Two years, three record heat waves in southeastern Australia

Summer 2009 — 2010 hasn’t even begun in Australia, and yet we are already sweltering under another record heat wave — the third in two years. Temperature records for the month of November have been broken across the region, caused by a blocking high pressure system over the Tasman Sea. This follows an abnormally hot winter, including Australia’s hottest August on record.

In my home city of Adelaide, we’re still experiencing the first official November heat wave since records began (a ‘heat wave’ being defined here as five or more consecutive days above 35°C). Last Saturday 7th Nov, the mercury climbed to 34.4°C, and on Sunday the heat wave officially commenced. From Sun 8/11 to Sat 14/11, the maximum temperatures have been 36.7°C, 37.0°C, 38.6°C, 39.2°C, 39.0°C, 38.7°C  and 39.5°C. The forecast for Sun 15/11 is 40 °C, after which the temperatures will drop back to the high 20s for a few days, and then another burst of days in the low-40s. If Sunday’s scorcher is realised (confirmed: 39.4°C), the heat wave will have lasted for 8 days [confirmed] (almost 9, with Sat 7/11 also almost reaching the threshold 35°C). Not a great time to hold a Christmas pageant — poor Santa!

Time for some context. The closest Adelaide has ever come to a spring heat wave was 4 days in a row 1894. This month’s event will double that — a doubling like this is not twice as unlikely, it’s orders of magnitude more unlikely. Consider that in prior to 2008, the record length for an Adelaide heat wave in any month was 8 days (all occurring in summer). Now, in the space of less than 2 years, we’ve had a 15 day event in Mar 2008 (a 1 in 3000 year event), a 9 day sequence in Jan/Feb 2009 (which included 8 days above 40°C and 13 consecutive days above 33°C), and now, another 8 day event in Nov 2009. How unusual is this? There have been 6 previous heat waves that lasted 8 days, many more of 7 days, more still of 6, and so on — the return time is logarithmically related to it’s length. Given these data, and the fact that the latest spring event has equaled previous all-time summer records (!), and the alarm bells should rightly be ringing. Statistically speaking, it’s astronomically unlikely that such a sequence of rare heat waves would occur by chance, if the climate wasn’t warming. But of course, it is.

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