Sydney and Melbourne can expect summer days when the mercury climbs to 50 degrees within a couple of decades if greenhouse gas emissions continue to rise, new research has found.
The study, led by Sophie Lewis at the Australian National University, analysed new models being prepared for the Intergovernmental Panel on Climate Change (IPCC) to examine the difference between a 1.5- and 2-degree warming limit compared with pre-industrial times.
At the upper end of the range – which would amount to a 1.1-degree rise from current global warming levels – NSW's record extremes would increase 3.8 degrees compared with existing records. Those in Victoria would rise by 2.3 degrees, the simulations showed.
For Sydney and Melbourne, populations could swelter in 50-degree weather even if the 2-degree global warming limit agreed in the 2015 Paris accord were achieved, according to the research co-authored by Andrew King from Melbourne University and published Wednesday in Geophysical Research Letters.
The current records for the two cities are 45.8 degrees for Sydney, set on January 18, 2013, and 46.4 degrees for Melbourne on Black Saturday, 7 February, 2009. Such hot days increase the risks of bushfires.
"If we warm average temperatures, we shift the whole distribution of temperatures, and we see a really large percentage increase in the extremes," Dr Lewis told Fairfax Media.
"What seems like a small increase in average temperatures, say 1 degree, can lead to a two- or three-fold acceleration in the severity of the extremes."
Temperature records are being broken frequently in Australia, with hot records 12 times more likely to be set than cold ones since 2000, Drs Lewis and King reported in a 2015 paper.
A heatwave last month smashed many records across eastern Australia with more than half the NSW stations with more than 20 years of data registering their hottest September day, the Bureau of Meteorology said.
Under a high carbon emissions scenario, 50-degree days could arrive "as early as the 2040s", Dr Lewis said, adding that even with a concerted reduction in pollution, those temperatures could be reached by about 2060.
Dr King said hospitals, electricity systems and infrastructure would all struggle during such heat spikes.
"For summer heat extremes in the future, there probably aren't any winners," Dr King said.
Reefs at risk
Along with other Australian regions which could see 50-degree days, the researchers also examined the temperature increases in the Coral Sea.
Even with less variability than land, oceans are also seeing significant warming. For temperature-sensitive coral reefs, the past few years of ocean heatwaves has triggered mass bleaching including on the Great Barrier Reef.
The researchers estimated autumn extremes of 0.3-0.8 degrees above the devastating 2016 record in the Coral Sea with the lower target of the Paris accord. At the 2 degrees warming goal, the Coral Sea will warm 0.6-1.2 degrees above the 2016 extreme.
Dr Lewis said the recent bleaching had "really smashed the reef", killing as much as half the Great Barrier Reef corals.
"If that's occurring now and we have additional warming on top of that - which will lead to an increase in seasonal temperatures - then that's a real worry for whether we'll have a reef at all, she said.
Dr King was also a co-author of a separate paper out recently in Environmental Research Letters that examined the impact on Europe if global temperatures reached that 2-degree upper limit agreed at Paris.
The paper found a heatwave of the scale of 2003 which killed 70,000 Europeans rapidly becomes more likely as the planet warms.
"What is startling is the speed of the change," Dr King said. "At 2 degrees, events like the 2003 heatwave could become so commonplace that they occur every second year."
While trends in regional seasonal rainfall levels are less clear, a warming world is likely to make single-day extreme rainfall events more likely. In Britain's case, a repeat of the July 2007 severe flooding would be 70 per cent more likely at 2-degrees warming compared with current conditions, he said.
Heatwaves by degrees
In a third recent paper by Australian-based researchers, published last week in Scientific Reports, Sarah Perkins-Kirkpatrick found the number of additional heatwave days will rise significantly depending on whether global warming is contained to 1.5 or 2 degrees, or beyond.
The research indicated heatwave days – based on a sequence of at least three consecutive days in the warmest 10 per cent for that date – will jump by an extra 14.8-28.2 per year for each degree of warming.
The heatwaves themselves will be 3.4-17.5 days longer per degree of global warming, with peak intensity of 1.2 to 1.9 degrees.
In the worst affected regions, such as parts of the tropics, summers will become a season-long heatwave, with large impacts on human health, productivity and ecosystems – even if warming can be kept to a global average of 2 degrees.
Dr Perkins-Kirkpatrick, who is a research fellow at the Climate Change Research Centre at the University of NSW, said some of the biggest increases in heatwaves will occur in poor nations who burn relatively little fossil fuels.
"It makes it really awful," she said. "They haven't done anything to deserve this, but these are the ones who are going to suffer the most."
While rating only "Buckley's chance" of keeping warming to that 1.5-degree lower Paris goal, Dr Perkins-Kirkpatrick said the science indicates emissions to be limited as much as possible.
"The impacts are going to be bad and we're not going to stop it," she said. "But it's never too late to try to change what we're doing."