The temperature warming trend in Darwin has been increased by a further 0.5C in the Bureau of Meteorology’s latest review of climate data, reigniting debate about a site long considered a “smoking gun” in how weather agencies treat historic records.
The Darwin record has been the subject of international concern by sceptics because it plays a significant role in the global climate picture.
The Weekend Australian revealed last Saturday that the bureau had rewritten Australia’s temperature records for the second time in six years, increasing the rate of warming since 1910 from 1C to 1.23C. The bureau has defended its homogenisation processes, which it said were needed to account for non-climate influences and changes in equipment.
Scientist Jennifer Marohasy said Darwin’s temperature record was important because the city was the only location in central northern Australia where temperatures had been measured since 1895 from within an instrument shelter. The Darwin record includes temperatures taken at Darwin post office from 1882 until 1941 and from Darwin airport from 1942 to the present.
Adjustments were made by the bureau to account for the site move and other factors, including shading from trees, that occurred after 1937. The bureau record was shortened to 1910.
Research by Dr Marohasy contends that cooling was not due to shading, but rather a cyclone in 1937 clearing vegetation that had screened the post office from the sea breeze. She said it was inappropriate to reduce all preceding temperatures as a result.
Dr Marohasy created a new minimally homogenised maximum temperature series for Darwin, correcting only for the move to the airport. This showed no overall cooling or warming trend for 1895 to 2014, consistent with the trend for Richmond in northwestern Queensland, which is the nearest site with a long, continuous temperature record.
A paper on the treatment of the bureau’s Darwin data by Dr Marohasy was accepted for publication in one of the world’s premier meteorological journals, but was pulled at the last minute.
The bureau has not accepted any challenge to its original homogenisation decisions.
“For the case of Darwin, a downward adjustment to older records is applied to account for differences between the older sites and the current site, and differences between older thermometers and the current automated sensor,’’ a spokesman said.
“In other words, the adjustments estimate what historical temperatures would look like if they were recorded with today’s equipment at the current site.”
Dr Marohasy said many would claim the raw record for Darwin must be wrong because it showed a general trend of cooling to about 1950, and then warming.