No increase in warm nights or mild winters at Bathurst

Around the world news headlines have attributed the recent bushfires in the Blue Mountains to climate change. Climate change theory explains that the link between bushfires and carbon dioxide relates to very hot days – to an increase in maximum temperatures. But interestingly there has been no increase in hot days at Bathurst, a town with an exceptionally long temperature record just to the west of the Blue Mountains.

After I plotted data by way of three charts showing that maximum temperatures had not increased over recent years at Bathurst, it was suggested in the comment thread that climate change may not necessarily manifest as more hot days, but rather as “milder winters” and “an increase in overnight temperatures”. Indeed Professor Andy Pitman, from the Centre for Excellence for Climate System Science, University of New South Wales, had already explained to Australians on the ABC’s 7.30 Report that: “we’ve just gone through a winter that is unprecedentedly warm” and therefore we have an “environment particularly conducive to fire”.

The technical scientific literature, and global warming theory, clearly explains that the link between bushfires and carbon dioxide relates to very hot days – to an increase in maximum temperatures, not to mild winters. But I was nevertheless interested to see whether in fact this winter had been mild at Bathurst and if in fact there has been an increase in “overnight temperatures”, etcetera. Such change would manifest as a change in mean monthly minimum temperatures and minimum daily temperatures.

I used the official homogenized and adjusted data from the Bureau of Meteorology ACORN-SAT program for the Bathurst Agriculture Station to explore monthly mean minimum values from 1909 to 2012. In addition I calculated mean monthly values from the daily values available for 2013 from unadjusted data for this same site, also available for download from the Bureau of Meteorology. I also plotted daily minimum temperatures for another site, Bathurst airport, also available for download from the Bureau of Meteorology.

There is no obvious warming trend for minimum temperatures for June, July or August over the period 1910 to 2013 for Bathurst, Chart 4. Monthly mean minimum temperatures for June have varied between -2.5°C and 5.1°C, 1927 and 1991 respectively. The mean minimum June temperature this year was 2.7°C. Similarly the mean minimum monthly values for July and August this year are not unusually warm at 0.8°C and 1.5°C, respectively, Chart 4. Furthermore there is no trend towards warmer minimum temperatures over time that would be evident in this data if nights and/or winters were becoming warmer, Chart 4. Indeed there is no evidence to suggest that winters are becoming warmer or that overnight temperatures are increasing.

It has also been claimed that this September was unusually warm; in particular the Bureau of Meteorology recently put out a special Climate Statement entitled ‘Australia’s warmest September on record’. This statement is based on the compilation of selected data from carefully chosen sites. Interestingly at Bathurst, considering just the mean minimum data, the warmest September on record with a mean minimum of 7°C occurred in 1921. This year the mean minimum temperature for September for Bathurst was 3.5°C, Chart 5. Neither the mean monthly minimum temperatures for September nor October show a warming trend, Chart 5. It could be argued, that because of the usually warm minimum temperatures in Septembers in the early 1920s, minimum September temperatures have cooled slightly over the last 100 years, Chart 5.

A plot of daily minimum temperatures for Bathurst airport from January 1, 1990, until October 27, 2013, also shows that there is no warming trend, Chart 6. Indeed, considering the 365-day moving average, it could be argued that average minimum temperatures have started to trend down over recent years. This is not consistent with popular claims that overnight temperatures are increasing, or the claim by Professor Pitman that winter temperatures are increasing, but rather the converse, that there has been some recent cooling.

Bathurst represents just one locality, but those who are familiar with temperature data for particular localities along the Australian east coast would know that the trends I report here for Bathurst are not atypical.  Indeed an assessment of minimum monthly temperatures for 21 localities in Australia with particularly long temperature records, and with data where dew point and minimum temperature averages correspond post 1957, shows a similar trend with a spike in the early 1920s, Chart 7.