Australia will endure more heatwaves, droughts and coral bleaching at 1.5 degrees of warming but the extremes will be considerably less than if global temperatures increase by 2 degrees, new research shows.
In some of the first research on the impacts for Australia of the 1.5 degree to 2 degree range agreed at the Paris climate summit in 2015, Melbourne University scientists have found the chances of a repeat of events such as the "angry summer" of 2012-13 are significantly reduced at the lower end of the warming scale.
That summer, which remains the country's hottest, was already about 10 times more likely than without the 1 degree of warming already experienced since pre-industrial times, said Dr Andrew King, a Melbourne University climate scientist and lead author of the paper published in Nature Climate Change.
At warming of 1.5 degrees, the odds of such a summer with its heat extremes and bush fire-conducive weather increases from about 44 per cent now to 57 per cent. The chance rises to 77 per cent in a 2-degree warmer world, the researchers found.
Australian droughts, too, are likely to be made worse with warming to 1.5 degrees - but less so than compared with heating beyond that level. For instance, the extremely dry year of 2006 would be about a 50-50 proposition in any year at 1.5 degrees, but almost a three-in-four years chance at 2 degrees.
Almost all of the increase in drought risks comes from warming temperatures adding to evaporation, rather than changes in rainfall deficits, the models show.
(See chart below of the likelihood of extremes in any year as temperatures rise.)
However Will Steffen, an emeritus professor at the Australian National University and a member of the Climate Council, said rainfall changes are harder to predict than temperature rises and models may be underestimating the shift.
"The drop in rainfall that we've seen in south-eastern Australia in the last 20 to 30 years - across Victoria, southern NSW and southern South Australia - is about where the models were predicting for 2030 or 2035," Professor Steffen said.
"We may actually experience bigger swings in rainfall than the models are capable of simulating."
The research, though, is valuable in demonstrating risks are unlikely to increase steadily but jump sharply in ways that affect wildlife, humans and agriculture alike, he said.
"A lot of ecosystems do not respond linearly to rainfall or water availability changes," Professor Steffen said. "There are thresholds and tipping points."
(See Bureau of Meteorology chart showing how more than half of Australia had summer heat in the top 10 per cent of years in 2012-13.)
Coral Sea dangers
Among the most extreme impacts of a warming world are already being witnessed on the Great Barrier Reef, where two hot summers have resulted in unprecedented coral bleaching with as much as two-thirds of reefs affected.
The research found a repeat of the marine heatwave in the Coral Sea in 2016 - which alone killed off more than one-fifth of the Great Barrier Reef corals - would rise from about a one-in-three chance at current conditions to a 64 per cent chance at 1.5 degrees of warming.
"If we follow high-emission scenarios, events like last year would be really cold events by the mid to late-21st century," Dr King, who is also a researcher at the ARC Centre of Excellence for Climate System Science, said. "It's really quite alarming."
Both Dr King and Professor Steffen said the threat facing the reef should prompt policymakers to act to ensure temperature rises are kept to 1.5 degrees. Drastic and urgent cuts to greenhouse gas emissions - beyond what was pledged at Paris - would be needed to reach that goal.
"When you look at Australia versus other OECD nations, our pledges are more like a 3.5 degree to 4-degree world, so we are woefully, woefully inadequate in terms of our action given these sorts of projections," Professor Steffen said.