If global average surface temperatures hit 1.5°C above pre-industrial conditions then a repeat of the coral bleaching that severely damaged the Great Barrier Reef earlier this year becomes more than twice as likely.
If the world pushes temperatures up to 2°C more than the pre-industrial world, then it almost triples the odds of another mass bleaching event.
These findings from University of Melbourne scientists at the ARC Centre of Excellence for Climate System Science, reported today in Nature Climate Change, are the result of research using NCI facilities to look at how Australian extremes in heat, drought, precipitation and ocean warming will change under a world 1.5°C and 2°C warmer than pre-industrial conditions.
"Most of the action occurred in changes to extreme heat, with a 13% increase in events like the Angry Summer of 2012/13 at 1.5°C above pre-industrial levels and 33% increase if it hits 2°C," said lead author Dr Andrew King.
"But the heat in the Coral Sea was unprecedented. There was no event in our modelling of pre-industrial temperatures where the Coral Sea was as warm as we saw in 2016 and as the globe warms these events will grow in frequency."
The researchers also looked at other extreme events like the southeast Australian drought of 2006 and the rain events that led to widespread flooding in northeast Australia in 2010 to see how they might alter as global temperatures increased.
Rainfall did not show any clear change because the impacts of natural variability like El Niño Southern Oscillation, monsoons, Indian Ocean temperatures and the Madden-Julian Oscillation influence it more than global temperature rises.
There were some increases in drought intensity as a result of increased heat but the lack of clarity around rainfall meant only a slight increase in the frequency of droughts was detected.
The results were the product of modelling thousands of years of climate activity under four different scenarios: pre-industrial conditions, current conditions, the world at 1.5°C and at 2°C.
Using the high-performance computing resources at NCI, the researchers then looked at four key extreme Australian events – the Angry Summer 2012/13; the Coral Sea marine heatwave of 2016; the severe rain event in NE Australia in 2010; and the 2006 drought in southeast Australia – to model how often these events would occur under each scenario.
The changing likelihood of Australian extreme events.
"It quickly became clear that keeping global temperatures under 1.5°C had a clear benefit for Australia in terms of reducing extreme events and the costs that come with them," Dr King said.
"It gave us a grim warning about what would happen to the Great Barrier Reef if we fail to act on the Paris agreement. Sea temperatures of the scale and frequency we have seen do not bode well for the future of one of our greatest natural wonders."