Bushfires — just chance, or climate change?

With Victoria's bushfire holocaust now confirmed as Australia's worst-ever natural disaster, people are reasonably asking: are these events linked to climate change?

The short answer is yes. Major bushfires are complex phenomena, requiring the right combination of temperature, wind speed, available fuel, low humidity and low preceding rainfall if they are to reach their full savagery.

By bringing more frequent and extreme hot weather events, and increasing the likelihood of droughts, climate warming now promises to make really severe bushfires much more frequent.

In a 2007 study, the Australian Bureau of Meteorology and the CSIRO sought to put a figure on the shortening odds of bushfire catastrophe.

Relating the scientists' findings, the Canberra Times reported on February 9: "By 2020, days of extreme fire danger are forecast to increase by 5 to 25 per cent if climate change is low and by 15 to 65 per cent if it is high."

Ominously, the 2007 study predicted that the worst changes would be seen in northern NSW. Until now this region has mostly been too humid, and its temperatures too mild, for bushfires of the intensity experienced further south.

The Victorian inferno of February 7 was the culmination of more than a week of record or near-record temperatures across much of south-eastern Australia.

In Adelaide, the maximum temperature of 45.7°C on January 28 was the third-hottest day ever recorded in the city.

Maximum temperatures in the South Australian capital remained above 40°C for six consecutive days, from January 27 to February 1. The last time Adelaide had experienced six days above 40°C was as far back as 1908.

The grilling suffered by Adelaide, however, was soon to be surpassed in Victoria. In most of that state, February 7 was the hottest day on record.

Hopetoun recorded the state's highest ever temperature of 48.8°C. Melbourne also set a new mark: 46.4°C.

With temperatures mounting, state political leaders groped for excuses that might explain paralysed public transport, rolling blackouts, and large numbers of heat-related deaths.

"We've got to remember, this is a one in 100 year heatwave", South Australian Premier Mike Rann lamely assured listeners to ABC radio on February 5.

Unfortunately for Rann, this supposedly one-off event had been spectacularly previewed less than a year before. In March 2008 — autumn, not summer — the maximum temperature in Adelaide had stayed above 35° for 15 consecutive days. The previous record, over 170 years of recording, was eight days.

If the fifteen-day heatwave were pure chance, the head of Adelaide University's Research Institute for Climate Change and Sustainability Professor Barry Brook later observed in his blog, it would recur on average once every 3000 years.

But come 2009, and south-eastern Australia was enduring another historic heat event.

As of February 4 this year, weather forecasts were predicting that Adelaide would undergo 12 consecutive days above 35°C.

If no underlying warming mechanism were involved, Brook calculated, the odds of a 15-day heatwave in Adelaide being followed within a year by another of 12 days were one in 1.2 million.

As it happened, the forecast 12-day streak was broken on February 4 when the maximum temperature in Adelaide fell about 1.5° below the arbitrary 35°C mark.

Making a rough recalculation, Brook revised his earlier figure. In the absence of a warming trend, the heat events of March 2008 and January-February 2009 would now have been likely to succeed one another, he concluded, about once every 150,000 years.

"But what's a couple of ice ages or 18 times the period of human civilisation between friends?" Brook remarked.

Then, as if to spite the deniers, temperatures soared back up again. On February 6, the Adelaide maximum reached 43.9°C, and next day, 41.5°C. So far, no one seems to have worked out the odds on 15 days above 35°C being followed by a run of eight out of 11 days above 40°C, all within less than a year.

"I would say confidently ... that [the March 2008 heatwave] is not a one in 3000 event any more", Brook told the February Adelaide Review. "It's much more frequent and it could end up being a yearly event within the next few years if we continue with our warming."

The startlingly longer and more intense Australian heatwaves have followed an average warming of the Earth's surface atmosphere that seems mild by comparison — around 0.8°C since the mid-19th century. Why have the changes in south-eastern Australia been so striking?

Global warming is not evenly distributed. Summer temperatures in continental interiors, such as inland Australia, are rising faster than the general increase.

When a high pressure system sits persistently over the Tasman Sea, as happened both this year and last, dry air moves slowly southward over the Australian inland, picking up heat as it goes. And with higher inland temperatures, there is now more heat to be picked up.

Also important is the fact that, as climate zones shift, Adelaide and even Melbourne are increasingly becoming part of Australia's inland in climate terms.

In earlier decades, bursts of cool air from the Southern Ocean would more or less reliably drive back hot inland air from the southern coastline before many days had passed. Heatwaves of more than a week were rare.

One of the results of climate warming, however, is that the cool mid-latitude westerly airstreams are contracting pole-ward.

The cool changes are weaker, and in the Southern Hemisphere, do not reach so far north. In southern Australia, there is now less and less reason why extreme heat events should not last for much of the summer.

And there is a catch: when a vigorous front does pass over southern Australia, there is a high chance that its approach will accelerate superheated inland air and send it spearing into forested areas. This is the type of event that doomed close to 200 people on February 7.

To reach full ferocity, bushfires also require unusually dry conditions. Prior to February 7, Melbourne had 35 days without rain, the second-longest period ever recorded.

Over the past 12 years, rainfall over most of the area affected by the fires has been about 20% below the long-term average.

Scientists now believe that the single largest influence on rainfall in south-eastern Australia is the recently discovered Indian Ocean Dipole (IOD), involving shifts in water temperatures in the Indian Ocean.

How the IOD might interact with global warming remains undetermined. But there is no longer any substantial doubt about the link between global warming and the pole-ward retreat of the westerly wind belts.

Cold fronts and showers embedded in the moist westerlies have historically been an important source of rain to the bushfire-prone regions of Victoria. This source is now waning.

If there is any lesson in the bushfire catastrophe, it is that climate change is well able to kill people, in large numbers, in Australia, right now.

Television viewers in the aftermath of February 7 watched a teary Prime Minister Kevin Rudd embracing bushfire victims.

We might all have been more impressed if he had set serious targets for reducing greenhouse gas emissions, while allocating the resources needed for success.