistical. On any given day, at a particular place and time of year, a range of temperatures is possible. When the average temperature increases by a degree, the whole distribution moves up a bit, enabling new record highs and triggering a disproportionate increase in the number of hot days – “a large shift in the hot tail” of the curve, as Lewis calls it.
When it comes to the frequency of extremely hot days “it only needs a small push for the change to be noticeable”, says Sarah Perkins-Kirkpatrick of the University of NSW, because they are so rare to begin with.
The Moree heatwave, for instance, is estimated to have been twice as likely due to the 1ºC of global warming that has already occurred, according to Kirkpatrick and King’s analysis. Nevertheless, their calculations suggest such an event should still only occur about once every 120 years.
Kirkpatrick has also looked at how global warming will affect heatwaves around the world. For each degree of warming, she calculates that everywhere on Earth can expect at least four more days of heatwave each year. Some places near the equator can expect as many as 34.
Worrying as such figures are, this kind of big-picture analysis may not capture the necessary level of local detail to make accurate projections, Pitman says, because the actual physical processes that drive heatwaves have only been studied “in a very piecemeal way”.
For instance, the stage was set for last summer’s prolonged heat by a large ridge of high-pressure air that settled in over south-east Australia. The unwelcome visitor stayed for much of January and February, blocking cold weather fronts from moving through. This meant heat built and built in the continent’s arid interior and was funnelled coastwards through smaller low-pressure troughs, causing unprecedented streaks of hot weather.
The high-pressure ridge itself was driven by larger forces: it had to do with “the temperature gradient between the equator and the south pole, coupled to complete changes in lots of really cool things”, Pitman says. Fully understanding those couplings between the big picture and the nitty-gritty of local extreme events is the mission of the new centre he is leading: “If you only study the averages you never get to look at that kind of phenomenon.”
For each degree of warming, everywhere on Earth can expect at least four more days of heatwave each year. Some places near the equator can expect as many as 34.
One aim is to look more closely at how frequent extreme events like last summer’s heatwave will become. If the once-in-120-years estimate is off because of changes to the underlying forces that create heatwaves, Pitman says “we have grossly underestimated how rapidly the climate will change and how big the change is going to be”.
Another challenge for researchers is to work on short-range seasonal forecasting, says Lewis: “It’s one thing to say we’ll have more hot spells by 2030 or 2050, but we really need to know about what will happen next summer.”
While the speed and intensity of warming will depend on efforts to cut carbon emissions – as King notes, even the commitments under the Paris accord will deliver something closer to a 3ºC increase despite the stated goal to limit warming to less than 2ºC – further warming is already unavoidable.
In Moree, Katrina Humphries seems untroubled by talk of climate change. “We’re geared for hot weather,” she says, recommending better water management and behavioural adaptations – a siesta through the midday heat, for instance – to cope.
“The climate’s been changing for 20,000 years,” she believes. “Mother Nature can be a bitch, she really can. We have to manage ourselves with what she does for us.”
How Humphries’ phlegmatic attitude will weather the coming decades remains to be seen. If nothing else, she can expect plenty of opportunities to quote Dorothea Mackellar: alongside heatwaves, both droughts and flooding rains will likely become more common, due to changes in precipitation patterns over the Pacific.