This morning (Saturday 23 August) the Weekend Australian published articles by Graham Lloyd, their Environment Editor, on homogenisation practices at the Bureau of Meteorology as questioned by Jennifer Marohasy. As I had a small part to play in bringing this to public light, here is a brief post to bring readers up to date.
The last paragraph in the second article reads:
“And the bureau says an extensive study has found homogeneity adjustments have little impact on national trends and changes in temperature extremes.”
This is laughable. Here is a graph of the national means of Raw and Homogenised minima data from 83 sites (out of 104) that I was able to compare directly. (I also analysed the remaining sites, finding 47% bias, but because large slabs of data had to be left out this is not reliable.)
Fig. 1: Australian mean minimum temperature anomalies 1910-2012
The ‘raw’ trend is +0.63C per 100 years. The adjusted trend is +1.05C. The effect of the homogenisation adjustments is an increase in the national trend of +0.42C or 66.6%. So much for “little impact.”
The article referred mainly to adjustments at Amberley and Rutherglen.
Fig. 2: Amberley minima
According to the BOM, the major adjustment was due to a pronounced discontinuity around 1980, that is, Amberley’s drop in temperature is not reflected in those of neighbouring sites, as is evidently correct.
Fig. 3: Amberley compared with the mean of 5 Acorn neighbours
However, the nearest Acorn site only 50km away, Brisbane Aero, also has a pronounced cooling trend, and a local cooling cannot be discounted.
An adjustment to the raw data before 1980 may be warranted, however, the size of the adjustment is questionable to say the least. The resulting trend at Amberley has now become greater than the trend of adjusted data at every one of the Acorn neighbours, and more than +0.86C greater than their mean.
Rutherglen in Victoria again shows cooling turned into warming.
Fig.5: Rutherglen minima
And again, the Acorn adjustments make Rutherglen’s trend greater than every one of its neighbours’ adjusted trends, as well as their mean:
Fig. 6: Rutherglen Acorn vs neighbours’ Acorn (mean)
The BOM is defending its territory, but this latest media exposure will mean increasing and critical scrutiny.