I have recently made a superficial analysis of temperature trends at York, Western Australia, the nearest weather station to my place at Gwambygine. York is approximately 100 kms inland from the Indian Ocean, on about latitude 32.
The weather data for York is interesting for two reasons: (i) there has been a continuously reporting weather station here since 1877; and (ii) in 1996 the station was relocated from the rear of the Post Office in the centre of town to a farm paddock two kilometres away. The Bureau of Meteorology (BoM) publishes separate weather data for each site. Thus it is possible to compare mean daily max and min temperatures for the period 1877-1995 with those for the period 1996-2006. (The 2007 data has not yet been published).
I found that the mean daily maximum temperature for the period 1996-2007 was 0.6 degrees warmer than the mean daily temperature over the previous 119 years. However, the mean daily minimum temperature for the decade 1996-2007 was 1.0 degree cooler than for the previous 119 years. This suggests that on average, overall, York has been marginally colder since 1996. In any case there is no evidence of “catastrophic warming” for this site.
Without the actual data (which is not freely available), it is impossible to test the statistical significance of these differences. In any case, I consider it more likely that any differences are due to the relocation of the weather station. The old Post Office site was surrounded by high stone walls and heat-absorbing/retaining brick buildings and car parks, whereas the new site is beyond the town in an open paddock.
I wrote to the BoM for comments on my analysis. In reply they presented a graph showing annual maximum and minimum temperature trends with a running 11-year mean combining both weather staions for York for the period 1910 to 2006. These reveal a roughly 1 degree increase in annual maximum temperature over the last 96 years and a roughly 0.3 degree increase in annual minimum temperature.
I wrote back to the BoM and asked why they chose 1910 as the starting point for their analysis. Their interesting reply was:
“A change in the type of thermometer shelter used at many Australian observation sites in the early 20th century resulted in a sudden drop in recorded temperatures which is entirely spurious. It is for this reason that these early data (pre-1910) are currently not used by the Bureau in monitoring climate change.”
I would be interested if anyone could refer me to an authoritative paper on the history, quality and anomalies in Australian weather records and the influence of the re-location of weather stations. I am aware, for example, that the Perth Western Australia weather station has been re-located at least three times over the years, each time to an area with an obviously different microclimate. How is this taken into account in determining real long term trends? And are there other key sites in the historical record for which temperature records have been artificially influenced by changes to thermometer shelters, or other technical aspects.
Roger Underwood is a former General Manager of the Department of Conservation and Land Management (CALM) in Western Australia, a regional and district manager, a research manager and bushfire specialist. Roger currently directs a consultancy practice with a focus on bushfire management. He lives in Perth, Western Australia.