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Hotter seasons to bring bountiful harvests for antipodean Grim Reaper

Date

Chris Hammer

Danger ... with an increase in background temperatures, climate scientists also expect an increase in climatic volatility.

Danger ... with an increase in background temperatures, climate scientists also expect an increase in climatic volatility. Photo: Jonathan Carroll

The Grim Reaper is a European, dressed in a heavy winter cloak. An Australian embodiment of death might look more appropriate in boardshorts and sunscreen.

For death in Australia – large-scale death – comes in the warmer months of summer and early autumn. That's when bushfires explode and cyclones come hurtling in from the Coral Sea.

The traditional image of the Grim Reaper comes from a different hemisphere and a different era, when the chill winds of winter brought death from exposure, hunger and disease. But that's not the case here; war, famine and pestilence rarely visit our shores.

Take a look at the worst-ever natural disasters, in terms of deaths, in recorded Australian history. One list of nine disasters includes four bushfires and four cyclones, with only Gundagai's winter flood of 1852 breaking the run.

A more comprehensive list identifies 18 natural disasters that have killed more than 100 people: 11 heatwaves, six cyclones and one bushfire. All occurred during the warmer months.

Of the 14 natural disasters in which between 50 and 99 people died, the breakdown is six cyclones, five bushfires, one heatwave and two floods, one of which occurred in summer.

This is why Australians should be deeply concerned about climate change – hotter summers mean this kind of death is likely to come visiting more often.

Climate scientists predict average temperatures are set to increase by somewhere between two and five degrees by the end of the century, but it's not average temperatures that create cyclones and bushfires. The big ones – those that kill scores of people and inflict hundreds of millions of dollars in property damage – occur in exceptional climactic conditions.

Those who lived through Victoria's 2009 Black Saturday bushfires, even if they were far from the destruction itself, will recall just how exceptional the weather was: years of drought, a two-week heatwave, followed by a day with temperatures in the mid-40s, and with extraordinarily low humidity and winds gusting at more than 100 kilometres an hour.

The weather throughout much of south-eastern Australia this week has been an unwelcome reminder of those conditions, in which a fire, once started, is almost impossible to control.

Australia's hot dry summers already ensure that such exceptional circumstances will inevitably occur. They occurred in Victoria and South Australia in the Ash Wednesday fires of 1983, and the Black Friday fires of 1939, long before climate change was perceived as a threat.

Some may take comfort in that, but it would be a misplaced comfort.

An average increase in summer temperatures will increase the frequency of bushfires, perhaps exponentially. The modelling cannot be precise on this, but the direction is clear.

This is because with an increase in background temperatures, climate scientists also expect an increase in climactic volatility. In other words, more exceptional weather events: drier droughts, wetter floods and more catastrophic bushfires.

Something similar is true of tropical cyclones. Cyclone formation is a complex process but one factor is essential: sea surface temperatures. As a rule of thumb, cyclones form only when sea surface temperatures exceed 26.5 degrees.

Once formed, the power of a cyclone is limited by that temperature. There is a concise mathematical formula that accurately determines the maximum potential force of a cyclone. The only variable is sea surface temperature and the formula matches 99 per cent of data recorded from cyclones.

So, while a gradual increase of global temperatures by two or three degrees may sound innocuous, it's not. A sea surface temperature of 26 degrees is unlikely to provide enough energy to form a tropical cyclone; at 29 degrees there's enough energy to power a category five storm with sustained winds exceeding 250km/h.

Scientists are divided on whether global warming will mean more frequent cyclones, but cyclone seasons are likely to be longer, cyclones could strike further south on Australia's coastline and, most importantly, those cyclones that do occur are likely to be more powerful.

Right now, the sea around Fiji has a temperature of 28 or 29 degrees. In the ocean beyond the reef east of Cairns, the surface temperature is 29 degrees. A cyclone might or might not form this summer, but if it does, there's already plenty of energy floating about out there to fuel it.

And this is the thing about climate change: it's impact might not be so gradual. Sure, sea levels may edge up by between two and 10 centimetres a decade, and average temperatures might increase by just one degree in 25 years, but that's not the threat.

The threat will come on those exceptional days, those days that once came every decade or two, those days that now threaten to visit us every few years. The European image of death is outdated; it's being replaced by a cruel new Australian version.

Chris Hammer is a Fairfax Media journalist. His latest book, The Coast, is published by Melbourne University Publishing.

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