Australasian Science: Australia's authority on science since 1938

Human Cause of Angry Summer

By Stephen Luntz

We can be more than 90% confident that human activity was involved in the “angry summer” of 2012–13, according to Dr Sophie Lewis of the University of Melbourne.

While it is not the first time climate scientists have seen the hand of global warming in extreme weather events, collaboration between CSIRO, the Bureau of Meteorology and National Computational Infrastructure Australia has greatly sped the analysis.

“Our research has shown that due to greenhouse gas emissions, these types of extreme summers will become even more frequent and more severe in the future,” Lewis says. She combined 100 years of observations with 90 climate model simulations to reach her conclusion, which has been published in Geophysical Research Letters.

While extreme events will happen in a stable climate, last summer was considered suspicious because it occurred at a time when El Niño Southern Oscillation conditions were roughly neutral. Previous hot summers have coincided with strong El Niño events.

Lewis is now studying the heavy rainfalls of 2010–12, including the Queensland floods, to see if they were purely the result of the strong La Niña prevailing at the time or if human influences contributed. She hopes in future it will be possible to reach conclusions shortly after extreme events occur.

Meanwhile, CSIRO Wealth from Oceans has added to its research on the southward shift of atmospheric circulation (AS, December 2012, p. 10) with research published in Scientific Reports. “What we are seeing,” says lead author Mr Guojian Wang, “is a ‘tug of war’ between stronger El Niños driving the winds north and the greenhouse gas-warming effect driving the winds south”.

Although many consequences are likely, the most noticeable so far has been an increase in sea level air pressure at mid-southern latitudes and decreasing pressure between 55° and 70°S. This pushes tropical and subtropical weather patterns south.