Comment

Long bow to blame NSW fires on man-made global warming

Natural forces can account for the severity of these fires and fires in the past.

It seems that every time there's a major bushfire in Australia there's also a queue of people who try to blame it on man-made warming. They easily forget that our history of fires dates from long before the rise in temperatures, and they seem ignorant about science in general and meteorology in particular.

In its latest report the IPCC claims that it is "likely" that heatwaves have increased in Australia, "likely" being just one step away from "as likely as not". Two of the three cited papers in the report appear to have a co-author who is also a lead author of that chapter of the IPCC report, which might be fortuitous.

A close check of the easiest obtained of the three references tells a less clear picture. It says that heatwaves have increased in some parts of Australia and have decreased or are unchanged in others, but it's talking of trends from 1971 to 2000, a period that covers a significant shift in the Pacific Ocean towards more El Nino events. The statement doesn't tell us what's happened over the past 15 years, a period when the global average temperature has been basically flat. Further, any attempt to link man-made warming and heatwaves should surely describe the cause of each heatwave rather than try to falsely imply that correlation is proof of cause.

The increase in atmospheric carbon dioxide increases the amount of heat being reflected back to Earth. Even few sceptics dispute this; the real argument is over how much, whether changes in other forces counteract any warming, and ultimately whether the total warming will be significant or negligible.

It's drawing a long bow to link this to heatwaves, which as both the latest IPCC report and the Bureau of Meteorology say, are caused by near stationary highs dragging warm air from one location to another. Highs have anticlockwise winds in the southern hemisphere so highs in the Tasman Sea and east of NSW will inevitably draw warm air from northern Australia across NSW and Victoria. If a low is following the high across Australia, then its clockwise winds will add force.

The old Climate Commission said that heatwaves would increase as temperatures rose. Even setting aside the different processes, that's a dubious claim. The term "heatwave" has no formal definition. If it is defined as days above 35 degrees then global warming, if it resumes, could easily increase that number. If it is defined as instances of three or more days with temperatures more than five degrees above average it's a different story because warming might cause the average to increase but not the number of times it is exceeded.

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Many factors, both man-made and natural, contribute to the severity of a bushfire. For the current NSW fires some key factors are clear.

Data for the Blue Mountains is not easy to find but as a whole, after nine years of below average rainfall, NSW rainfall totals in 2010 and 2011 were 47 per cent and 21 per cent above average, with 2012 very close to average. This pattern will have encouraged plant growth. Not surprisingly, other bad bushfires were preceded by one or two years of good rainfall.

Temperatures in August and September were above average in NSW. The Bureau of Meteorology attributes this to highs crossing the east coast being slightly higher than usual, for reasons unknown, with the anti-clockwise winds drawing warm tropical air from further north than usual down onto the state. This would lead to early drying of vegetation, although this seems not without precedent because major NSW bushfires have previously occurred in October and November.

The spread of a fire is a crucial factor in how severe it becomes. When the bushfires began they were driven by strong, hot and very dry winds, just as the Victorian fires of 2009 were. Bushfires on calm days can be difficult enough to control and extinguish but a rapidly expanding or very large fire front, fanned by strong winds, quickly becomes a challenge to firefighting resources. The Rural Fire Service mentions several major fires in the past and in some cases specifically notes very strong winds.

The human factors can be summarised in one word – preparedness. Maintenance of firefighting access trails, removal of fire fuel and having adequate firefighting resources immediately to hand can all play a part in minimising the destruction, but even these can be overwhelmed by a rapidly moving fire.

It is drawing a very long bow to claim that man-made warming contributed to the NSW bushfires. Natural forces can account for the severity of the current fires and of other fires in the past. The history of bushfires in NSW also shows there is nothing unusual about these fires.

John McLean is the author of three peer-reviewed papers on climate and was an expert reviewer for the IPCC's latest climate report.