Guest commentator on Real Climate, David Karoly of Univ. of Melbourne: Bushfires and extreme heat in south-east Australia

From Real Climate, February 16, 2009

Bushfires and extreme heat in south-east Australia

Guest commentary by David Karoly, Professor of Meteorology at the University of Melbourne in Australia

On Saturday, 7 February 2009, Australia experienced its worst natural disaster in more than 100 years, when catastrophic bushfires killed more than 180 people and destroyed more than 2000 homes in Victoria, Australia. These fires occurred on a day of unprecedented high temperatures in south-east Australia, part of a heat wave that started 10 days earlier, and a record dry spell.

This has been written from Melbourne, Australia, exactly one week after the fires, just enough time to pause and reflect on this tragedy and the extraordinary weather that led to it. First, I want to express my sincere sympathy to all who have lost family members or friends and all who have suffered through this disaster.

There has been very high global media coverage of this natural disaster and, of course, speculation on the possible role of climate change in these fires. So, did climate change cause these fires? The simple answer is “No!” Climate change did not start the fires. Unfortunately, it appears that one or more of the fires may have been lit by arsonists, others may have started by accident and some may have been started by fallen power lines, lightning or other natural causes.

Maybe there is a different way to phrase that question: In what way, if any, is climate change likely to have affected these bush fires?

To answer that question, we need to look at the history of fires and fire weather over the last hundred years or so. Bushfires are a regular occurrence in south-east Australia, with previous disastrous fires on Ash Wednesday, 16 February 1983, and Black Friday, 13 January 1939, both of which led to significant loss of life and property. Fortunately, a recent report “Bushfire Weather in Southeast Australia: Recent Trends and Projected Climate Change Impacts”(ref. 1) in 2007 provides a comprehensive assessment on this topic. In addition, a Special Climate Statement(ref 2) from the Australian Bureau of Meteorology describes the extraordinary heat wave and drought conditions at the time of the fires.

Following the Black Friday fires, the MacArthur Forest Fire Danger Index (FFDI) was developed in the 1960s as an empirical indicator of weather conditions associated with high and extreme fire danger and the difficulty of fire suppression. The FFDI is the product of terms related to exponentials of maximum temperature, relative humidity, wind speed, and dryness of fuel (measured using a drought factor). Each of these terms is related to environmental factors affecting the severity of bushfire conditions. The formula for FFDI is given in the report on Bushfire Weather in Southeast Australia. The FFDI scale is used for the rating of fire danger and the declaration of total fire ban days in Victoria.

Fire Danger Rating           FFDI range
High                          12 to 25
Very High                     25 to 50
Extreme                         >50

The FFDI scale was developed so that the disastrous Black Friday fires in 1939 had an FFDI of 100.

To understand the environmental conditions associated with the catastrophic bushfires on 7 February 2009, we need to consider each of the factors and the possible role of climate change in them.

Maximum temperature: This is the easiest factor to consider. Melbourne and much of Victoria had record high maximum temperatures on 7 February (2). Melbourne set a new record maximum of 46.4 °C, 0.8 °C hotter than the previous all-time record on Black Friday 1939 and 3 °C higher than the previous February record set on Ash Wednesday 1983, based on more than 100 years of observations. But maybe the urban heat island in Melbourne has influenced these new records. That may be true for Melbourne, but many other stations in Victoria set new all-time record maximum temperatures on 7 February, including the high-quality rural site of Laverton, near Melbourne, with a new record maximum temperature of 47.5 °C, 2.5 °C higher than its previous record in 1983. The extreme heat wave on 7 February came after another record-setting heat wave 10 days earlier, with Melbourne experiencing three days in a row with maximum temperatures higher than 43 °C during 28-30 January, unprecedented in 154 years of Melbourne observations. A remarkable image of the surface temperature anomalies associated with this heat wave is available from the NASA Earth Observatory.

Increases of mean temperature and mean maximum temperature in Australia have been attributed to anthropogenic climate change, as reported in the IPCC Fourth Assessment, with a best estimate of the anthropogenic contribution to mean maximum temperature increases of about 0.6 °C from 1950 to 1999 (Karoly and Braganza, 2005). A recent analysis of observed and modelled extremes in Australia finds a trend to warming of temperature extremes and a significant increase in the duration of heat waves from 1957 to 1999 (Alexander and Arblaster, 2009). Hence, anthropogenic climate change is likely an important contributing factor in the unprecedented maximum temperatures on 7 February 2009.

Link to the rest of the article at realclimate.org: http://www.realclimate.org/index.php/archives/2009/02/bushfires-and-climate/