Darwin warming claim triggers challenge to BoM

Bureau of Meteorology’s decision to increase temperature warming trend in Darwin reignites debate over climate ‘smoking gun’.

Scientist Jennifer Marohasy.

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.

“Yet this is actually what most high-quality records, particularly from rural northern and eastern Australia, show,’’ she said. “This also corresponds with what was a widespread fear of global cooling until about 1960.”

The bureau said the adjustment of historical temperatures in its ACORN-SAT dataset did not suggest records at any point in time were too high or too low. “Both upwards and downwards adjustments have been made at many ACORN-SAT sites to ensur­e past observations are consistent with modern conditions,” the bureau spokesman said.