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| Unprecedented bushfires burned out about 20,000 hectares of Tasmania's World Heritage Area last month. (Supplied: Dan Broun) |
Bushfires in Tasmania are happening more frequently now than in the past 1,000 years, and human impacts on climate are likely to blame, researchers say.
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"My conviction is that the current trend is evidence of anthropogenic forces," said Dr Michael-Shawn Fletcher of the University of Melbourne, who studies long-term interactions between humans, climate and vegetation.
A belt of westerly winds north of Antarctica brings rain to southern parts of Australia including Tasmania. The movement of these winds north and south is known as the Southern Annular Mode (SAM).
During a negative SAM, westerlies move north, but during a positive SAM the winds move south and drier conditions prevail on land.
Previous research has linked a shift towards a positive SAM and the southward shift of westerlies, with depletion of the ozone layer, which affects the upper atmosphere.
Positive SAM linked to increased fire frequency
In the first ever study of its kind, Dr Fletcher and PhD student Michaela Mariani found the southerly shift in westerlies was associated with an increase in fire activity in south west Tasmania.
Their study included an analysis of charcoal deposits from fires over the past 1,000 years in an area of western Tasmania that includes areas where fires are currently burning.
Large-scale fire events in Tasmania in the past have been very rare and we're beginning to see them more frequently ... There's evidence we're seeing anthropogenic climate change. Professor David Bowman, fire ecologist, University of TasmaniaThe researchers compared the amount of charcoal at different time periods to changes in SAM and found there was more charcoal at times when the westerlies had shifted southward.
The rise and fall in the amount of charcoal closely matched the SAM index, with a steady rise in both charcoal and the SAM index since around 1500, and a particularly steep rise in recent times.
"Correlation isn't causation," said Dr Fletcher. "But it's a well-described phenomenon that ozone depletion has caused a positive trend in SAM and we've got clear evidence that the SAM index in the 21st century has exceeded anything that's occurred in the last 1,000 years."
The researchers also found a close match between actual SAM and fire records on a shorter time scale, over the past 30 years.
"If you have a dry autumn, winter, spring you've got more fires in summer," said Dr Fletcher.
The research has been accepted for publication in Geophysical Research Letters.
Current Tassie fires in study area
Dr Fletcher said while the impact of El Nino-Southern Oscillation (ENSO) on fire frequency has recently been recognised, relatively little attention has been paid to the impact of SAM.
"We've ignored the fact that in Tasmania, at least, fires are also modulated by this other system," he said.
He said the current fires are likely to be a result of a combined effect of ENSO and SAM.
"What we have here in this current year is the fifth strongest El Nino event on record and the second positive SAM event on record," said Dr Fletcher. "It's a double whammy."
SAM could act as an 'early warning'
A related report appearing this week in the journal Environmental Research Letters argues that it is hard to predict fire activity in Australia because of its highly variable climate.
However senior author Professor David Bowman, a fire ecologist from the University of Tasmania, welcomed Dr Fletcher's research for showing the role SAM can have in providing "early warning" of bad fire seasons.
"You can't predict precisely, but you can use climates modes like SAM to help you forecast bad fire seasons," said Professor Bowman. "It's just a guide, but it's important."
Professor Bowman said claims that humans are increasing fire frequency were supported by different lines of evidence over long time periods over time.
"Large-scale fire events in Tasmania in the past have been very rare and we're beginning to see them more frequently," he said. "There's evidence we're seeing anthropogenic climate change."
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