Homogenisation more often then not increases warming trends. ACORN-SAT is based on 112 stations, many of which are combinations of nearby stations which are then all homogenised. However there are actually a total 1805 individual stations scattered about Australia, most of which are excluded from ACORN-SAT. These all have varying time coverage, but most of them still have sufficient data between 1961 and 1990 to calculate temperature anomalies based only on the recorded temperatures. As a result I have been able to compare the raw measurement result from all these stations against the ACORN-SAT homogenised result.
The main advantage of the triangulation method of integrating spatial data is that it copes very well with a changing mix of stations, since it avoids any binning. It also seamlessly combines stations at the same location, replacing older stations with newer ones automatically over time. Figure 2 shows the spatially averaged ‘Raw’ results as compared to ACORN-SAT, both calculated identically.
First of all it is clear that Australia has warmed over the last 100 years. However the net warming in the raw data is about 0.3C less than that advertised by ACORN-SAT. The selected 112 homogenised stations have increased the apparent warming by about 30%.
Here is an animation of the raw spatial data showing large year to year variability , but with an overall gradual warming. Note how the mix of stations is constantly changing. This is simply because each one has a different time range. Cities like Sydney and Melbourne have 4 or 5 stations each contributing.
In the previous post I tried to show using 3 examples that homogenisation algorithm itself is responsible for a slight increase in warming trends. I was then accused of cherry picking. Since then I have looked at a further 18 ACORN stations. Of these none show a reduced trend while 12 show clear linearised increases. Here is one example. Click reload if animation does not rerun automatically.
The raw temperature data from 1805 stations shows that Australia has warmed by about 0.8C over the last 100 years. This is about 0.3C less than ACORN-SAT.
Homogenisation of the data of the 112 selected sites results in a linear increase of apparent warming with time. This is visible in ~70% of the stations. The pair-wise comparison algorithm is the likely cause, because it adjusts station data so as to agree with nearby ones. An analogy would be like placing a weak magnet under a paper containing iron filings.