130: UHI #3 - Perth (Western Australia)

The population of Western Australia is only about 2.67 million but two million of that total live in the state capital Perth. So 75% of the state's population live in Perth even though Perth accounts for less than 0.25% of the area of Western Australia. Perhaps this is why temperatures in Perth appear to have risen at more than twice the rate of the rest of the state. By 1990 temperatures in Perth had risen more than 1.5°C since 1900 compared to less than 0.7°C in Western Australia as a whole (see Fig. 130.1 below). That looks like classic urban heat island (UHI) behaviour. The only caveat is that the main weather station for Perth at Perth Regional Office (Berkeley Earth ID: 4321) ceased operations in 1992 just as the UHI was taking off.

Fig. 130.1: The change to the 5-year average temperatures of Perth (red curve) and Western Australia (blue curve) since 1900.

In Post 22 I examined the temperature trends for Western Australia. The mean temperature change since 1900 is shown in Fig. 130.2 below and it indicates that Western Australia has warmed by about 1°C since 1990. The best fit for 1991-1990 indicates a temperature rise of less than 0.64°C in 90 years while the 5-year average suggests a rise of about 0.67°C for the same period.

Fig. 130.2: The mean temperature change for Western Australia since 1900 relative to the 1961-1990 monthly averages. The best fit is applied to the monthly mean data from 1901 to 1990 and has a positive gradient of +0.71 ± 0.11 °C per century.

The mean temperature anomaly (MTA) for Western Australia shown in Fig. 130.2 above is the result of averaging monthly temperature anomalies from nearly hundred stations as Fig. 130.3 below demonstrates (see here for a full list of all stations). However, before 1900 there are less than ten available stations so the MTA is less reliable and more prone to error from statistical variability. For more details and analysis of the complete data for Western Australia see Post 22.

Fig. 130.3: The number of station records included each month in the mean temperature anomaly (MTA) trend for Western Australia in Fig. 130.2.

The oldest weather station in Western Australia is Perth Regional Office (Berkeley Earth ID: 4321). It has data stretching back as far as 1852, and continuous data from 1876 to 1992. In fact it is the only station within Perth with over 480 months of continuous data between 1876 to 1992, hence its significance as a case study of the urban heat island (UHI) effect.

Compared to the rest of Western Australia, Perth Regional Office shows much more significant and continuous warming since 1900 (see Fig. 130.3 below). The best fit for 1901-1990 indicates a temperature rise of more than 1.47°C in 90 years while the 5-year average suggests a rise of over 1.5°C.

Fig. 130.4: The mean temperature change for Perth Regional Office since 1900 relative to its 1961-1990 monthly averages. The best fit is applied to the monthly mean data from 1901 to 1990 and has a positive gradient of +1.63 ± 0.19 °C per century.

Summary

The following temperature changes were observed from 1901 to 1990.

Western Australia: 0.67°C (trend 0.64°C).

Perth: 1.53°C (trend 1.47°C).

So Perth warmed by almost 1°C more than the surrounding state of Western Australia in the ninety years up to 1990, or more than twice as fast. A classic UHI!

25. A summary of temperature trends in Australia

In the seven posts previous to this I analysed the temperature trends in each of the seven major Australian states (ACT was combined with NSW) since instrumental temperature records began in about 1840. The results are summarized below.

In the following graphs the blue curve is the 10-year average temperature for the state relative to the decade 1991-2000 based on the actual unadjusted raw temperature data. The red line is the best fit to that data for the 100 year interval 1901-2000. The orange curve is the trend based on Berkeley Earth adjusted data. This is also defined relative to its mean for the decade 1991-2000.

Fig. 25.1: Temperature trends for NSW. The gradient of the best fit line is 0.08 ± 0.11 °C per century.

Fig. 25.2: Temperature trends for Victoria. The gradient of the best fit line is -0.04 ± 0.09 °C per century.

Fig. 25.3: Temperature trends for Tasmania. The gradient of the best fit line is 0.72 ± 0.11 °C per century.

Fig. 25.4: Temperature trends for South Australia. The gradient of the best fit line is 0.40 ± 0.11 °C per century.

Fig. 25.5: Temperature trends for Western Australia. The gradient of the best fit line is 0.76 ± 0.11 °C per century.

Fig. 25.6: Temperature trends for Northern Territory. The gradient of the best fit line is 0.09 ± 0.09 °C per century.

Fig. 25.7: Temperature trends for Queensland. The gradient of the best fit line is 0.69 ± 0.13 °C per century.

