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'Extremes more common' with climate change

By: Garry Shilson-Josling
09 January, 2013

Small rises in the average temperature can mean a big rise in the number of scorching hot days.

At first glance, that might not seem logical.
Shouldn't small changes in the temperature mean small changes in the frequency of scorchers?
Unfortunately, the answer is no.
Take Tasmania, for example.
Temperatures have been rising in the island state.
In the first 20 years of data available from the Bureau of Meteorology, 1911 to 1930, the temperature across Tasmania averaged 10.3 degrees Celsius, or 50.7 degrees Fahrenheit for those still preferring the old scale.
But thanks to the accumulation of greenhouse gases, trapping more of the sun's energy in the Earth's atmosphere and oceans, the average crept higher.
Over the most recent 20 years, 1993 to 2012, the average was 10.93C or 51.7F.
"Warming in the mean (average) over Tasmania is consistent with global warming," said climate scientist Professor Andy Pitman, director of the ARC Centre of Excellence for Climate System Science at the University of New South Wales.
But the increasing frequency of extremes well away from the average, rather than the higher average itself, can be what does the damage.
Recent Tasmania-wide averages above 30C (86F) set the scene for the latest wave of bushfires.
Such temperature extremes are still seen as highly unusual.
One senior firefighter described such conditions as "once in a generation".
And once upon a time, that description might have been apt.
But times have changed.
In the first 20 years of data for Tasmania, 1911 to 1930, there was only one day when the average temperature across Tasmania maxed out above 30C.
That was on January 11, 1928.
In latest 20 years up to and including 2012, the state-wide average beat the 30C mark no fewer than 19 times.
In other words, state-wide highs over 30C used to be a once-in-a-generation — or at least once every 20 years — rarity.
But they now happen almost once a year.
There are plenty of complicating factors — the effects of climate change are not spread evenly through time or among regions — but the same broad trends can be seen clearly right across the country.
From 21.44C in the first 20 years covered by the data to 22.15 in the latest 20 years, the average temperature across Australia has risen only about three quarters of a degree.
Not much, you might think.
But between the earlier period and the later one, the number of days the average has beaten 37.8C (a sweltering 100 in the old Fahrenheit scale), has risen nearly five-fold.
It happened nine times in the first 20 years covered, but 46 times in the most recent two decades.
What used to happen less than once every two years now happens on average every five months.
And there's nothing mysterious about it.
As with many natural phenomena, most daily temperatures are clustered around the average, with temperatures much higher or lower than the average very uncommon.
Graph that, and it looks like an upside-down bell — the famous "bell curve" favoured by statisticians.
But when the average moves up, those hot extremes become much less uncommon.
Something which has become a once-in-a-decade event rather than a once-in-century event might still be rare.
But it is now 10 times more likely than it was.
Prof Pitman said this effect is a "classic property" of any variable, like temperature, which can be described using the bell curve.
"It is why climate scientists have been saying for over a decade that a small increase in mean temperature is not remotely something to be complacent about," he said.
"Yes, this behaviour is precisely what we expect to see due to rising mean (average) temperatures."
Source: AAP

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