A quirk or two in the Bureau of Meteorology database nearly set fire to the global warming debate this morning.

A member of the Weatherzone forums noticed that the historic high temperature reading for the Bathurst Agricultural College of 41.2 C on 11 January 1939 had been replaced by a reading of 40.1 in 2004.

This touched on a much gnawed bone of contention in the climate change debate as to whether claims of all time record highs in the 90s and noughties were ignoring the truth of inconvenient historic highs in the 30s or earlier decades of the last century in other parts of the world.

The short answer is “No”, or certainly not in Australia, but there was an interesting side bar to that when Blair Trewin, BOM climatologist and records custodian was asked what was going on.

Trewin explained that the bureau is engaged in a long term project to digitise all of its records, but that in many cases, it works back from the current date.

And in the case of Bathurst AG, only back to 1966 thus far, making the 2004 reading appear on the digital database as the hottest on record.

He said the 1939 historic record would be identified as the hottest reading for the site yet taken when it was picked up by the digitisation process, unless of course, a hotter reading is recorded before that work is completed.

Trewin also said “I don’t think anyone seriously disputes that the January 1939 event still stands as the most significant individual heatwave on record in most of SE Australia.”

He says some older historic high readings have been scrubbed because they were made using dubious procedures, including thermometers mounted on beer crates, or inside houses, or placed where heat radiating up from the ground could increase the reading.

A long term experiment at Adelaide showed that the type of instruments used in colonial times exaggerated maximums by 1-1.5 C compared to the Stevenson screens used on official weather stations since around 1910.

(The writer is a member of the Weatherzone forums.)