Australia's summer of 2012 was known as "the angry summer," with record heat, extreme bush fires and flooding striking the nation in quick succession.

Those heat extremes, the hottest in the country's observational record, were likely caused by man-made climate change, according to a new study accepted for publication in the journal Geophysical Research Letters.

The study shows, with 90 percent confidence, that such extreme summers in Australia are five times more likely due to an increase in greenhouse gases, said paper co-author David Karoly, an atmospheric scientist at the University of Melbourne and the Australian Research Council Center of Excellence for Climate System Science.

Since climate change is already leading to higher average temperatures overall, the finding that extremes are also more likely was not surprising, said Sophie Lewis, a climate scientist at the University of Melbourne and the climate system science center and the lead author on the paper.

"Given how our study fits into this broader context, the substantial human influence on our hot, angry summer is not an unexpected result," she said.

What was surprising, said Karoly, was the magnitude of the change in the probability of extremes, given that the Earth has not yet warmed a full degree Celsius.

"To us, that was already a very large change," he said.

'Projections are becoming observations'
To get these results, the scientists used output from nine climate models. Each model had run simulations that included anthropogenic climate influences like human-released greenhouse gases and aerosols as well as simulations run without those human influences.

They compared the probability of the extreme summer of 2013 in the natural-only experiments with the ones that had human influences in them.

Normally, studies trying to attribute climate change to extreme events are published much later after the event, noted Karoly.

This time, the scientists were able to quickly analyze the summer of 2013 because the model data sets were publicly available through a program called the Coupled Model Intercomparison Project Phase 5.

Being able to analyze a recent extreme event so clearly might help capture the attention of the public and decisionmakers.

"The public still remembers the hot summer," said Karoly.

Will Steffen, a climate scientist and executive director of the Australian National University Climate Change Institute, who was familiar with the study, said the research methodology used was very well-established.

Steffen was not surprised by the finding that climate change led to an increase in the likelihood of such an extreme summer.

The study "shows without a doubt that the projections of climate scientists a few decades ago that the risk of extreme heat would increase are now becoming the reality. The projections are becoming observations," he said.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500