129: UHI #2 - Melbourne (Victoria)

The Australian state of Victoria has a total population of 6.7 million, of which 5.1 million live in the city of Melbourne. That means that 76% of the population of Victoria live in its capital city even though Melbourne accounts for only 4.4% of the area of Victoria. It is not really surprising then that the temperature trends for Melbourne and Victoria over the last 100 years are markedly different. In fact while the state of Victoria has cooled slightly for most of the last 120 years, Melbourne has warmed by over 2°C (see Fig. 129.1 below). So like Sydney in the previous post, Melbourne looks like a classic urban heat island (UHI).

Fig. 129.1: The change to the 5-year average temperatures of Melbourne (red curve) and Victoria (blue curve) since 1900.

 

In Post 19 I examined the temperature trends for Victoria. The mean temperature change since 1880 is shown in Fig. 129.2 below and it indicates that Victoria has exhibited no significant warming. In fact the best fit for 1886-2005 indicates that temperatures actually declined very slightly, although the 5-year average over the same period suggests that they may have risen slightly by about 0.47°C with most of this rise occurring after 1990.

Fig. 129.2: The mean temperature change for Victoria since 1880 relative to the 1966-1995 monthly averages. The best fit is applied to the monthly mean data from 1886 to 2005 and has a slight negative gradient of -0.02 ± 0.08 °C per century.

The mean temperature anomaly (MTA) for Victoria shown in Fig. 129.2 above is the result of averaging monthly temperature anomalies from over fifty stations as Fig. 129.3 below demonstrates (see here for a list of all stations). However, before 1900 there are less than twenty 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 Victoria see Post 19.

Fig. 129.3: The number of station records included each month in the mean temperature anomaly (MTA) trend for Victoria in Fig. 129.2.

The oldest weather stations in Victoria is Melbourne Regional Office (Berkeley Earth ID: 151813). It is located in the heart of Melbourne and has continuous data stretching back as far as 1855. It is also the only major station within 20 km of the city centre that has continuous data extending back before 1940 (the next best is Laverton Aerodrome), hence its significance as a case study of the urban heat island (UHI) effect.

In contrast to the rest of Victoria, Melbourne Regional Office shows significant and continuous warming since 1880 (see Fig. 129.4 below). The best fit for 1886-2005 indicates a temperature rise of more than 1.4°C in 120 years while the 5-year average suggests a rise of over 2.1°C.

Fig. 129.4: The mean temperature change for Melbourne Regional Office since 1880 relative to its 1966-1995 monthly averages. The best fit is applied to the monthly mean data from 1886 to 2005 and has a positive gradient of +1.23 ± 0.10 °C per century.

Summary

The following temperature changes were observed from 1886 to 2005.

Victoria: 0.47°C (trend -0.02°C).

Melbourne: 2.1°C (trend 1.48°C).

So Melbourne has warmed by at least 1.5°C more than the surrounding state of Victoria, or up to four times faster. A classic UHI!

Given that both Sydney and Melbourne appear to be great examples of UHIs, one might expect the same of similar large cities, Adelaide and Brisbane. Yet this appears not to be the case. Neither of these cities exhibits greater warming than the rest of their respective states even though over 70% of the South Australian population of 1.8 million live in Adelaide. For Brisbane and Queensland, though, the proportion is only 44%, although Brisbane is almost twice the size of Adelaide by population. The reason both Adelaide and Brisbane do not exhibit striking UHI properties could be that they are too small. Adelaide has a population that is less than a quarter of that of Sydney. That said, the population of Perth in Western Australia is just less than two million and yet as the next post will show, it too appears to be an urban heat island (UHI).

19. Victoria (Australia) - temperature trends PARABOLIC

The state of Victoria in Australia has 65 sets of weather station data that are longer than 480 months (the equivalent of 40 years), of which 15 are long stations with over 1200 months of data. While this is not as extensive a set of station data as is seen for New South Wales (NSW) and was discussed in the previous post, it is still more than was seen for New Zealand. In this post I will essentially repeat the analysis that was performed on the NSW data last time but with data from Victoria. The aim is to see if the trends seen in NSW and Victoria are just local, or whether they are indicative of a common trend across the whole of Australia.


i) Weather station distribution

The long stations in Victoria are fairly evenly distributed across the state, as illustrated in Fig. 19.1 below. So, in their totality, they are fully representative of the overall climate of the state.

