Ken Stewart, 28 January 2012
In March 2011 I analysed trends in Australian Maximum Temperatures from 1910-2010, and now I am following up with Minima. Since that time I have looked at adjustments made by the Bureau of Meteorology (BOM) and in a paper co-written by Dr David Stockwell proposed an alternative network of climate sites we call the Minimally Adjusted Network (MAN). This network comprises sites included in BOM’s High Quality (HQ) network but makes adjustments by comparison of overlapping raw data only.
I have further revised this network by removing all sites where there is less than two years of overlapping data, where there is less than 80 years of data, where the record shows spurious data, or where metadata supplied by BOM indicates serious problems. I have deliberately included all urban sites including all capital cities except Darwin, usually at a nearby airport. The MAN dataset for Minima now comprises 89 sites.
There is good overlap of 10 years from 1943 to 1952. The graph shows Cairns Airport minima are warmer than the Post Office, and in fact the mean difference is 0.29 degrees. Therefore the Post Office raw data may be adjusted up +0.29 degrees so that the two series are comparable, to give this graph of the spliced data:
No assumptions are made about the data, and no subjective decisions are made about whether to adjust and by how much, unlike the adjustments used in BOM’s HQ datasets. No other adjustments are made. If a site meets the criteria but the metadata indicates serious problems (e.g. Nhill) or the data appears to be spurious (e.g. Halls Creek), it is excluded. As more metadata becomes available it may be that further sites will be excluded.
To compare temperature across Australia I convert all data to anomalies from the 1961 to 1990 mean for each site. I then calculate the mean anomaly for each year for a selection of sites. While I show data from 1910 to 2011, much 2011 data has still not been “quality assured”, so many annual means were not available. For these sites I found the annual mean from the daily means, which is not the correct method, but is close.
Caveat: There is considerable uncertainty in all temperature data. BOM says that Automatic Weather Station (AWS) data has an accuracy of +/- 0.3C. Celsius thermometers (mercury in glass) have an accuracy of at best +/- 0.5C. Fahrenheit thermometers, certainly no more accurate, were used before September 1972 and temperatures recorded before then were converted to Celsius for modern comparison. The records show that in the Fahrenheit era, many observers used whole degrees even though BOM’s instructions to observers were to record temperature in tenths of a degree. That’s hard enough on a Celsius max/min thermometer. A number of quite recent observations appear to be recorded in whole degrees. The change to Celsius in 1972 may have led to an artificial warming relative to pre 1972. Certainly the Australian temperature record must be treated with great caution. Therefore my figures are subject to the same uncertainty.
While Australian temperatures have been rising, it has been by no means a steady increase. There was an enormous jump up in the 1970s. The 4th order polynomial trend line shows the distinct fall and rise. Are we following a natural cycle? The next few years will be decisive.
What’s with WA?
WA is clearly a different climate zone.
City vs Country
Not such a great difference. Urban sites are warming slightly faster- UHI?- cooler up to 1950s, and in the 1990s, and are a little more variable, which is surprising.
Beach or Bush
Sites within 100km of the coast have much less variability than inland sites, due to the oceanic influence. Inland sites have greater extremes, but the overall trends are very similar, except that coastal sites are getting warmer, whereas inland sites’ extreme peaks have been decreasing since 1973. The extremes are at identical times, but the standout differences are 1973, 1976, and of course 2011. Note the late 1970s step up is very clear.
North and South
The boundary line is the South Australia/ Northern Territory border. First southern Australia:
Wildly different! Note the warmest year by far was 1998, and 2011 was colder than any year since the 1970s. Now let’s combine:
Note that northern minima- further from the South Pole- are warming faster than the southern. The north has much greater extremes and appears to have peaked.
Maxima and Minima
I have plotted Maxima and Minima only to 2010.
While the linear trend for Minima is greater than for Maxima, and after initially decreasing, Minima anomalies have been rising since the 1940s, in the past decade (“the hottest on record”), minima have flattened. The record means have been caused by much higher maxima. This is especially visible in a plot of the annual Max-Min range, which shows the linear trend steadily decreasing as is expected with the Greenhouse Effect. However, the range has increased steeply since the 1980s which is not what is expected. This is not the signature of Anthropogenic Global Warming, at least in Australia.
The Minimally Adjusted Network provides an alternative network of 89 long term sites with few adjustments to the raw data. Uncertainties in the data accuracy of at least +/- 0.3C mean that the temperature record must be viewed cautiously.
There is little difference between Urban and Non-Urban minima.
Western Australia has a completely different climate to the rest of Australia.
Coastal and Inland trends are not greatly different, except for variability, until 2011.
Much can be learned from looking at Means, but an analysis of Minima yields further interesting information. Remember: Anthropogenic Global Warming due largely to the greenhouse effect should be visible in Minima. Temperature increase should be greater at night, in winter, and towards the poles. It is plain, however, that in Australia this is not the case.
Since the 1940s, very cold years have been getting progressively milder, but since 1973 very mild years have been getting progressively cooler. Further, the increase in minima is more pronounced in northern Australia than in the south- and while southern Australian minima have been steadily increasing, northern minima have a much greater variation and appear to be now decreasing. The Range bewteen Maxima and Minima does not show the greenhouse Effect signal. My guess is that El Nino/ La Nina conditions have a very large influence on northern Australian (largely Queensland) minima and consequently Australian minima.
The huge variations we see from the 1970s to about 2000 have decreased. The huge step up of the 1970s (The Great Climate Shift) is clearly visible in the minima, but the next few years will show if it has run its course. I expect it has.
And finally, 2011 was not particularly unusual- except in inland and northern Australia where it was the coldest since 1994.
Appendix: Minimally Adjusted Network
|Longreach Air||Rottnest Island|
|Rockhampton Air||Cape Leeuwin|
|Sandy Cape Lighthouse||Jarrahwood|
|Cape Moreton Lighthouse||Katanning|
|Launceston Ti tree||Tamworth|
|Cape Otway Lighthouse||Newcastle|
|Wilsons Promontory Lighthouse||Bowral|
|Gabo Island Lighthouse||Echuca|