Updated 11 March
Here we look at some subtle changes in trends that result from homogenisation. It is often the case for long time spans that a station has either been moved or else two different stations in the same location have been combined. In this case a small offset in temperature values is to be expected. The objective is where possible to create a continuous temperature series from 1910 to the present time. However in every case I have looked at the homogenisation procedure itself has extended way beyond the join and always increased the apparent warming. Here are three examples.
- Launceston, Tasmania. The ACORN time series is actually a combination of 3 nearby sites in the city. a) Launceston Pumping Station from 1910-1946. b) Launceston Airport (original site) from 1939-2009 and c) Launceston Airport (current site) from 2004 onwards. The animation below consists of 3 frames. Frame 1 is the raw average temperatures (monthly and annual) from the 3 sites is the middle trace in 3 colours – red, green and pink. Above is the maximum annual recorded temperature and the bottom trace is the minimum recorded temperature. Frame 3 is the same thing for the homogenised data – single red colour for monthly, blue for annual, purple for maximum and light blue for minimum. Frame 2 is a mix of the two.
The joining of the 3 bands at first sight looks to be fine, but closer inspection shows that the maximum and minimum temperatures in the central section are differentially being shifted so as to produce a linear rising temperature trend where there was none apparent before. There are no obvious kinks in the raw data to justify this.
2. Alice Springs
A similar animation is displayed below for Alice Springs
Alice Springs consists of a merge between the Post Office station (1910-1953) and the Airport since 1953. Note how the animation shows an increase in minimum temperatures at the airport resulting in a linearisation of the trend, neither of which has any direct connection with the merge.
Darwin is tropical and shows little warming since 1910, but here again we see small adjustments extending up to 1980 from a merge in 1940.
Darwin is a merge of 3 different sites, the post office before 1942 and two airport sites. This should be a straightforward linear shift between the two stations but again the shape of the early data is completely changed producing a small linear warming trend.
There is evidence of warming in the raw temperature measurement data. However, I strongly suspect this has then been boosted in ACORN-SAT by their ‘homogenisation’ process. My guess is probably by about 33%. This must also apply to GHCN, CRUTEM and BEST, since they all basically use the same algorithm.