Fig. 19.1: The locations of long stations (large squares) and medium stations (small diamonds) in Victoria. Those stations with a high warming trend since 1871 are marked in red.

The station locations shown in Fig. 19.1 also differentiate between those stations that have warming trends and those where the trend is negative or stable. I have defined a warming trend to be one where the slope of the best fit to the temperature trend is positive and more than twice the error in the gradient (i.e. 95% confidence). Based on the distribution of stations in Fig. 19.1, it appears that most of the warming in Victoria is found in and around Melbourne. A similar trait was seen in New South Wales around Sydney, suggesting that urban heating is a significant issue.

 Fig. 19.2: The amount of warming for long and medium stations in Victoria. The stations with definite warming trends are shown as red squares.

It can be seen in Fig. 19.2 that only 4 of the 15 long stations have a warming trend. For stations with more than 600 months of data only 15 of the 42 stations have a warming trend. For shorter length stations the warming is more pronounced, but these stations add little to the overall trend for the region because of their limited length.


ii) The trend in mean temperature

Fig. 19.3: Temperature trend for long and medium stations in Victoria since 1850. The best fit to the data has a gradient of -0.05 ± 0.08 °C per century.

Adding the temperature anomalies from all stations with more than 480 months of data yields the trend shown in Fig. 19.3 above. In this case the monthly reference temperatures (MRTs) were calculated for the period 1981-2010. This period is longer than1981-2000 period used for the New Zealand data so the MRT values are likely to be more stable. The period is also 20 years later than the 1961-1990 period favoured by most climate scientists. The 1981-2010 period was chosen because it allows most of the medium station data to be used. However, overall these changes make little difference to the final result.

The trend in Fig. 19.3 very similar to that for New South Wales shown in Fig. 18.3 previously. In this case the overall temperature trend from 1890 to 2007 (as indicated by the red line) is -0.05 ± 0.08 °C per century. In other words, there is no warming taking place.


iii) The Berkeley Earth (BE) mean temperature trend 

Fig. 19.4: Temperature trend for long and medium stations in Victoria since 1850 derived using the Berkeley Earth adjusted data. The best fit to the data has a gradient of +1.64 ± 0.08 °C per century.

However, if we repeat the averaging process for the temperature data, but use Berkeley Earth adjusted data, we get the trend shown above in Fig. 19.4. This also initially shows a slight downward trend, but only before 1940, after which there is a strong positive trend of +1.64 ± 0.08 °C/century that raises the overall temperature by over 1.1 °C before 2010. The trend in Fig. 19.4 is almost identical to the plot shown on the Berkeley Earth site (see Fig. 19.5 below), and also resembles both the IPCC "hockey stick" and the instrumental temperature record since 1850, at least qualitatively.

Fig. 19.5: Temperature trend for Victoria since 1840 according to Berkeley Earth.

The difference between the data in Fig. 18.5 (or Fig. 18.4) and that in Fig. 18.3 is almost entirely due to the adjustments made to the data by Berkeley Earth. These are shown in Fig. 18.6 below.

Fig. 19.6: The Berkeley Earth breakpoint adjustment for Victoria since 1840 together with the difference between the Berkeley Earth adjusted anomaly and the raw anomaly (in blue). The best fit (red line) to the difference data is +0.60 ± 0.02 °C per century. The yellow curve is the contribution to the difference from breakpoint adjustments

The adjustments made to the data by Berkeley Earth appear to be of two main types. The most significant are the breakpoint adjustments that I have discussed previously. These are supposed to compensate for measurement errors in the original data. However, there appears to be a second adjustment that is introduced when the MRTs are calculated. The source of this is unclear, but I suspect it arises from a homogenization process being used to determine the MRT for each dataset, rather than just averaging the data from within that dataset as I have done. The homogenization process probably uses data from adjacent local stations to help refine the MRT. Whatever the source, it is clear from Fig. 19.6 that these adjustments are not neutral and they significantly alter the temperature trend.

What we can see in Fig. 19.6 is that the total effect of all the adjustments is to introduce a positive warming of +0.60 ± 0.02 °C/century for the period 1881-2010, of which +0.39 ± 0.02 °C/century is due to the breakpoint adjustments. This equates to a temperature rise of at least 0.78 °C being added to the original data between 1881 and 2010 by the Berkeley Earth data processing.


iv) Noise and scaling behaviour

Fig. 19.7: The change in standard deviation of the Victoria mean temperature anomaly after smoothing with a moving average of size N. The gradient of the best fit line is -0.257 ± 0.015 and R² = 0.9842.

