This last summer has been hot in south-eastern Australia. But was it the hottest ever? Summer 80 years ago was arguably as hot, if not hotter.

Australia’s Environment Minister, Melissa Price, also recently claimed this summer’s bushfires as a consequence of climate change. I grew up with stories from my late father of terrible bushfires – infernos – back in 1939. The Black Friday firestorm of 13 January 1939 destroyed four times the area of farmland and forest as the devastating February 2009 fires – and twenty times as much as burnt this last summer.

But it is actually now near impossible to know which summer was the hottest ever summer – because of the extensive remodelling of our temperature history.

The extensive remodelling is not denied by the Australian Bureau of Meteorology. Rather it is justified on the basis that temperatures are now measured using a non-standard method (spot readings) from non-standard equipment (custom built probes in automatic weather stations). Apparently, we need to know how hot it was back then, relative to the equipment used now – so temperature are remodelled. To be clear, there are three factors that potentially confound how hot it was back then – or now: the equipment, how it is used, and the remodelling, which is often referred to as homogenisation.

The largest single change in the new ACORN-SAT Version 2 temperature database is a drop of more than 13 degrees Celsius at the town of Wagga on 27 November 1946.

But let’s begin with Rutherglen. The Rutherglen agricultural research station has one of the longest, continuous, temperature records for anywhere in rural Victoria. Minimum and maximum temperatures were first recorded at Rutherglen using standard and calibrated equipment back in November 1912. Considering the first 85 years of summer temperatures – unadjusted/not homogenized – the very hottest summer on record at Rutherglen is the summer of 1938/1939.

While this last summer of 2018/2019 was hotter according to Minister Price, such a claim would not pass scrutiny if assessed for the Guinness Book of records – because of all the changes to the way temperatures are now measured at Rutherglen relative to back in 1938/1939.

At Rutherglen, the first big change happened 29 January 1998. That is when the mercury and alcohol thermometers were replaced with an electronic probe – custom built to the Australian Bureau of Meteorology’s own standard, with the specifications still yet to be made public.

According to Bureau policy, when such a major equipment change occurs there should be at least three years (preferably five) of overlapping/parallel temperature recordings, except the mercury and alcohol thermometers (used to measure maximum and minimum temperatures, respectively) were removed on exactly the same day the custom-built probe was placed into the Stevenson screen at Rutherglen, in direct contravention of this policy.

In 2011, the Bureau made further changes in that it stopped averaging one-second readings from the probe at Rutherglen over one minute. The maximum temperature as recorded each day at Rutherglen is now the highest one-second spot reading from the custom-built probe. That is correct – spot reading.

So, to reiterate, we now have a non-standard method of measuring (spot readings) from non-standard equipment (custom-built probes) making it impossible to establish the equivalence of recent temperatures from Rutherglen – or any of the Bureau’s other 695 probes in automatic weather stations spread across the landmass of Australia – with historical data.

Then there is the remodelling – with the most recent remodelling creating Version 2 of ACORN-SAT. This has resulted in an overall 23 per cent increase in the rate of warming between Versions 1 and 2 for the 112 weather stations that comprise ACORN-SAT. This is the database used by the Bureau and the CSIRO to monitor climate change across Australia.

At Rutherglen, a modest rate of warming in the raw maximum temperatures of 0.7 degrees Celsius per Century has been changed to 1.3 degrees Celsius in ACORN-SAT Version 2. Changes to the minimum temperature trend are more dramatic: a slight cooling trend of 0.3 degrees Celsius has been changed to warming of 1.9 degrees in ACORN-SAT Version 2 for Rutherglen.

This remodelling – known as homogenisation – involves the detection of discontinuities and then adjustments which generally result in past temperatures being cooled relative to the present. By cooling the past, present temperatures appear hotter. For example, considering maximum temperatures at Rutherglen, the largest single drop-down (adjustment) to daily temperatures occurs from 1 January 1938 back in time. The Bureau classifies the hot summer of 1938/1939 as a ‘discontinuity’ that is ‘statistical’ in ‘cause’ and then cools all the days before 31 December 1938 by 0.6 degrees Celsius back to 1912 – the beginning of the record.

