Global warming and bushfires: what to expect

September 7, 2009

In the last week of winter, something strange happened: bushfires raged across New South Wales, with major fires in the Shoalhaven and Eurobodalla in the south of the state.

The week before, the north-east of NSW and southern Queensland — even parts of southern Sydney — had total fire bans declared.

In many areas of NSW, fire season started a month earlier than the usual October 1. As authorities predict extreme weather conditions for the coming season, images of the devastating bushfires in Victoria earlier this year spring to mind.

Why do we see such weather conditions arriving so early, and why are they becoming so extreme?

The simple answer is that the climate is changing. The last two years have seen records tumbling for the hottest days and the length of heatwaves, particularly in the southern states. The hottest 14 years on record have occurred in the last 20 years.

Victoria's February bushfire disaster was more than just a freak occurrence. Hotter conditions and extreme bushfire weather are the result of a change in the climate due to human factors.

The reason is the strengthening and extension of the southern subtropical ridge, an atmospheric phenomenon caused by warm air from the equator sinking towards the ground around 30° south of the equator. This creates a band of warm, dry high pressure cells and keeps low pressure cells and storm fronts below this latitude and maintains westerly trade winds.

A recent study by the Bureau of Meteorology and the CSIRO showed the subtropical ridge over Australia has indeed strengthened over the last 13 years, expanding southwards and pushing storm cells towards a more southerly latitude, away from the continent.

Scientists used sophisticated computer modelling to show the effects of normal climatic variations and found that the subtropical ridge did not change significantly. When human influences such as ozone depletion and greenhouse gas emissions were factored in, the modelling showed a dramatic strengthening of the ridge, which mimicked observations by meteorologists.

The result is the current drought, which began in 1996, bringing a drastic reduction of rainfall over southern Australia. Melbourne's dams are now getting about one third less water than they did before 1996, the Age said on August 30.

The ridge of high pressure cells has also led to more hot, dry air in the southern states. This air moves ahead of eastward-bound storm fronts, creating hot, dry winds. As more warm air rises from the equator, particularly during the dry summer months, the winds caused by the high pressure cells strengthen.

This tendency will get worse as the surface of the Earth heats up due to global warming.
It was hot, dry conditions coupled with strong winds that led to the fatal bushfires in February.

It's not just a small increase in average temperatures that will dictate how severe a bushfire season will be. That small increase will lead to a greater increase in both intensity and frequency of extreme weather conditions. As these phenomena tend to strengthen together, it brings about a significantly larger increase in the bushfire threat.

This is not new information. Following the January 1994 bushfires in NSW, Green Left Weekly reported: "These high temperatures also occur in a wider context that is ominous … 1993 was the sixth hottest year on record. The seven hottest years on record have all occurred since 1980 … [T]he increasing frequency of extreme climatic events is consistent with global warming."

When fire authorities talk about bushfire behaviour, they relate it to the Fire Danger Index (FDI). This index was worked out by CSIRO scientists and relates to fire behaviour according to temperature, relative humidity, wind speed and long- and short-term drought effects.

The FDI scale is not linear — more logarithmic or exponential — and the equation to work out the FDI is very complicated, involving various matrices. It is usually done in the field with a sliding meter.

However, fire behaviour is linear in relation to the FDI: the rate of spread and flame height double as the FDI doubles and the spotting distance (the distance embers can be carried by wind to start another fire ahead of the front) increase at a significantly greater rate.

The FDI gives the totality of the weather conditions for the fire. The quantity of available fuel also has a dramatic effect on fire behaviour, but it is not part of the FDI calculations. As weather conditions worsen (prolonged drought, rising temperature etc.), the FDI can rise dramatically.

A small increase in any of these conditions can mean a larger increase in the FDI and thus fire behaviour.

The FDI benchmark of a maximum of 100 was set following the Victorian Black Friday bushfires in 1939, which killed 71 people. This was considered the worst that conditions could possibly get.

The Canberra fires of 2003 were a wake-up call to fire authorities: the FDI was above 100 — it was closer to 140!

This year, on February 7 and 8, when fires ripped through Victoria, the FDI recorded in many centres was 300 or higher.

In light of this data, it's easy to understand why those bushfires were so devastating. That is, unless you are the Victorian government.

When the state Labor government established the 2009 Victorian Bushfires Royal Commission, its terms of reference didn't include anything on the weather or climatic conditions that led to the fires being so devastating. Climate change was not to be investigated as being a potential factor in the events that led to 173 people being killed.

In August, the commission released 51 interim recommendations, some of which have already been implemented by the Victorian government. But there were many factors involved in the conditions that generated the super intensity of the fires, and the response to the fires, that were not dealt with. Some of these may yet come out in the final report.

Ignoring climate change as a factor in increased temperatures and extreme weather events is a recipe for increasingly destructive bushfires. Such folly bodes badly for the future as we face greater catastrophes. Firefighters on the front line will increasingly be trying to defend communities against conditions they are ill-equipped to face.

As the planet heats up, even if only by fractions of a degree in average temperatures, we will see more frequent extreme weather events of increasing intensity and we will see more and more destructive bushfires.

How we react to prevent such extreme weather events from worsening and how we set ourselves up for the battles ahead will determine how many people we will be able to save from disaster the next time.

[Shaun McDonald is a volunteer firefighter in NSW and a member of the Socialist Alliance.]

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