UPDATED, Ken has now finished the full tally of comparisons and the adjustments to minima increase trends by 47% . (Headline changed from 60% to 50% to reflect the shift.) See the new details of the last few stations at KensKingdom.
Billions of dollars, climate models, predictions, and hundreds of press releases depend on the BOM records of Australian temperatures. There were so many inconsistencies, inexplicable adjustments and errors that we put in a Senate request for the ANAO to audit the records. In response, to dodge the audit, the BOM dumped its HQ (“high quality”) dataset entirely, and established a new “best practise” ACORN dataset.
Independent volunteer auditors have been going through the ACORN records — thanks especially to Ken Stewart who is publishing his findings on his site as he works through the set. He’s analyzed 84 out of 104 sites, and finds that ACORN is just as bad as the HQ set. At Kenskingdom he shows that so far, the adjustments used to create the official Australian temperature record increase the warming trend by13% for maxima and a whopping 66% for minima. (Note the caveats in the conclusions below.)
The raw Australian data suggest the nation warmed by 0.6 °C over the last century. The BOM adjustments lift that to 1.05 °C.
The BOM wants the Australian public to think it is impartial, neutral and honest
Some adjustments are necessary. Perhaps these are, but the BOM does not explain on a station by station basis why they are justified. The BOM also claim their adjustments are neutral but a simple comparison with the raw records shows the adjustments themselves create a large part of the warming trend.
The first time around a bias in adjustments could have been inadvertent. But after critics pointed out the inexplicable bias, and the dataset was redone, the BOM issued a carefully crafted wording, that was too-clever-by-half.
For the old HQ set, the BOM said the adjustments were neutral. But contrary to what the head of the BOM said, Ken Stewart found they increased the trend by 40%.
Dr David Jones, Head of Climate Monitoring and Prediction, National Climate Centre, Bureau of Meteorology, stated clearly that the adjustments made “a near zero impact on the all Australian temperature”.
For the new ACORN set, the official CAWCR Technical Report No. 04 carefully wordsmiths their position on adjustments so it is technically true, but misleading at the same time:
“There is an approximate balance between positive and negative adjustments for maximum temperature but a weak tendency towards a predominance of negative adjustments (54% compared with 46% positive) for minimum temperature.”
The number of positive versus negative adjustments is not what matters. What matters is the change to the trend. The size of the positive adjustments is a lot larger than the negative ones. It’s not balanced at all.
A few examples of unexplained adjustments:
Ken Stewart has analyzed 83 ACORN sites of the total 104 sites.
Figure 4 shows my plot of annual mean minima for the same period (calculated as a straight average- the BOM graph is area averaged) for 83 sites ‘raw’ compared with Acorn.
Figure 4:
The average difference between raw and adjusted records shows a strange pattern (see Figure 5 below). Apparently the thermometers before 1971 were recording temperatures that were overestimating temperatures. Generally the better modern thermometers after that were underestimating the temperatures. What bad luck!
In addition, the urban population grew, cities retained heat, and airports got more traffic and larger tarmacs. It makes sense (if you are bonkers) that those thermometers near hotter bigger buildings and more bitumen would shift from a “warm bias” to a “cold bias” right? Really…
Note: the ‘raw’ trend is +0.63 °C per 100 years. The adjusted trend is +1.05 °C. The difference of +0.42 °C represents an increase of 66.6 %.
Figure 5 is a plot of the annual average difference in temperatures.
Note there is only one year (1959) before 1971 that has on average greater positive adjustments, and there are no years after 1971 of average adjustments below zero. In other words, the record before 1971 is cooled and after 1971 it is warmed.
The maxima anomalies also got warmer, but not by much — though we note they start in 1910 — the coldest year in many places around the world. (I’ll say more on that soon).
Figure 8: Annual mean maxima for 84 sites (‘raw’ vs Acorn)
Acorn adjustments have increased the maxima warming trend by +0.09C, or 13 %.
Amberley in Queensland
Conclusion
While the number of positive and negative adjustments made by the creators of Acorn may be balanced or nearly so, their effect on the minimum temperature record is enormous. Analysis of a not insignificant sample of 83 of 104 Acorn sites shows a warming bias in adjustments to minima of 45 %, which has the effect of increasing the network-wide temperature trend by 66.6 %. The adjustments have predominantly cooled pre-1971 temperatures and warmed post-1971 temperatures. For maxima, the increase in trend is 13 %. This result casts doubt on the veracity of the Acorn temperature record, and its usefulness for climate analysis.
Please note: I make no judgement about the justification or lack of it for the individual adjustments. Nor am I claiming that my calculation of +0.63 °C per 100 years is the true trend in minima for Australia. Far from it: that figure is based on only 83 stations, not evenly distributed, many of which have much less than 100 years of data and/or many years of missing data. I’m saying no one knows for sure, but that the adjustments to the ‘raw’ data at CDO, in order to create the Acorn dataset, result in a massive and unexplained difference.
I welcome any comments or arguments that can show how I may remove errors from this finding, or how I may improve my analysis.
The bottom line is that obviously Australian temperatures have risen in the last century, but by how much? The trends are fed into global climate models, they’re used to estimate climate sensitivity, and forward projections are based in part on previous rises. Then there is the constant hammer of press releases trumpeting headlines with “records” that sometimes depend entirely on the adjustments.
It is time it was audited properly.
Posts on Australian Temperatures