To repeat, the Bureau does not deny making these changes. Rather it claims such changes to Rutherglen’s temperature history are necessary to show what the temperature would be back then, using today’s equipment. But. There was no actual change in the equipment between versions 1 and 2 of ACORN-SAT for Rutherglen. So, this reason could not actually be considered reasonable.

So why did, for example, the Bureau drop the minimum daily temperatures by a further 2.6 degrees Celsius on the day of the Black Friday bushfire? To be clear, the minimum temperature on the day of the Black Friday bushfire at Rutherglen was measured as 28.3 degrees Celsius. This value is changed to 27.8 degrees Celsius in ACORN-SAT Version 1, a reduction of 0.5 degrees Celsius. In Version 2, the temperature is reduced further, now archived as just 25.7 degrees Celsius for 13 January 1939 – a reduction of 2.6 degrees from the original temperature as actually recorded on that day.

There is a real history of rural Victoria: 71 men and women perished in that bushfire back on 13 January 1939. According to my late father, it was extraordinarily hot.

The Bureau has never put a media release out letting the Australian public know that there is a Version 2 of ACORN-SAT, with even cooler historical temperatures for Rutherglen and most of the rest of Australia than in Version 1 that was only published in 2012.

Just a few years ago, the minister then responsible for the Bureau, Greg Hunt, was claiming that ACORN-SAT Version 1 was the world’s best practice and the correct temperature history of Australia.

Just to the north of Rutherglen is Wagga, and the largest single cooling of any temperature in ACORN-SAT Version 2 was made to this temperature record. Specifically, on 27 November 1946, the minimum temperature of 21 degrees Celsius in ACORN-SAT Version 1 is changed to just 7.6 degrees Celsius in Version 2. This is a drop-down (a cooling of the past) of 13.4 degrees Celsius for a single day.

A temperature probe replaced the mercury and alcohol thermometers at Wagga on 1 November 1996. There was another equipment change on 10 January 2001, when the small Stevenson screen was replaced with a larger screen.

There have been no changes to the site or the equipment since then, since 2001. Yet there is a further overall one-degree increase in the rate of warming at Wagga between the Version 1 (published in 2012) and Version 2 of ACORN-SAT.

Not only does the Bureau somewhat arbitrarily appear to increase the rate of warming, but it also makes-up/invents 32 years of temperature recordings for Wagga Wagga airport.

The first temperatures ever recorded at this official ACORN-SAT bureau weather station (number 072150) were in January 1942. Yet the homogenized ACORN-SAT series for Wagga airport begins on 1 January 1910. This is done by joining the Wagga airport with another temperature series (number 072151), and then homogenising with data from other weather stations including numbers 74114, 73038, 73127, 73019, 72023,72000, 73009, 75028, etcetera. The pattern and trend in temperatures in the homogenized temperature series (ACORN-SAT Versions 1 and 2) for Wagga bear no resemblance to the original temperature measurements.

The remodelling by the Bureau is industrial-scale: this is necessary to generate a consistent global warming trend that does not exist in the raw unhomogenized data.

My late father was eight years old and living not far from Rutherglen on 13 January 1939. He remembered the hot wind blowing from the north-west on that day. I grew up with his memories of that time. My father described hot and hungry years – just as John Steinbeck described farm life in the mid-west of the US in the 1930s in his famous ‘Grapes of Wrath’. There was hardship, and there were dust storms in the US and also in south-eastern Australia.

Indeed, in rural Victoria, the summer of 1938-1939 was on average at least two degrees hotter than anything measured with equivalent equipment since.

Yet Minister Price denies this history – my late father’s history.

There are consequences for future generations in this remodelling. It affects how we understand the relationship between climate and bushfires. Also, by continually reducing past temperatures, there is potential for new record hot days, record hot summers and hottest years for even cooler weather. This is nonsense – consistent with how the Bureau now measures, archives and remodels our temperature history.

Jennifer Marohasy is a Senior Fellow at the Institute of Public Affairs and blogs at jennifermarohasy.com, where this piece also appears.

Illustration: Bureau of Meteorology.

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