This follows on from my last post where I showed a quick comparison of Rutherglen raw data and adjusted data, from 1951 to 1980, with the 17 stations listed by the Bureau as the ones they used for comparison when detecting discontinuities.
Here is an alternate and relatively painless way to check the validity of the Bureau’s homogenisation methods at Rutherglen, based on their own discontinuity checks. According to the “Manual” (CAWCR Technical Report No. 49), they performed pair-wise comparisons with each of the 17 neighbours to detect discontinuities. An abbreviated version of this can be used for before and after comparisons. For each of the 17 stations, I calculated annual anomalies from the 1961-1990 means for both Rutherglen and the comparison site, then subtracted the comparison data from Rutherglen’s. I did the same with Rutherglen’s adjusted Acorn data.
A discontinuity is indicated by a sudden jump or drop in the output. The ideal, if all sites were measuring accurately and there are no discontinuities, would be a steady line at zero: a zero value indicates temperatures are rising or falling at the same rate as neighbours. In practice no two sites will ever have the same responses to weather and climate events, however, timing and sign should be the same. Therefore pairwise differencing will indicate whether and when discontinuities should be investigated for possible adjustment.
Similarly, pairwise differencing is a valid test of the success of the homogenisation process. Successful homogenisation will result in differences closer to zero, with zero trend in the differences over time. The Bureau has told the media that adjustments are justified by discontinuities in 1966 and 1974. Let’s see.
Fig. 1: Rutherglen Raw minus each of 17 neighbours
Note: there is a discernible drop in 1974, to 1977. There is a very pronounced downwards spike in 1967 (ALL differences below zero, indicating Rutherglen data were definitely too low.) There also a step up in the 1950s, and another spike upwards in 1920. Rutherglen is also lower than most neighbours in the early 1930s. Also note several difference lines are obviously much higher or lower than the others, needing further investigation, but the great majority cluster together. Their differences from Rutherglen are fairly consistent, in the range +/- 1 degree Celsius.
Now let’s look at the differences AFTER homogenisation adjustments:
Fig. 2: Rutherglen Acorn minus the neighbours: The Test
The contrast is obvious. The 1920 and 1967 spikes remain. Differences from adjusted data are NOT closer to zero, most of the differences before 1958 are now between 0 and -2 degrees Celsius, and there is now an apparent large and artificial discontinuity in the late 1950s. This would indicate the need for Rutherglen Acorn data to be homogenised!
Compare the before and after average of the differences:
There is now a large positive trend in the differences when the trend should be close to zero.
There are only two possible explanations for this:
(A) The Bureau used a different set of comparison stations. If so, the Bureau released false and misleading information.
(B) As this surely can’t be true, then if these 17 stations were the ones used, this is direct and clear evidence that the Bureau’s Percentile Matching algorithm for making homogenisation adjustments did not produce correct, successful, or useful results, and further, that no meaningful quality assurance occurred.
If homogenising did not work for Rutherglen minima, it may not have worked at the other 111 stations.
While I am sure to be accused of “cherry picking”, this analysis is of 100% of the sites for which the identities of comparison stations have been released. When the Bureau releases the lists of comparison stations for the other 111 sites we can continue the process.
A complete audit of the whole network is urgently needed.