August 2009 was Australia’s warmest on record. Temperatures averaged over the country were 2.47C above the long-term average, nearly a degree above the previous August record set in 1998, and 25% of the country had its hottest August day on record at some stage during the month.
Some places, such as Collarenebri and Murwillumbah in NSW and Gatton in Queensland, broke their previous August records by 5C or more. Temperatures reached as high as 37.8C at Mungindi in NSW and 38.5C at Bedourie in Queensland, both of which were all-time state records for August.
Such an exceptional month leads to many people to ask: is this climate change?
The converse question is – when we have extreme cold, does this mean that climate change isn’t happening? Extreme cold events have occurred in recent years too; earlier this year an Australian record for April of -13C was set at Charlotte Pass, and in June 2007 exceptionally cold days occurred across large parts of the Australian tropics, with daytime temperatures in single figures as far north as Mount Isa and Tennant Creek.
A single event in isolation does not tell us very much about long-term trends in the climate. Instead, we can analyse what has been happening to the frequency of record temperatures over time. A recent Bureau of Meteorology study analysed the years in which monthly record high and low temperatures occurred across Australia, using only locations which had long-term, complete observations and were outside major cities. Both daytime maximum and overnight minimum temperatures were analysed.
While there is some fluctuation from year to year, in the period from 1957 to 2007 analysed in the graph above, there is a clear trend towards more high temperature records (the red line), and fewer low temperature records (the blue line).
In the first ten years, from 1957 to 1966, low records outnumbered high records by a ratio of 1.5 to 1 for maximum temperatures, and 2.2 to 1 for minimum temperatures. Conversely, since 1997, high records have outnumbered low records by a ratio of 2.4 to 1 for maximum temperatures, and 2.1 to 1 for minimum temperatures.
How well does this match the overall warming trend? Average temperatures in Australia have warmed by about 0.8C over the last 100 years, most of it in the last 50. Assessing how this trend affects extremes at individual places is tricky, but extreme months – those where the national average is more than 1.5C above or below normal – give us another insight into this question.
As one might expect, the frequency of extreme warm months has increased greatly over the last 60 years, and that of extreme cold months has dropped equally sharply (graph above), but once the effect of the overall warming trend is removed, there is no clear trend in extremes, leaving aside a lack of extreme cold months in the 1950s, as seen in the graph below.
This suggests that the number of extreme months is consistent with what we would expect, given the overall warming trend. In other words, while we can’t blame global warming for any individual extreme warm event it is certainly making these events more common.
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