The Bureau of Meteorology is one of Australia’s most important national public institutions; so much depends on our knowledge of past climate and our ability to predict future trends and events. The Bureau’s recordings of maximum and minimum temperatures across Australia provide information that is important to our understanding of such trends and patterns in a changing climate. The Bureau does not, however, simply report this information. Rather, complex remodelling is undertaken in the development and compilation of the official climate change statistics. In this chapter, I consider the results of the Bureau’s ‘homogenisations’ of historic temperature measurements through two case studies: Darwin and Rutherglen.
The Bureau undertook some remodelling in 2018 that increased the overall rate of warming by 23% between Versions 1 and 2 of the Australian Climate Observations Reference Network − Surface Air Temperature (ACORN-SAT) database (Trewin 2018). The Bureau claims that this remodelling of temperatures is justified for two reasons: because of changes to the equipment used to record temperatures; and because of the relocation of the weather stations. However, there have been no changes to equipment and no relocations since the release of ACORN-SAT Version 1 for either Darwin or Rutherglen.
Case study: Rutherglen
At an agricultural research station near Rutherglen in south-eastern Australia, maximum and minimum temperatures have been recorded since November 1912 in a Stevenson screen in a paddock, as shown in Figure 16.1. This is an official Bureau of Meteorology weather station with values used to calculate official statistics. The trend in the raw minimum temperatures is for a slight cooling, which is a consequence of land-use change: specifically, the staged introduction of irrigation into the region for cropping, vineyards and orchards (Marohasy 2016).
In 2014, Graham Lloyd, Environmental Reporter at The Australian, quoting me, explained how the cooling trend in the minimum temperature record at Rutherglen had been changed into a warming trend by the Bureau by progressively reducing temperatures from 1973 back to 1912, as shown in Figure 16.2. For the year 1913, there was a large difference of 1.8 °C between the mean annual minimum temperature, as measured at Rutherglen using standard equipment at this official weather station, and the remodelled ACORN-SAT Version 1 temperature. Remodelling the data to cooling the past relative to the present in this way has the general effect of making the present appear hotter.
The Bureau responded to Lloyd, claiming the changes were necessary because the weather recording equipment had been moved between paddocks. This is not a logical explanation where the local terrain is flat and, furthermore, the official ACORN-SAT catalogue clearly states that there has never been a site move (Bureau of Meteorology 2012).
Nevertheless, readers might want to give the Bureau the benefit of the doubt and let them make a single set of changes. But just six years later, the Bureau again changed the temperature record for Rutherglen. In Version 2 of ACORN-SAT, the minimum temperatures as recorded before 1970 at the Rutherglen Research Station have been further reduced, making the present appear even warmer relative to the past. The warming trend is now 1.9 °C per century.
The text and figures above are drawn from the first part of “Rewriting Australia’s Temperature History,” Chapter 16 of Climate Change The Facts 2020. A PDF of the chapter can be downloaded here. The entire book is available for purchase here.