The hypothesis tested.We test this hypothesis:
Heatwaves in Australia are becoming more frequent, hotter and are lasting longerbecause of climate change.
(The claim was made in a Climate Council report of Jan 2014. From other publications, it seems to be perceived wisdom among authorities from the Australian Bureau of Meteorology and CSIRO, who help to guide national policy.)Here, we examine the daily maximum temperatures of 5 State capitals, Perth, Adelaide, Melbourne, Sydney and Hobart. These were chosen because many people live close to these weather stations and because their observations cover many decades. Brisbane has too much missing temperature data and Darwin is already hot.
We use simple algebra and 5 sites only because of limited resources. However, more complicated analysis must still explain the findings of simple tests.
Here, for ease of calculation, a heatwave is simply defined as a string of consecutive days whose average of the maximum temperatures is anomalously high. We look at past heatwaves of 4, 5, 6 and 10 consecutive days. We select the Top 20 hottest heatwave years and then rank them in various ways.
Thus, we present a bundle of graphs, being 5 cities x 2 data sets X 3 heatwave day lengths for a total of 30 graphs. Averaged maximum daily temperatures in degrees C always form the Y axis, years always form the X axis.
The primary finding:
Findings in more detail.
For most of the graphs, the years are shown in chronologic order of oldest to most recent. The linear regression line has no great mathematical meaning; it is inserted to help the eye to see if there is a discernible trend over the years. The line is coloured blue for a cooling trend, yellow for essentially zero trend and red for a warming trend.
To avoid having to count from the 30 graphs, here is a summary table of trends:
Test One: Are heatwaves becoming more frequent?
There is not a strong pattern for Melbourne, Sydney, or Hobart.
The hypothesis that heatwaves are becoming more frequent is then supported by one city only of the 5 tested, Perth. Note that the results for Perth and indeed all stations, can vary if a different selection of local weather stations is chosen.
Test Two: Are heatwaves becoming hotter?
The outcome is shown in this table. In all but one case out of 15, (Sydney, Acorn, 5 day) these data show heatwaves are getting cooler with the passing of time.
Test Three: Are heatwaves becoming longer?
In this example, it can be seen that the long, 10 day heatwaves are no more prevalent in the second half of the history than the first half, 1887 to 1950. It is concluded for this case that heat waves are not becoming longer. The following table summarises all 15 CDO cases calculated.
The hypothesis that heatwaves are becoming longer is not supported by this analysis of these 5 important cities.
CONCLUSIONS.It is easy to raise objections to the methodology of this analysis.
It is not easy to explain why perceived wisdom supports the opening hypothesis of longer, hotter and more frequent heatwaves, when this simple exercise falsifies it in the first instance.