Tom Quirk takes a close look at the long historic station of Melbourne. As we would expect, things have changed around the sensor since 1855 when records started. Amazingly he finds the maximum trend in Melbourne was largely flat from 1855 – 1995. The minimums shows a classic warming from 1945.
To find out how much of the warming in Melbourne may be due to the Urban Heat Island effect (UHI) we can compare the minima at the CBD station to one on the outskirts — and Laverton is 20 kilometers away. The site near the CBD is warming at 0.2C per decade faster than the site on the outskirts. It amounts to a whole degree warmer over 50 years, though the rate may be tailing off now. It’s hard to fit in more concrete or more skyscrapers than there already are.
Tom has a close look at the adjustments and finds plenty of questions but few answers. These adjustments are done as step changes, and Tom (and I) wonder why the gradual increase in concrete would warm Melbourne “step-wise” rather than as a slope change. Tom also wonders why the BOM say that one change is due to a “time of observation” shift. He points out that the thermometers will hold the maximum or minimum for 24 hours, so the exact moment someone goes out to read the thermometer may only affect which date the reading applies too (and really only offset it by one day), rather than raise or lower the figure. The mystery?
Taking Melbourne’s Temperature
Guest Post Tom Quirk
The Melbourne temperature record is one of the “long time” instrumental records of Australian temperature. It starts in 1855 and continues to the present day. Originally measurements were made in the Flagstaff Gardens, then when the Melbourne observatory was established in 1863 near the Botanical Gardens, the measurements were taken at that location until 1907 when there was a move to the present location on the corner of Victoria and Latrobe Streets in central Melbourne.
My great-uncle Pietro Baracchi served as the Victorian government astronomer from 1895 to 1915. He was a meticulous experimental scientist so I am following the classic rule for scientific analysis – go look at the measurements.
The raw annual average measurements are shown in Figure 1. There were no thermometer changes between 1907 and 2000 for the minimum-temperature thermometer and between 1907 and 2001 for the maximum-temperature thermometer. As we shall see, the BOM high quality homogenized data set, ACORN-SAT, does not seem to follow the classic rule.
The Bureau of Meteorology (BOM) now tells us that we can regard only the measurements after 1910 as being reliable. We might note in passing that the raw record itself does not suggest anything wrong with the earlier data.
The maximum temperature (which generally occurs in the middle of the afternoon) shows almost no trend at all until about 1995. However the minimum temperature (which generally occurs in the early morning before sunrise) shows an upward trend starting at about 1945. With no sunlight on the ground, the night air cools but the heat emitted by buildings and human activities (the “urban heat island” effect) lessens the cooling. The upward trend of minimum temperature since 1945 perhaps reflects the change in the number and size of the buildings in the area surrounding the location of the thermometers.
We can compare the Melbourne measurements with those starting in 1944 at Laverton, some 20 km from the Melbourne Regional Office location. See Figures 2 and 3. The Laverton instruments were moved some 1.2 km from the original site in 1997. One year of overlap measurements at Laverton show no significant change at that time in maximum-temperature readings, but do show a 0.2 degree decrease in the minimum-temperature readings.
Figure 2 shows an increase in the minimum temperatures (according to the raw data) at the Melbourne location compared to that at Laverton from 1944 to the present. The comparison (roughly an increase of about 2 degrees compared with an increase of about 1 degree) is almost certainly an indication of the much larger urban heat island effect in Melbourne. Figure 3 shows both the maximum temperatures and the trends of maximum temperature in Melbourne to be much the same as in Laverton.
The third temperature series in each of Figures 2 and 3 is of the average annual temperatures recorded in the ACORN-SAT data which has been “homogenized” by the Bureau of Meteorology. A comparison of the measured and adjusted temperature increases from 1944 to 2013 is shown in Table 1.
