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Heatwaves in Australia: in many ways they are not hotter, longer or more common. Why won’t BOM and ABC say that too?

Posted By Joanne Nova On February 13, 2015 @ 7:42 pm In Global Warming | Comments Disabled

Heatwaves are a wonderful headline generator. That’s because the term sounds scary, yet the “wave” itself is undefined. A hundred different types of heatwave are theoretically possible, but they all sound the same in a headline. It means an activist team could pick and choose the particular one that scores a “record”. Heatwaves can be 3, 4, 5, 7 or 10 days. They can be measured by town, city, state or national data and they are can be above 35, 37, 40 degrees or… pick a number. A heatwave can be measured as days above some percentile of average. That means a few warm days in a cold town can be defined as a heatwave.

Geoff Sherrington, drawing no dollars from the taxpayer, takes a simple and obvious approach, and looks at 5 capital cities with the BOM raw and adjusted ACORN data. He considered 4, 5, and 6 day heatwaves to see if there was a trend. With 5 cites, 2 data types, 3 lengths of heatwave, Sherrington created 30 graphs. After testing all those different combinations of heatwaves, there were only three graphs out of 30 that showed an increasing trend. Over half of the heatwave graphs showed no trend, and a third showed a cooling trend. This is not what the press headlines are telling Australia.

But are heatwaves becoming more frequent? Of the five capitals, only Perth shows a higher number of heatwaves in the last 60 years than in the first half of the last century. Adelaide had fewer heatwaves in recent times, and things stayed about the same in the other three capitals.

The hottest heatwaves in Perth were in 1933,  1956 and 1961. The hottest in Adelaide was 2009, then 1939. In Sydney, 1960 was a standout.  In Melbourne, 2009, 1959, and 1912. The hottest heatwave years in Hobart were 1994,  and 1955. The old raw records give the record to 1897.

If the BOM were scientists would they issue press releases saying that one particular definition of heatwave showed a warming trend without also mentioning that there were 20 other definitions of heatwave that didn’t show it?

If the ABC were journalists they would not parrot the unscientific BOM press releases without asking about the other forms of “heatwave” and the effect of adjustments.

  1. “Does this increase in heatwaves hold for other lengths and cutoffs of the definition of heatwave?
  2. Why don’t the BOM mention those types of heatwave? Don’t people in Adelaide, for example, have a right to know that they had more 4, 5, and 6 day heatwaves early last century?
  3. Why does it take an unpaid volunteer to tell the complete story on heatwaves when the Australian people pay the Bureau $300 million to do that?

Here’s the questions this blogger wants the ABC to answer:

  1. The ABC budget is $1.1b. Why does it take an unpaid volunteer to ask the questions the Australian public want to know?
  2. What is the ABC doing to make sure corruption, falling standards and confirmation bias are not destroying our most valued public institutions, for example, the BOM?

The Bureau of Meteorology was given $344m in the 2014-15 budget (Australian Dept of Environment, budget statement 2014). It suffered cuts of $10 million over 4 years. That’s a cut of less than 1%.

– Jo

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Are heatwaves in Australia becoming more frequent, hotter or lasting longer?

By: Geoffrey H Sherrington

Scientist

The hypothesis tested.

We test this hypothesis:

Heatwaves in Australia are becoming more frequent, hotter and are lasting longer because 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.

There is no settled definition of ‘heatwave’ yet. Australia’s Bureau of Meteorology, BOM, is currently creating more complex definitions, commonly in terms of 3 day heatwaves.

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.

There are two relevant data sets, both from the Australian Bureau of Meteorology (BOM) whose compilation of the historical record is acknowledged. The first set is the longer one, called CDO for Climate Data Online. This is essentially raw data as recorded. The second set is the BOM ACORN-SAT, or Acorn for short, which is an adjusted, homogenised set that usually commences in 1910. As time goes by, more announcements are made in Acorn terms, when sometimes it is more appropriate to use raw data, so we test both.

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:

The hypothesis is falsified for the cities tested.

That is, it is wrong to claim that in these important cities at least, there has been such a change of the characteristics of heatwaves as defined.

Findings in more detail

Here is one example of the 30 graphs used in the first stage of the analysis. The temperature data are from the BOM CDO for the Sydney Observatory station 66062.

Heatwaves, Sydney, Australia

To create this graph, the daily maximum temperature, after some minor infilling of missing data, was searched for every value of the hottest 4 consecutive days. Each 4 day average was ranked from hottest to coldest, then a Top 20 hottest selection was made, with any year appearing only once. (Rarely, some years have 2 Top 20 events, but we used only the hotter).

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:

 Trend
4 day 5 day 6 day
Perth CDO COOL COOL COOL
ACORN cool cool cool
Adelaide CDO ZERO ZERO WARM 3 cases out of 30 show warm
ACORN zero cool cool 10 cases out of 30 show zero
Melbourne CDO COOL COOL ZERO 17 cases out of 30 show cool
ACORN zero zero zero
Sydney CDO COOL WARM ZERO
ACORN zero warm warm
Hobart CDO COOL COOL COOL
ACORN cool zero cool

Test One: Are heatwaves becoming more frequent?

We answer this by counting the number of heatwaves in the first half and second half of the years of each chronological Top 20 data set. The longer, CDO data set is used. Here is a table to summarise the findings.

NUMBER OF HEATWAVES, CDO DATA SET
4 day 5 day 6 day
Perth 1897-1955 EARLY

4

1

1

1956-2013 LATER

16

19

19

Adelaide 1887-1950 EARLY

13

14

15

1951-2013 LATER

7

6

5

Melbourne 1856-1935 EARLY

8

10

10

1936-2013 LATER

12

10

10

Sydney 1859-1936 EARLY

6

8

8

1937-2013 LATER

14

12

12

Hobart 1882-1956 EARLY

11

10

10

1957-2013 LATER

9

10

10

Findings: There is a mixture of results depending on site location. Perth has more heatwaves since 1956, than before then. Adelaide has the opposite, with many more heatwaves before 1950, than after 1950.

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?

This is answered by trends of the heat wave Top 20 in chronological order.

Each chronological list was divided into the earliest 10 of the Top 20 years and the later 10 years. The average temperature of each early set of 10 was compared with the average of each later set of 10. This later number, for each of the 5 sites, was subtracted from the earlier number to show a temperature rise or fall over the history. A positive number is taken to mean that there is cooling over the years.

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.

Heating of Top 20
from early to late
SITE

4 DAY

5 DAY

6 DAY

PERTH Acor

0.27

0.60

0.87

ADELAIDE Acorn

0.12

0.34

0.53

MELBOURNE Acorn

0.34

0.02

0.46

SYDNEY Acorn

0.66

-0.46

0.01

HOBART Acorn

0.40

0.25

0.47

Test Three: Are heatwaves becoming longer?

We introduce 10 day heatwave calculations through graphs such as this one for Adelaide.

Adelaide, heatwaves, australia

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.

NUMBER OF
TOP 20 CASES
SITE EARLY HALF LATER HALF
Perth

1

19

Adelaide

12

8

Melbourne

6

14

Sydney

7

13

Hobart

20

0

The result is a mixed bag. This method of analysis suggests that later heatwaves have been dominantly longer in Perth, because only 1 case appears before the half-way point. Conversely, early heatwaves are dominant in Hobart, with not a strong signal in Adelaide, Melbourne or Sydney.

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.

SUPPLEMENTARY DATA

Excel spread sheets are available on request.

 Headline edited for clarity a few hours after posting.

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