Bureau of Meteorology ‘cools the past, warms present’

The Bureau of Meteorology has remodelled Australia’s official temperature record for the third time in nine years and found things to be warmer than readings had measured.

A series of updates to the ACORN data have added 0.228C mean temperature warming if comparing 1910-19 with 2010-17. Picture: AFP

The Bureau of Meteorology has remodelled Australia’s official temperature record for the third time in nine years and found things to be warmer than thermometer readings had measured.

The latest revisions to records at 25 Australian Climate Observations Reference Network – Surface Air Temperature (ACORN-SAT) weather stations has resulted in a slightly greater ­increase in both average maximum and minimum temperatures.

The bureau did not announce the changes but details of them were published on the bureau’s website.

A bureau spokesman said ACORN-SAT was the dataset used to monitor long-term temperature trends in Australia. “Each year, the Bureau of Meteorology updates the dataset to include the most recent data from that year,” the spokesman said.

The last update, ACORN-SAT version 2.2, was made publicly available online in December. Independent analysis of the latest changes show they added 0.06C to maximum warming and 0.11C to minimum warming from 1910-19 to 2010-19.

A series of updates to the ACORN data have added 0.228C mean temperature warming if comparing 1910-19 with 2010-17 (2017 being the final year of ACORN 1).

Researcher and journalist, Chris Gillham, said the impact of adjustments to ACORN versions 1, 2, 2.1 and 2.2 was to cool the past and warm the present. He said the latest changes were “not overall large changes, but changes nevertheless”.

The bureau declined to comment on independent analysis of data it had published. But in an addition to its ACORN website page, it said adjustments had been made to temperature records at 25 sites.

Adelaide and Sydney records were altered due to changes in ­location of recording equipment.

The bureau said changes were made to 20 sites on the basis of statistical analysis. According to the bureau, statistical analysis is used to identify an abrupt warming or cooling at a particular site, relative to other sites in the region.

“A significant change relative to other sites indicates a non-­climatic driver, which sometimes has an easily identifiable cause (e.g. a new building near a site) and sometimes does not (often these will relate to local vegetation or land surface changes)”, the bureau said. “In carrying out this statistical analysis, the bureau uses 10 years’ worth of data from multiple sites to quantify the size of the change. Adjustments are only applied where a significant change has been identified. BoM said the latest adjustments “have not altered the long term warming trend in Australia”.

Adjustments are applied to all data prior to the date of the change.

Scientist Jennifer Marohasy, an outspoken critic of the bureau’s homogenisation process, said: “I have shown repeatedly, ­including in peer-reviewed publications, that without scientific justification historical temperatures are dropped down, cooling the past. This has the effect of making the present appear hotter – it is a way of generating more global warming for the same weather.”

Dr Marohasy said the daily maximum and minimum values in the national temperature dataset were different from the actual recorded historical value, often by several degrees, usually cooler.

“The bureau has now remodelled the national temperature ­dataset three times in just nine years,” Dr Marohasy said.

“Most recently in December 2021, but without giving any indication of how this will affect the overall trend as reported each year in the Annual Climate Statement.”

In a media release on January 6, BoM said that in 2021, Australia’s mean temperature was 0.56C above the 1961 to 1990 climate reference period. It was the 19th warmest year since national records began in 1910, but also the coolest year since 2012.

The bureau says its analysis methods for ACORN-SAT have been published in international peer-reviewed journals and subject to external reviews in 2011, 2015 and 2018. “These external reviews expressed overall confidence in the bureau’s practices and found its data and analysis methods to be among the best in the world”, a spokesman said.