Conclusions

1) The Berkeley Earth adjusted data trends appear to closely follow the actual data trends for the period after 1970. However, before 1970 the two trends often diverge significantly.

2) Even after divergence, the Berkeley Earth adjusted data trend has almost identical features to the corresponding actual data trend. These features include the patterns of peaks, and the large changes in gradient at similar points in time. In fact the adjusted and unadjusted trend lines appear to show near identical instances of abrupt discontinuities and rapid temperature changes at various points along their respective records.

3) Linear regression fits to temperature record data cannot adequately represent the complexity of the data. For example, a 100-year best fit to the NSW data in Fig. 25.1 yields a gradient of 0.08 °C per century for the period 1901-2000, yet for the interval 1881-1980 this changes to -0.41 ± 0.11 °C per century. Likewise, changing the interval of the best fit for Northern Territory to 1886-1985 also changes the gradient to -0.41 ± 0.11 °C per century, while a best fit interval of 1871-1970 for South Australia results in a gradient of -0.60 ± 0.12 °C per century. In other words, the gradient of the best fit line depends strongly on the measurement interval chosen for the best fit.

4) The temperature trends based on actual data (not the Berkeley Earth adjusted data) suggest that for most states, except maybe Queensland, average temperatures in the latter part of the 20th century were lower than those in the latter part of the 19th century.

5) In most states the majority of any climatic warming has occurred after 1980.

Links to all posts with my original analysis are listed below or on the sidebar.

New South Wales (and ACT)

Victoria

Tasmania

South Australia

Western Australia

Northern Territory

Queensland

22. Western Australia - temperature trends 1°C WARMING

Western Australia is the first of the tropical states in Australia that I am going to examine. The Tropic of Capricorn (latitude 23.44° S) cuts through the state, separating the upper third from the rest. However, this does not appear to have any significant impact on the overall temperature trend which is similar to the that of other Australian states I have examined so far.


i) Weather station quality and distribution

Western Australia has the largest area of any Australian state (2.646 million km²), and the second lowest population density after Northern Territory. Despite its relatively low population, it still has 24 long stations with more than 1200 months of temperature data up to 2013, and 37 station records with more than 1000 months of data. In total, there are close to 100 long and medium stations with more than 480 months of data (see here).

Unfortunately, there is virtually no data prior to 1880. Only one station has data that precedes 1880 and that is Perth Regional Office (Berkeley Earth ID - 4321). This has some data that dates from 1852 (see Fig. 22.1 below), but only 40 months of it are before 1876 (see Fig. 22.1 below). The implication of this lack of early data will be discussed later in the post, but it certainly impacts on the interpretation of the overall trend. However, the trend for Perth Regional Office clearly exhibits the same features that we have seen for the regional anomalies in Victoria, South Australia and New South Wales. There is a minimum in the data around 1940 followed by a rise in temperature of about 1 °C. This rise is still less than the rise going back in time for the period from 1940 to 1880, thereby indicating that temperatures in the 1880s were probably higher than the present.

Fig. 22.1: The temperature anomaly for Perth Regional Office according to Berkeley Earth.

If we look at the distribution of weather stations in Western Australia we see that it is more uniform than was the case for South Australia and Tasmania, but not as uniform as was seen in Victoria and New South Wales. The greatest deficiency is the lack of station data for the interior of the state close to the borders with South Australia and Northern Territory. There are only 5 inland stations east of longitude 122E that are required to represent an area of over one million square kilometres. In addition, half the long stations are clustered in the south-west corner around Perth and Albany.

Fig. 22.2: The locations of long stations (large squares) and medium stations (small diamonds) in Western Australia. Those stations with a high warming trend since 1851 are marked in red.

ii) Temperature trend based on long and medium station records

If we average the long and medium station temperature anomalies for Western Australia we get the trend curve shown below in Fig. 22.3. These anomalies were calculated relative to their monthly reference temperatures (MRTs). In turn the MRTs were calculated from the 1961-1990 monthly average for each of the 12 months. 

Th trend curve in Fig. 22.3 is almost identical to that determined for South Australia (see Fig. 21.2). This similarity is easier to recognise if we consider the 5-year moving average (yellow curve). Common features include the peaks at 1910, 1920 and 1935, and the multiple peaks at 1960. In addition, both trends show similar magnitudes for the temperature rise from 1950 to 2010, and a similar fall from 1875 to 1910.

