Since 2010 I have been documenting problems with different versions of Australia’s official temperature record as produced by the Bureau of Meteorology (BOM). Since the High Quality (HQ) dataset was quietly withdrawn in 2012 we have seen regularly updated versions of the Australian Climate Observation Reference Network- Surface Air Temperature (ACORN-SAT or Acorn). We are now up to Version 2.2. In this Part I shall show the effect of these changes on temperature trends. In Part 2 I will show how record length affects trends, and in Part 3 I will look at the record since 1970 at some individual stations.
Figure 1 is from the BOM Climate Change Time Series page.
Figure 1: Australian Official Temperature Record 1910 to 2021
The linear trend is shown as +0.13℃ per decade, or 1.3C per 100 years. My colleague Chris Gillham of WAClimate has provided me with archived Acorn 1 annual mean temperature data to 2013 which allows this comparison:
Figure 2: TMean: Acorn 1 and Acorn 2
The result of introducing Acorn 2 has been a much steeper trend: Acorn 1 trend to 2013 was 0.9℃ per decade. The trend has now become 0.13℃ per decade. (The extra 9 years have added an extra 0.017C per decade to the trend.)
Figure 3 shows when and how large the changes were:
Figure 3: Difference between Acorn 1 and Acorn 2
Acorn2 is cooler than Acorn 1 before 1971 and warmer in all but three years since. Since these were based on the same raw temperatures (with some small additions of digitised data and a couple of changes to stations) the changes were brought about entirely by adjustments to the data.
I calculated running trends from every year to 2013 for both datasets. As trends shorter than 30 years become less reliable I truncated the running trends at 1984. Figure 4 compares thre trends to 2013 of Acorn 1 and Acorn 2.
Figure 4: Acorn 1 and Acorn 2 running trends per decade to 2013
The weather fluctuations of the mid-1970s to 1980s played havoc with trends.
Figure 5 shows the difference between the trends.
Figure 5: Difference between Acorn 1 and Acorn 2 Trends
The difference ranges from +0.024C per decade for 1910 to 2013, to +0.039C for 1950 to 2013. Having increased warming by from 0.25C to 0.4C per 100 years (just by making different adjustments) Acorn 2’s trend is much more alarming than Acorn 1’s.
Conclusion:
This is from the BOM’s explanation for Acorn:
“A panel of world-leading experts convened in Melbourne in 2011 to review the methods used in developing the dataset. It ranked the Bureau’s procedures and data analysis as amongst the best in the world. ‘The Panel is convinced that, as the world’s first national-scale homogenised dataset of daily temperatures, the ACORNSAT dataset will be of great national and international value. We encourage the Bureau to consider the dataset an important long-term national asset.’” ACORN-SAT International Peer Review Panel Report, 2011.
Acorn 1.0 was apparently such an important long-term asset that it was quickly superseded by Acorn 2 with a much more alarming trend.
Tags: Acorn, adjustments, Australia, bom, Bureau of Meteorology, temperature
kenskingdom 
April 8, 2022 at 8:09 pm
Ken
This is a piece of information to tuck away for when you are looking at the ozone layer or other variation factor for solar radiation.
Around May 1974, in Darwin, I and many other outside workers who were tanned very brown and never got sunburned, got blistered after about one and three-quarter hours exposure to the suns, from 10:30 to middayish.
We later discovered that the ozone layer measuring device at Darwin airport was knocked out by Cyclone Tracy 7 months later and was not replaced for 20 years.
We never figured out what the implications were.
April 9, 2022 at 12:03 pm
Interesting article. One wonders why there is a need to make changes which in reality have little to no value and are probably wrong anyway. One also wonders why ALL and ANY adjustment invariably leads to an INCREASE in the temperature and NEVER a reduction! This is actually further evidence that the whole thing is a cunningly manipulated hoax.
April 9, 2022 at 5:36 pm
It would be interesting to see the ACORN data compared to the raw data, including the data going back to 1880, using the high standard ‘colonial-supervised’ data. BoM has refused for a long time to use data earlier than when they were established, yet when they were first established they used that data to provide the Federal government with a report on the weather/climate for establishing Canberra and the ACT.
April 16, 2022 at 7:09 am
In the AOTR graph above, 2001 is shown above average and 2011 on average (mean temps). However, if you look at the yearly summaries written at the time, both were below the average mean. http://www.bom.gov.au/climate/current/annual/aus/2011/
1914 was originally 0.5C above average and has been adjusted down almost 0.4C.
April 22, 2022 at 6:44 am
Sorry – should read ‘adjusted down to 0.14C’. My bad.