Table 1 Temperature increases from 1944 to 2013
|Locations||Minimum temperature increase 0C per decade||Maximum temperature increase 0C per decade|
0.18 ± 0.02
0.13 ± 0.03
0.35 ± 0.02
0.21 ± 0.03
0.14 ± 0.03
0.16 ± 0.03
|Raw Melbourne – Raw Laverton||
0.21 ± 0.03
0.05 ± 0.05
Two conclusions can be drawn from this analysis:
- There is a clear heat island effect in central Melbourne that is detectable in the minimum temperature measurements. It may be as much as 0.2 degrees per decade. (Or 1 degree over 50 years!).
- The adjustments made to obtain the homogenised ACORN-SAT Melbourne data reduce the apparent long-term temperature increases. (So these adjustments compensate somewhat for the UHI effect.)
In more detail, the ACORN-SAT Melbourne minimum temperatures before 1990 are shifted up relative to the raw data. This seems fairly strange. Surely one might expect that, if one is in the business of adjusting raw data, the later minimum temperatures should have been moved down to take account of the urban heat island effect. An upward correction is also applied to the maximum temperatures, but is applied only to the past and not the present. This echoes the old Polish saying from Soviet times – the future is certain, only the past is unpredictable.
The Bottom line
A step adjustment does not compensate for a gradual rise from UHI. The effect of the actual ACORN-SAT adjustments to the raw data for Melbourne is illustrated in Figure 4. There are sharp breaks rather than gradual changes that would be expected from the slow growth of the urban landscape. The breaks do no appear to coincide with instrument changes. The Bureau explains the adjustments for the maximum and minimum temperatures as being “statistical”.
Would a time of observation change need a step adjustment? As well, they say that the change in 1964 is due to the reading time of the thermometers taking place at midnight before 1964 and at 9.00 am after 1964. This seems a remarkable adjustment. If the thermometers were read at midnight then the minimum and maximum would be for that day while a shift to a 9.00 am reading would give the minimum temperature for the day of the reading but the maximum for the previous day. How could this give rise to an adjustment? Even if the temperature was recorded for the wrong day only one day in a year would be wrong and then only by a small amount.
The explanations of the ACORN-SAT data adjustments do not provide much support for their validity. Only one of the many adjustments seems to be specifically linked to a change in procedure, and the term “statistical” conveys no physical or procedural change as justification.
My great-uncle Pietro Baracchi would be somewhat put out by the sometimes intemperate exchanges between the “experts” over the Australian temperature record as detailed by Graeme Lloyd in The Australian over the last month. One can only guess what Pietro would have made of the manipulation of the direct measurements.
One of his duties was making weather forecasts and his attitude to this function is well put in this extract from the Australian Dictionary of Biography:
“Baracchi was best known to the general public as official weather-forecaster for the colony, a role that he did not like. To him, ‘popular meteorology’ was ‘of little practical value except as an amusement, and of doubtful credit to science’. In 1902 he supported the opinion that meteorological work carried out by astronomical observatories should be placed under Commonwealth control, and that the observatories, relieved of these duties, should remain independent State institutions. In 1906 the Meteorology Act gave control of weather services to the Commonwealth and by the end of 1907 the Melbourne Observatory was freed of its meteorological function—and never regained its former status”.
In our family he was known to have been greatly upset by his barber who told him how well he grew vegetables by reading the weather forecast in the Argus newspaper and then preparing for the opposite. It seems that the Australian coal industry is dealing with climate change forecasts with a similar approach to that of the barber some hundred years earlier.
I understand that the Melbourne measurements are not used to derive the continent-wide Australian temperatures. Even so I think that great-uncle Pietro might conclude, looking down on us now, that if the treatment of the Melbourne temperature measurements is an indication of the quality of the other homogenized measurements then the Bureau should think again about the value of this work. In the words of Pietro Baracchi this work of the Bureau may be “of doubtful credit to science.”
ACORN-SAT information and data: http://www.bom.gov.au/climate/change/acorn-sat/