Finally, if we look at the effect of data smoothing on the noise level, we again see strong evidence of scaling behaviour (see Fig. 19.7 above). This time the noise (as defined by the standard deviation) again scales as N ^-a with the exponent a = 0.257 ± 0.015. Once again, this is very similar to the scaling seen for New South Wales previously where a = 0.272 ± 0.005, and implies a noise level (i.e. standard deviation) on 100-year averaged temperature data of at least 0.16 °C. This is similar to the value predicted for individual stations such as Newcastle Nobbys Signal Station (Berkeley Earth ID - 152044) that was studied in Post 17. That study indicated that fluctuations in the 100-year temperature average of over 0.5 °C could be commonplace over 2000 year time periods (see Fig. 17.8).


v) Conclusions

1) Based on the original station data, there is no evidence of any rise in overall temperatures in Victoria since 1881 (see Fig. 19.3).

2) The overall temperature trend for Victoria since 1850 is very similar to that seen for New South Wales.

3) The long-term temperature trend for Victoria exhibits fluctuations of more than ±2 °C over timescales of more than 100 years (see Fig. 19.3). Even the 5-year moving average has fluctuations of at least ±0.5 °C, and maybe as much as ±1 °C. This adds to previous evidence that suggests that what we are probably seeing in these temperature trends is primarily low frequency noise or random fluctuations.

4) Breakpoint adjustments and other adjustments (possibly from homogenization) can completely change the form and shape of the long-term temperature trend (see Fig. 19.4). They are not neutral.

5) Breakpoint adjustments added at least 0.5 °C to the long-term temperature trend for Victoria (see Fig. 19.6). Other adjustments increased this to nearly 0.8 °C.

6) The noise in the regional temperature average for Victoria scales in a similar way to that seen for New South Wales except that the power law is N ^-0.26, where N is the size of the sliding window in the moving average (see Fig. 19.7).

7) As the standard deviation for the 60-month smoothed temperature anomalies is 0.36 °C, this means that there is still a 50% probability of a temperature rise of more than 0.65 °C occurring over the course of a century in Victoria purely by random chance, as I explained here.

8) The statistical results presented here, and for New South Wales, imply that chaotic effects in the temperature record are important, and probably dominant in many cases, even over long (i.e. more than 100 years) timescales. 

vi) Addendum

One noticeable feature of the temperature trend in Fig. 19.3 is the large dip in temperature values before 1875. It may tempt some to think that this is evidence of an upward warming trend. It is not. It is because there are only two temperature records in Victoria with data before 1877, Melbourne Regional Office (Berkeley Earth ID: 151813) and Cape Otway Lighthouse (Berkeley Earth ID: 151786). 

The Cape Otway data exhibits no warming. So the mean temperature before 1880 is similar to that after 2000. However, the Melbourne data exhibits a strong warming trend of more than 1.5 °C from 1860 to 2010. This warming trend is cancelled after 1875 by the data from other stations that are generally stable or cooling. However, without these other stations, as is the case before 1875, the Melbourne data is able to dominate. The net result is that the overall trend suddenly jumps by almost 1 °C after 1875. For this reason the data before 1876 cannot be regarded as representative of the temperature of the entire state because it is dominated by one station: Melbourne Regional Office.

It should also be noted that changing the period for the MRT calculation can also affect the data slightly, either by changing the number of stations that qualify for the final averaging process, or by altering the offsets of the different datasets relative to each other. These effects are usually small but noticeable.

 

Fig. 19.8: Temperature trend for long and medium stations in Victoria since 1876. The best fit (red line) to the temperature data for 1880-2007 is -0.02 ± 0.07 °C per century.

For example, the trend in Fig. 19.8 above was derived by calculating the MRT for the period 1966-1995. This increases the number of stations incorporated into the trend by one to 66. The peak in the 5-year moving average at 1880 is now slightly higher than the one at 1890 so the overall trend (red line) becomes less negative, -0.02 ± 0.07 °C compared with -0.05 ± 0.08 for the data in Fig. 19.3 above. Note also that the data in Fig. 19.8 is truncated at year 1876. It is therefore much more representative of the overall trend in Victoria than the data shown in Fig. 19.3.