The most significant difference between the two trends is that the South Australia trend is longer. It extends back to 1857, and as a result it reveals the extent to which temperatures over that period were greater than they are today. This is not as evident if we look only at the Western Australia data in isolation. However, it is clear that the Western Australia data fits into the same consistent pattern that we have seen in South Australia, Victoria and New South Wales, and possibly Tasmania as well.

Fig. 22.3: Temperature trend for long and medium stations in Western Australia since 1876. The best fit linear trend line (in red) is for the period 1951-2010 and has a gradient of +1.19 ± 0.17 °C/century.

If we look at the temperature rise post-1950 in isolation we see that it amounts to 1.19 ± 0.17 °C per century (see red curve in Fig. 22.3). This amounts to a total temperature rise from 1951-2010 of about 0.7 °C. But as we know, this is only half the story, because it fails to account for the earlier temperature decline.

An additional feature of the data in Fig. 22.3 is the behaviour of the noise level. It appears to remain more or less constant in magnitude from 1880 up to 2010. Yet if we look at the number of stations in the average over this time-frame, this is not constant. Instead, it rises significantly from about 4 in 1880 to over 80 for the period 1960-2010.

Fig. 22.4: Number of stations per month included in the regional average for the Western Australia temperature anomaly.

Normally we would expect the noise to decrease with increased averaging, scaling as 1/√n where n is the number of terms in the average. In this case this should be manifested as a reduction in the noise level over more than a factor of 4. Yet none is seen. I have already speculated that this could be because of correlation effects between station data.


iii) The Berkeley Earth (BE) mean temperature trend

In all previous analyses we have seen that the Berkeley Earth temperature trend deviates significantly from that which one would expect based on the raw data. The data for Western Australia is no exception.

Fig. 22.5: Temperature trend for all long and medium stations in Western Australia since 1875 derived using the Berkeley Earth adjusted data. The best fit linear trend line (in red) is for the period 1901-2010 and has a gradient of +1.02 ± 0.03 °C/century.

Summing the Berkeley Earth adjusted monthly averages yields a far longer and more consistent temperature rise over the course of the 20th century than is found for the raw data in Fig. 22.3. The gradient of this rise is +1.02 ± 0.03 °C and it results in a total temperature rise over the period 1901-2010 of more than 1.1 °C. This is 40% more than is seen the raw data. In addition, the peak in temperatures before 1900 is significantly reduced in size to only about 0.4 °C. The result is that Berkeley Earth graphs suggest that temperatures in 1880 were at least 0.7 °C below the present values (see Fig. 22.6 below), whereas the raw data suggests they were almost comparable.

Fig. 22.6: Temperature trend for Western Australia since 1840 according to Berkeley Earth.

The difference between the data in Fig. 22.5 and that in Fig. 22.3 is again primarily due to breakpoint adjustments. There are, however, noticeable differences between the data in Fig. 22.5 which I have reconstructed from the Berkeley Earth adjusted station data, and that shown in Fig. 22.6 which is the weighted average according to Berkeley Earth. While the trends are the same, and the 10-year moving average in each case have features that are generally coincident, there are bigger differences in the 12-month moving averages than were seen in the data for Victoria and New South Wales. This is probably due to the fact that the distribution of stations in Western Australia is less homogeneous than is the case for Victoria and New South Wales. However, it also shows that while the station distribution has affected the 12-month moving average, it has much less impact on data with longer smoothing intervals.


iv) Breakpoints and other adjustments 

Fig. 22.7: The difference between the data in Fig. 22.5 and Fig. 22.3 together with a linear best fit for the period 1902-2010 (red line). The gradient of the best fit line is +0.23 ± 0.03 °C per century. The yellow curve represents the total breakpoint adjustments.

Finally, if we look at the source of the difference between the results in Fig. 22.3 and 22.5, namely homogenization and breakpoint adjustments, we see that both appear to add to the warming trend between 1901 and 2010. The overall magnitude of this adjustment is about 0.25 °C, of which approximately 0.19 °C is due to the breakpoint adjustments.


iv) Conclusions

1) Temperatures in Western Australia before 1880 were probably similar to current temperatures. There is no evidence of significant anthropogenic climate change (see Fig. 22.3).

2) The temperature trend for Western Australia is consistent with that seen for all other states in Australia.

3) Temperatures in Western Australia were much lower in the 1940s than they are now (see Fig. 22.3).

4) The noise level in the regional average of monthly anomalies (see Fig. 22.3) is similar to the noise level in the individual records. Not only does the averaging process for the regional trend have little effect on the noise level, but the number of stations included in the average has little effect as well.