Extreme weather or extreme exaggeration?
The recent floods and cyclone in Queensland, fires in Western Australia and various blizzards in the northern hemisphere have had alarmists wailing about “weather extremes” becoming more frequent with global warming.
According to UK data, the first three months of 2010 were cooler than the same months in 2002 and the rest of the year was cooler than 1998, but the average temperature across 2010 made it one of the warmest. That’s statistics for you. Annual global average temperatures in nine of the last ten years were reported as being between 0.4 and 0.5 degrees higher than the 1961-90 average, but rather than show a steady increase those temperatures bounced around.
Australia’s annual average temperatures have likewise bounced around. Year 2010 was the coolest since2001 and in the intervening years we had a major bushfire, driven by very hot and strong northerly winds and fueled by excessive vegetation, but no big floods and just one major cyclone, which occurred 32 years after the previous one. Despite the claims, extreme weather seems to be less frequent as temperatures rise, a point also reportedly made by the Bureau of Meteorology about cyclones.
There’s another aspect to the claims more extreme weather that’s intrigued me for some time – the repeated assertion that heat waves will become more frequent, and by implication the notion that heat waves are influenced by human activity.
David Jones, of the Bureau of Meteorology, said in 2009:
“People will need to get used to these sorts of heatwaves. … As the climate heats up through this century, they are going to become much more common.” 
Kevin Hennessy, principal CSIRO research scientist, in 2008:
“We would anticipate in future more heatwaves will occur…”
The Bureau of Meteorology’s regional director for South Australia, Andrew Watson said he
“… was confident the current record heatwave – which yesterday stretched into its 13th day – was a result of global warming, …”. [also 3]
Monash University’s Neville Nicholls, an IPCC Lead Author, was reported by The Age as saying that
“Climate change is causing heatwave records to be smashed in ways that would have been considered fantasy just a few years ago” and paraphrased him as saying that the situation would get worse in future. Nicholls also made similar comments to a CAWCR government workshop in 2009 .
Tim Flannery made similar claims in his book The Weathermakers, so too has Andy Pitman of the UNSW .
Ross Garnaut’s report said:
“Apart from increasing heatwaves and decreasing frost frequency, little can be said with confidence concerning the details of how extreme weather and climate events will change in the future.”. (This section of his report was written by Amanda Lynch, Neville Nicholls, Lisa Alexander and David Griggs, all of Monash University but Garnaut should take ultimate responsibility for accepting it.)
I think that these people, among them several recognised proponents of man-made warming, have some explaining to do because by my reckoning their comments are very suspect on at least two important grounds.
Firstly, notions of what constitutes a heat wave are very subjective. It might be a surprise to you but there appears to be no scientific or common definition of what a heat wave is. My general dictionary describes it only as an “unbroken spell of abnormally hot weather” and my science dictionary doesn’t even list the term. An online dictionary service quotes the Random House saying “a period of abnormally hot and usually humid weather” and the fact that many Australians would disagree with those final few words shows how subjective it is.
The word “abnormal” in the definition doesn’t help much either. Abnormal relative to when? Last week, last month, last year, or the last 20 years?
Some definitions try to lock in particular temperature thresholds but that would undermine the degree of abnormality if, as some folk claim – not that I believe them – that temperatures will continue to rise. If a situation is no longer particularly abnormal then it’s hardly likely to be perceived by the general population as a heat wave.
So if what constitutes a heat wave is highly subjective then counting heatwaves is more subjective than scientific. Moreover, if higher temperatures are the norm then the perception of what’s abnormal also changes.
The second flaw in claims that heat waves will become more common flies in the face of exactly what a heat wave is.
Perhaps I shouldn’t have used the word exactly because even the nature of heatwaves seems to vary slightly. In the USA heatwaves are often said to be stationary pools of warm humid air that have typically come up from the Caribbean, but Australians and Europeans see things differently.
The Bureau of Meteorology’s Monthly Weather Review for Victoria in January 2009, the month of the bad bushfires, says:
“Perhaps the most important synoptic feature during the month, and the one responsible for Victoria’s heatwave, was a blocking high pressure system in the Tasman Sea which developed from about the 26th. The system directed a northerly air flow over the southeast of Australia, moving an extremely hot air mass over the southeast of the continent.”
And of November 2009 the corresponding review for South Australia said:
“As the high pressure system entered the Tasman Sea it slowed dramatically and became nearly stationary during the next week, 7th to 14th, bringing heatwave conditions over the vast majority of South Australia.”.
Of heatwaves in Perth, Western Australia, we are told:
“Perth’s summer patterns often follow a typical sequence. A ridge of high pressure south of the state combines with a deepening trough off the west coast to direct east to northeasterly winds over the Perth region. This pattern causes rising temperatures over successive days. … Prolonged spells of hot days occur when this pattern is slow moving, the high being maintained south of the state and the west coast trough remaining off the coast. On such occasions, the east to northeasterly winds prevent the early arrival of the seabreeze and cause temperatures well above the average.”
And for Brisbane and the Gold Coast:
“In South-East Queensland, heat waves typically occur between November and February, but days of excessive heat can occur between October and March. During these events the predominant wind is generally from the south-west to the north-west, i.e. from the interior of the Continent. Winds from these quarters have the potential to nullify the cooling effects of any sea breeze.” 
Chapter 3 of the IPCC 2007 report said of the 2003 European heat wave:
“The 2003 heat wave was associated with a very robust and persistent blocking high-pressure system that may be a manifestation of an exceptional northward extension of the Hadley Cell (Black et al., 2004; Fink et al., 2004).”.
The message of all these is clear – heatwaves are caused by quasi-stationary High or Low pressure cells either moving warm air (Australia and Europe) or causing warm, humid air to remain in one place (USA).
I ponder whether Jones, Hennessy, Nicholls and the others mentioned above are trying to claim that High and Low pressure cells will in future be more prone to slowing or to coming to an outright stop. Surely they aren’t claiming that a few extra molecules of carbon dioxide will cause this grinding halt.
As I said, I think they have some explaining to do.
 Geosciences Australia (2001) – “South-East Queensland Community Risk Report”, (chapter 10, “Heat Wave Risks” by Ken Granger and Michael Berechree) online at http://www.ga.gov.au/image_cache/GA4213.pdf
 IPCC 2007 report, WGI contribution, Chapter 3, Section 3.8.4 (Box 3.6)