DARWIN TEMPERATURES: UNSCRAMBLING THE ACORN SAT 2 SERIES
By Bob Irvine
There have been a number of adjustments made to the Darwin temperature record in recent years that have significantly increased its trend from 1910 to 2016. This trend increase is largely due to the lowering of historical temperatures.
The BOM has achieved this significant lowering of historical Darwin temperatures by, in my opinion, using badly correlated comparison stations and doubtful statistical methods.
Grounds for being suspicious of the Australian BOMs methods are outlined in the discussion below.
The Darwin station history and adjustment justifications can be accessed at the link below.
Raw Darwin temperatures have been changed 6 times before inclusion in the Acorn Sat 2 series. This post will not deal with the three most recent changes in 1990 and 1995. The earlier three adjustments were on 1/1/37, 1/2/41 and 1/1/80. Each of these adjustments lowered all previous readings back to the beginning of the record by the amount specified. In total the Darwin mean temperature prior to 1937 was lowered by approximately 1.26C by these three adjustments.
Each of these three adjustments will be dealt with separately starting with the earliest in 1937.
1/01/1937 MAXIMUM TEMPERATURE ADJUSTMENT
All maximum recordings before 1/1/37 at Darwin P.O. (014016) were lowered by 1.02C on this date due to changes in the local vegetation. These changes were made by the BOM based on maximum temperatures taken from the comparison station Wyndham Port (001005).This series shows a significant and consistent increase in temperature from the mid-1920s to the 1950s that does not show up in the Darwin PO or Darwin Airport series (0145015).
In Fig. 2 it is obvious that the record has been compromised after 1936, probably by the vegetation issue identified by the BOM. A Pearson correlation of less than 0.5 disqualifies a comparison station based on monthly data according to the BOM methods. For the accuracy required by this post yearly data should be good enough and will almost certainly be confirmed if anybody wants to wade into the monthly data.
For the 10 years from 1927 to 1936 the correlation between Darwin PO and Wyndham Port is 0.39 which disqualifies Wyndham Port as a comparison station. Wyndham Port also has very poor or no correlation with Darwin Airport from 1942 to 1945 (correlation 0.03). The BOM should not have used Wyndham Port as a comparison station.
The problem then becomes, how do we change the Darwin PO maximum record to more accurately reflect the changes in vegetation after 1937 and the station move in 1941. The best and most obvious available data set to do this is the Darwin PO minimum record. Vegetation does not generally affect minimum temperatures as there is no shading problem at night.You can see that Darwin PO minimum temperatures have no trend from 1941 back to about 1910. This is also the case for the Darwin PO maximum temperatures from 1927 to 1936, Fig, 2. A best estimate maximum average for the years from 1927 to 1941 is, therefore, obtained by continuing the maximum average from 1927 to 1936 through to 1941.
The BOM have averaged the 1937 to 1941 maximums to get a figure of 31.54C. This they have subtracted from the 1927 to 1936 average of 32.56C to justify an adjustment of 1.02C. This process is not legitimate for three reasons. Firstly, the record from 1937 to 1941 is compromised by the vegetation issue and should be discarded. Secondly, they have used the non- correlated Wyndham Port series as justification. And thirdly, 1939, 1940 and 1941 were colder than average and have biased the 5-year 1937 to 1941 average down. This is confirmed by the nearby Port Keats Police Station (014905) maximum record. Port Keats is the only series nominated as a comparison series by the BOM that covers the period 1939 to 1041.
All Australian raw station records can be accessed by station number at the following link;
A more accurate way of quantifying the adjustment is to use the difference between the Darwin PO maximum temperature average from 1927 to 1936 and the average from 1942 to 1952 in the Darwin Airport record. This method, incidentally, has the advantage of including any changes to maximum temperatures that are needed to allow for the 1941 station move to Darwin Airport.
The adjustment then becomes the Darwin PO maximum average from 1927 to 1936 of 32.56C minus the Darwin Airport maximum average of 31.69.
By these methods, the adjustment should have been 0.87C instead of the BOMs figure of 1.02C.
1/02/1941 MINIMUM TEMPERATURE ADJUSTMENT
All minimum temperatures prior to 1/2/41 were lowered by the BOM by 0.83C at Darwin PO to allow for the station move to Darwin Airport. The BOM used three comparison stations to justify this adjustment.
These stations were Port Keats (014905), Daly Waters (014626) and Wyndham Port (001005).
Fig. 4, shows Darwin PO minimums. The other four relevant minimum temperature graphs are printed below.
The first thing to notice is that the Wyndham Port series from 1942 to 1950 is negatively correlated with the minimum temperatures from Darwin Airport and should immediately be discarded.
Daly Waters has no record for the first four months of 1939 which affect correlation as this can change with the seasons. This leaves only one year of overlap with Darwin PO (1940). It also has a correlation of only 0.49 with Darwin Airport from 1942 to 1945. For these reasons the Daly Waters data has also been discarded.
Port Keats minimums, on the other hand, have a correlation of 0.62 with Darwin Airport from 1942 to 1945. Port Keats also has two years of overlap with Darwin PO that has a good correlation.
If Port Keats is used as a bridging station, then the adjustment between Darwin PO and Darwin Airport is 0.67C. This is arrived at by using the common 1939 and 1940 data with Darwin PO and the common data between 1942 and 1952 with Darwin Airport.
Darwin PO minimums from 1931 to 1940 average 23.7. Darwin Airport minimums from 1942 to 1952 average 22.94. This method gives an adjustment for minimum temperatures of 0.76 as an allowance for the station move. This is in reasonable agreement with the adjustment derived using the Port Keats data.
Taking these two methods together we arrive at a reasonable adjustment to Darwin minimum temperatures of about 0.7C. This should give a reasonable approximation of the change in minimum temperatures caused by the move from Darwin PO to Darwin Airport.
1/01/1980 MAXIMUM TEMPERATURE ADJUSTMENT
All maximum temperatures prior to 1/1/80 were lowered by 0.67C by the BOM. Their stated reason for doing this was “statistical”. They quoted four comparison stations as justification for this change.
They were Jabiru Airport (014198), Oenpelli (014042), Katherine Council (014902) and Cape Don (014008). These maximum temperature records are printed below and should be compared to the raw Darwin Airport maximums in Fig. 4.
All these stations are hundreds of kilometres from Darwin. They all show a temporary drop in temperature from the early 1970s to the late 1970s.
I see no reason why all maximum temperature readings going all the way back to the beginning of the 20th century for Darwin should be lowered by 0.67C based on these comparison stations.
Unless someone can think of a reason for this adjustment, I think it is reasonable to simply ignore this 0.67C. The direct measurements taken at Darwin Airport are obviously more accurate than this type of speculation.
If the methods outlined in this post were used, then these three adjustments would lower the Darwin mean temperatures prior to 1/1/37 by about 0.79C. This is consistent with a total increase in Darwin temperature of approximately 0.85C for the period 1910 to 2016 when combined with the other three more recent BOM changes.
If the BOMs methods are used, then these three adjustments would lower Darwin mean temperatures by 1.26C. This increase is an outlier when compared to other global datasets and has been arrived at largely by using badly correlated comparison stations as well as the doubtful statistical adjustment mentioned above.