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.
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