Major heatwaves are under way across much of the northern hemisphere.
Canada has faced its worst wildfire season in history. Tourists have flocked to California’s Death Valley where temperatures reached a near-world record of 53.3C on Sunday.
On the same day, China issued several temperature alerts, including its highest-ever temperature of 52.2C in the remote Xinjiang township of Sanbao.
In Europe, a second heat storm, which follows the Cerberus heatwave that began on 10 July, could see temperatures approach the European record of 48.8C, while wildfires burn in Greece and Spain.
The current extreme weather comes after previous heatwaves that swept Europe, northern Africa and south and south-east Asia in April and May.
“I’m not surprised but I’m very concerned, because these events are just persisting for so long,” says Associate Prof Sarah Perkins-Kirkpatrick, a climate scientist at the University of New South Wales Canberra who focuses on heatwaves.
“The Mediterranean area is warming twice as fast as the global average, so it’s no surprise that they’re seeing these supercharged heatwaves year on year.”
What causes heatwaves?
Regardless of location, all heatwaves are driven by high-pressure weather systems – also called anticyclones – which bring calm, stable air over a region, with little wind and few clouds.
“Two effects happen: it allows radiation from the sun to come in which is absorbed by the Earth and re-emitted – and that’s what heats the atmosphere,” Perkins-Kirkpatrick says. “Also, you get lots of sinking air come down the centre, which also warms the local area.”
Dr Andrew King, a senior lecturer in climate science at the University of Melbourne, says heatwaves are influenced by large-scale climate drivers like the El Niño Southern Oscillation (Enso) and also smaller weather systems and local factors such as dry soils or the urban heat island effect.
In the northern hemisphere, the polar jet stream – a belt of westerly winds – can sometimes result in “blocking highs”: systems that allow heat to build in specific areas. A “split jet” pattern with blocking highs is currently affecting parts of Europe and Asia, King says.
The climate crisis is intensifying heatwaves – making them more frequent and longer-lasting. “We can already see this in our observations and we know this will continue as we continue to warm the planet,” King says.
What can Australia expect when the southern hemisphere summer arrives?
“It’s hard to draw a straight line between what’s happening with [the northern hemisphere] and what’s coming for our summer,” Perkins-Kirkpatrick says. “While we were having really bad La Niña summers and lots of floods, Europe was still having extreme record weather the summer immediately prior.”
A study published in the journal Nature last week has linked the 2022 European summer – the hottest on record to date – to more than 61,000 deaths.
Enso, the biggest year-to-year climate fluctuation globally, also has a greater effect on Oceania than areas such as Europe.
The Bureau of Meteorology has not yet declared an El Niño this year, giving it a 70% chance of forming, but international organisations including the UN’s World Meteorological Organization have already declared the phenomenon is under way.
Globally, El Niño years are on average slightly warmer – adding up to 0.2C to surface temperatures. In Australia, they increase the chances of a drier and hotter summer.
“If we have an El Niño supercharged by climate change, then we could be in for a very bad summer,” Perkins-Kirkpatrick says.
Another climate driver, the Indian Ocean dipole (IOD), is predicted to enter a positive phase in the coming months. “It’s quite common to have El Niño and positive IOD events occur together,” King says. “They do reinforce each other a little bit because they’re both associated with drier, warmer conditions for much of south-east Australia.”
Australia has warmed by 1.4C over the past century, compared with a global average of 1.1C.
“What we’re seeing in the northern hemisphere is so incredibly extreme,” King says. “It should be a good incentive to reduce our greenhouse gas emissions.”
