February 10, 2009

experts comment on the Australian wildfires

The wildfires swept through the Australian state of Victoria, causing significant damage and loss of life.

Prof David Petley, Professor of Geography and Wilson Chair in Hazard and Risk at Durham University, said:

“Wildfires are quite common events, but the number of fatalities that this particular episode has caused is really quite unusual. Looking at data from the CRED EM-DAT database, the average annual number of deaths in wildfires worldwide is 59.5 fatalities per annum. Care is needed in the interpretation of the data as CRED only record events that kill ten or more people, so these values consistently underestimate the true toll. Nevertheless, with 171 reported deaths from this event so far the unusual impact is clear.

“It is interesting to think through the likely long term impact of these fires in terms of erosion and landslides.

“From looking at previous research into the impact of wildfires in SE. Australia it is clear that although the fires have caused mass fatalities, devastated vast areas, and made thousands homeless, there should not be a serious increase in erosion in the burnt areas. This is because of the rapid rate of plant growth in the aftermath of fires plus the resistance of the soil to erosion. This will help greatly in the post-fire recovery of the burnt areas. This is a pattern that is quite different from that seen in for example California, where heavily burnt areas often suffer from devastating erosion and landslides.”

Nick Reeves, Executive Director of Chartered Institution of Water and Environmental Management, said:

“This is further evidence of the reality of climate change and the increasing frequency of catastrophic weather events around the world. Extreme heat and fire are likely to become a more frequent aspect of life in certain parts of Australia as average global temperatures soar. New and more appropriate mitigation and management techniques will have to be learned to avoid the human and environmental tragedy the world has witnessed in recent days. This may involve tough decisions on the location of some human settlements as unbearable heat and fire render them unsustainable. What we are seeing now is a terrible tragedy that must provide lessons for the future, for us all.”

Prof Bill McGuire, Director of the Aon Benfield UCL Hazard Research Centre at University College London, said:

“Australia is experiencing its worst drought for over a hundred years and this, combined with record-breaking temperatures in the last few weeks, makes it hardly surprising that this season’s wildfires are worse than anything that has gone before. With climate change models predicting an even drier and hotter Australia in coming decades, the scene could well be set for even worse to come.”

Clifford Jones, Reader In Engineering at the University of Aberdeen, said:

“The Australian continent has been occupied by Europeans since January 1788, when the First Fleet arrived. Ever since, bush fires have been an important feature of life there, and in modern Australia suburban as well as rural local authorities have fire control centres. One cannot live in Australia over a summer without being conscious of the risk of bush fires, and governments have the authority to impose a ban on barbecues on days when the fire risk is high. Risk is assessed by the quantity of the forest debris (‘litter’) per unit area of the ground underneath the tree canopies and also by the temperature. Humidity is also a factor.

“What distinguishes bush fires in Australia from forest fires in other parts of the world is the fact that Eucalypts, which dominate the Australian landscape, release very flammable oil. This is previously stored in ‘glands’ in the leaves and is released when the leaves start to heat up. The factor of Eucalyptus oil undoubtedly adds to the vigour – and therefore the hazards – of bush fires in Australia. The primary fuel, much more abundant than Eucalyptus oil, is however the trees themselves including barks and stems. Additionally to Eucalyptus oil, a factor in accelerating a fire it is wind and it is on warm windy days that precautions need to be most stringent.

“It has already been mentioned that barbecues are prohibited on high risk days. It is believed that a very serious bush fire in the early 1980s was caused by the shorting of electrical conductors running above a bed of forest litter. This is what is termed an inert ignition source: once a blob a metal created by the shorting has entered the forest litter it transfers heat to it but will itself cool. By contrast there can be a ‘reactive ignition source’. This is an element of forest material having ignited which when it enters a bed of forest litter will continue to react and itself get hotter. Clearly reactive sources are of much smaller physical size than inert ones, and roughly speaking a reactive source of a few millimetre dimension might be as dangerous as an inert one the size of a tennis ball.”

Prof John Dold, Chair of Applied Mathematics at the University of Manchester, said:

“Bushfire (wildfire) behaviour is strongly associated with two things, the existence of vegetation to burn (particularly dead dry vegetation) and the weather conditions at the time of the fire. Drought in parts of Australia has helped to make the vegetation vulnerable to fire, but in milder weather conditions the fires would still have been much less dangerous.

“The factors introduced by the weather conditions are:

– high temperature and low atmospheric humidity which dry the vegetation even further. Temperatures were up in the 40s Celsius in south-eastern Australia over the weekend

– strong winds that drive the fire rapidly in the direction of the wind. The faster the fire spreads, the more intensely it burns, the larger the flames grow and the more fierce becomes the radiation from the fires. Rapidly spreading fires are obviously the most dangerous and the most difficult for firefighters to control.

– if the winds are steady then at least one knows which direction to use in order to run from the blaze, but if the winds are erratic (as they seem to have been in the latest Australian fires) a “safe”
direction can suddenly become extremely dangerous.

– it is also known that an unstable atmosphere makes for more extreme fires. The lower atmosphere becomes unstable when the temperature falls with altitude (without condensation) at a rate of about 1 degree Celsius per 100 metres. Normally, the temperature falls at a lower rate of about 1 degree per 150 metres, making the atmosphere stable.

– close to “unstable” conditions, the typically stronger higher altitude winds are able to penetrate down to ground level. The hot air produced by the fire is also able to develop a stronger convection plume. With such a high ground-level temperature, as there was recently in south-eastern Australia, it is very likely that parts of the lower atmosphere became unstable.

– embers produced by the fire in strong erratic winds are able to be carried further, causing spotfires, or new fires ahead of an existing fireline. If the atmosphere is not stable then the plume can also lift these embers higher, enabling them to travel yet further. Obviously, spotfires can make the job of containing an existing fire very difficult.

– embers in strong winds are also believed to be one of the most dangerous ways in which bushfires set houses and buildings alight.

“The advice often given to householders in Australia is to stay at their house, protecting it from embers entering in any way (for example windows, igniting debris in gutters, etc.). The speed and ferocity of the fires this last weekend, however, seem to have been overwhelming.

“A big question concerns the frequency with which such conditions are likely to arise in the future. Climate change might well be gradually increasing the probability of having extremely dangerous fire-weather conditions at any one time. The great tragedy seems to be that some warped individuals actually lit some of these fires deliberately when the conditions for extremely bad fire behaviour were well known to be exist. Without an ignition source, deliberate or accidental, it would just have been a very hot, dry and windy day.”