As it is now two weeks (nine business days) since I questioned Dr Vertessy on his claims in his ABC Radio interview, it appears an answer is still to be given, so I shall post what I have found so far.
Dr Vertessy claimed that we are seeing “of the order of five times the number of very serious heatwaves” as in the middle of last century. Not knowing Dr Vertessy’s definition of a “very serious heatwave”, I have used the following metric:
“Three days or more in a row in summer (December- February) where the maximum temperature is in the top 5% of temperatures for that day at that location, with daily benchmarks calculated using daily maxima for each month from 1961 to 1990.”
I have also used as an absolute metric of very hot days the Bureau’s own definition, days above 40 degrees Celsius. I have used ACORN-SAT maxima to 31 December 2014 downloaded directly from the Bureau’s Acorn site, and daily maxima from 1 January to 28 February this year for each site, downloaded from Climate Data Online.
Note that this does not consider other serious factors such as humidity or night time minima.
I have initially looked at all state capitals, and will later look at other locations.
I have calculated decadal running counts of the number of days meeting the criterion to show how hot weather has changed over time.
So what did I find to be the answer to “Are we getting five times more heatwaves than we did 60 years ago”? Mostly no, but it depends where you look.
Fig. 1: Decadal count of heatwave days in Adelaide
Yes, but the peak may be past.
Fig. 2: Decadal count of >40C days in Adelaide
This shows a distinct recent rise.
Fig. 3: Decadal count of heatwave days in Brisbane
A peak 10 years ago, dropping to zero heatwaves in the decade to 2015.
Fig. 4: Decadal count of >40C days in Brisbane
One day, 22 February 2004.
Fig. 5: Decadal count of heatwave days in Darwin
As Darwin doesn’t have “summers”, the count was of all days. Note the 1930s and 1970s. Darwin is not seeing more heatwaves. Darwin has never had a day over 40C.
Fig. 6: Decadal count of heatwave days in Hobart
Hobart has had no heatwave days in the past decade, compared with five in the 1950s.
Fig. 7: Decadal count of >40C days in Hobart
Hobart has fewer extremely hot days than in the past.
Fig. 8: Decadal count of heatwave days in Melbourne
Melbourne has fewer heatwave days than the middle of last century.
Fig. 9: Decadal count of >40C days in Melbourne
Melbourne has more very hot days than it did in the 1950s, but less than the 1940s.
Fig. 10: Decadal count of heatwave days in Perth
Perth has had 10 heatwave days in the past decade. In the decade to 1955 it had 6- but in the 1960s it had three times the current number.
Fig. 11: Decadal count of >40C days in Perth
The recent peak was one more than in the 1960s. The warming since the 1970s is clearly visible.
Fig. 12: Decadal count of heatwave days in Sydney
Four days in the last 10 years, compared with zero in the 1940s and 1950s- but less than the 1960s.
Fig. 13: Decadal count of >40C days in Sydney
The current peak of seven days in the past 10 years of very hot days is about the same as the 1940s and 1960s, but much less than the 1980s.
Technically, Dr Vertessy is correct in his claim of “of the order of five (four to six?) times the number of very serious heatwaves” as in the middle of last century, at Adelaide, Darwin, and Sydney, but not at Brisbane, Melbourne, Hobart, or Perth. However, Sydney had far more in the 1960s, and Darwin had as many in the 1970s and far more in the 1930s. Adelaide alone shows a clear picture of many more heatwave days in the past 10 years.
In several of the records it is possible to see cycles of 15 – 20 years duration. While there is an argument that heatwaves and extremely hot days are weather events, not climatic, resulting from blocking highs or the lateness of sea breezes, these apparent cycles indicate a climatic influence. What would cause blocking highs to be more persistent, or sea breezes to be consistently later, for 10 years or more? Atmospheric circulation patterns, including the location of the sub-tropical ridge, would appear to be the major influence.
The longer term analysis from 1910 shows a more complete picture than since the 1950s. Wouldn’t it be good to use “carefully curated” Acorn maxima from before 1910.
In a future post I will look at other locations, as a continent’s climate extremes can’t be usefully analysed with only seven sites. As well, this analysis has used ACORN-SAT data only. What will the raw data show? Therefore I will also compare results for Acorn and raw. Bourke might prove interesting.
Meanwhile, I am waiting patiently for Dr Vertessy’s response. Apart from Adelaide, the state capitals certainly don’t support his claim.