The Australian Bureau of Meteorology (BOM) revises the raw temperature data it collects from its weather stations. The process—which the BOM calls 'homogenisation'—adjusts the temperature readings at a given location to take into account nearby measurements. The homogenised temperatures form the ACORN-SAT dataset on which all official reporting on climate change is based.
Due to the lack of transparency surrounding the raw data and the adjustments that are made to them, homogenisation has proved to be highly controversial.
Tom Quirk, an AEF Director, has sought to throw light on the issue by comparing satellite temperature measurements of the lower troposphere over Australia with the near-surface temperatures published by the Australian Bureau of Meteorology (BOM) for the period from 1979 to 2017. The satellite data were sourced from the University of Alabama in Huntsville (UAH) in the US. To account for differences due to altitude in these two datasets, Tom used standardised differences in the temperature anomaly—the difference between the temperature recorded at a particular time and the long-term average for the place in question.
Tom found a significant loss of correlation between the UAH and BOM temperature series in the late 1990s. This was when automatic weather stations with electronic temperature measurement replaced manned stations, which used mercury and spirit thermometers. The sources of the temperature differences are the instrument changes and particularly the step-changes in temperature adjustments introduced as part of the homogenisation process.