26. The temperature trend in Australia since 1853 - PARABOLIC

In Posts 18-24 I calculated the temperature trend for each Australian state using all the available instrumental temperature records with more than 40 years of data, and then compared the results with those claimed by Berkeley Earth. These results were summarized in my last post.

The overall picture, based on the actual raw data, was that most states had experienced significant warming since 1980 but that the current temperatures were generally the same as those seen in 1880 or earlier. In most cases, the temperature records exhibited large swings in temperature between 1890 and 2000, typically a large decline in temperature up to 1950 and then a recovery; only Queensland exhibited a temperature trend that was anything like that advanced by the IPCC in their global instrumental temperature record, but Queensland has no data before 1887. And only in the case of Queensland did the temperature construction using the raw data qualitatively agree with the equivalent Berkeley Earth version. In the case of all other states the two sets of data (i.e. the one based on raw anomalies and the Berkeley Earth adjusted anomaly version) diverged significantly the further back in time before 1950 that you look, and generally the divergence was even greater in the 19th century.

What also became abundantly clear was that due to the large fluctuations seen over time intervals of more than 50 years, only temperature records that are at least 150 years in length allow the observer to put those fluctuations into their true context and to draw any meaningful conclusions. The temperature record is not as most climate scientists and the IPCC appear to claim: namely stable and constant before 1900 and exponentially rising thereafter. What has become clear to me is that the data is largely random but on multiple timescales. There are short-term fluctuations in the monthly means of up to ±5 °C, and long-term fluctuations on timescales of many decades or centuries that are often in excess of ±0.5 °C. In previous posts (see Post 9 and Post 17 in particular) I have speculated on the nature of these fluctuations, their scaling properties and whether they are fractal in nature.

In this post I will complete the analysis for Australia by using the regional temperature trends for each state to construct an average for the whole of Australia. The method will be straightforward: to weight each state's overall trend by the area of that state relative to that of the total area of Australia. Based on this methodology the weightings for each state are as follows:

NSW Victoria Tasmania South Australia Western Australia Northern Territory Queensland

0.1004 0.0284 0.0085 0.1229 0.3306 0.1776 0.2316

In addition, the trend from each state is renormalized with a fixed temperature offset in order to ensure that the mean anomaly of each state for the period 1961-1990 is zero. After the scaling by weighting and the renormalization, the contributions from each state are added. The resulting trend in the monthly mean temperature for the whole of Australia is shown below in Fig. 26.1 together with the 5-year moving average. As a result of the offsetting process, the temperature changes in Fig. 26.1 are all defined relative to the 1961-1990 average.

Fig. 26.1: The temperature trend for Australia since 1853. The best fit is applied to the interval 1871-2010 and has a gradient of 0.24 ± 0.04 °C per century. The temperature changes are relative to the 1961-1990 average.

The data in Fig. 26.1 shows that the five year average of the regional temperature for Australia, relative to the average for 1961-1990, decreased from a peak of +0.37 °C in 1879 to a minimum of -0.46 °C in 1909. It then recovered from a low of -0.38 °C in 1948 to reach another peak of +0.47 °C in 2007. So while it is true that temperatures rose by 0.85 °C from 1948 to 2007, it is also true that there was an almost equal but opposite fall of -0.75 °C during the 70 years prior to 1948. So, when put into context, the temperature rise since 1948 does not appear that cataclysmic. Yet when you look at the Berkeley Earth adjusted data for the same period, the picture is very different.

Fig. 26.2: The temperature trend for Australia since 1853 based on Berkeley Earth adjusted temperature data. The best fit is applied to the interval 1951-2008 and has a gradient of 1.51 ± 0.06 °C per century. The temperature changes are relative to the 1961-1990 average.

Fig. 26.2 shows the result of the same averaging process for the temperature anomalies in Australia as previously outlined for the data in Fig. 26.1. The only difference is that Fig. 26.2 uses Berkeley Earth adjusted anomaly data, and Fig. 26.1 uses the original data with the correct anomalies calculated for each station. Yet the two datasets look completely different. Whereas the raw data in Fig. 26.1 combines to form a U-shaped curve or parabola, the adjusted data forms a highly asymmetric curve, with little or no change in temperature before 1950 and a steep rise afterwards. That rise has a gradient of 1.51 ± 0.06 °C per century, which equates to a total rise from 1951 to 2008 of 0.88 °C. This is similar to that seen in Fig. 26.1 for the same time period, but without the preceding decline in temperature. However, while the adjusted data in Fig. 26.2 differs significantly from the raw data in Fig. 26.1, it agrees quite well with the trend published by Berkeley Earth and shown below in Fig. 26.3.

Fig. 26.3: The temperature trend for Australia since 1840 according to the Berkeley Earth website.

I believe that the level of agreement between the 12-month and 10-year moving average curves in Fig. 26.3 with those in Fig. 26.2 validates my approach, and therefore also validates the data in Fig. 26.1. The other point to note about the data in Fig. 26.1 is the gradient of the best fit line. This is 0.24 ± 0.04 °C per century and therefore equates to a total temperature rise of about 0.34 °C from 1871 to 2010. This is far more than the actual change of about 0.10 °C and illustrates the caution that needs to be applied to linear trends that are applied to non-linear data. 

Fig. 26.4: A comparison of the actual ten year average temperature trend for Australia since 1853 and the Berkeley Earth adjusted temperature version. The best fit is applied to the actual raw data over the interval 1859-2008 and has a gradient of 0.184 ± 0.056 °C per century. The temperature changes are defined relative to the 1991-2000 average.

If we contrast the raw data in Fig. 26.1 with the adjusted data in Fig. 26.2 by comparing the 10-year moving average of each (see Fig. 26.4 above) we see that after 1980 there is relatively close agreement between the two. However, as we go back in time before 1980, we see the curves diverge. If we compare the temperature change from 1880 to 2008 in each case, we see that the raw data shows evidence of a rise of only about 0.2 °C, while for the adjusted data the rise is over 0.6 °C. This difference is principally due to adjustments made by Berkeley Earth to the data. These adjustments are shown below in Fig. 26.5.

Fig. 26.5: The contribution of Berkeley Earth adjustments to the anomaly data after smoothing with a 12-month moving average. The linear best fit to the data is for the period 1901-2010 (red line) and the gradient is +0.295 ± 0.016 °C per century. The orange curve represents the contribution made to the blue adjustment curve by breakpoint adjustments only.

The total adjustments in Fig. 26.5 (blue curve) were determined by subtracting the raw monthly data in Fig. 26.1 from the equivalent curve for the sum of the adjusted data. The data was then smoothed with a 12-month moving average which removed over 90% of the noise. The breakpoint adjustment curve in Fig. 26.5 (the orange curve) is just the weighted sum of the breakpoint adjustment curves from the different states (see Posts 18-24). The breakpoints adjustments are determined from the Berkeley Earth station data by subtracting the Berkeley Earth raw anomaly (which is different from my raw anomaly because it uses homogenization) from the Berkeley Earth adjusted anomaly.

The curves in Fig. 26.5 above indicate that summing the Berkeley Earth adjustments to the anomaly data results in a curve that has a trend of gradient +0.295 ± 0.016 °C per century for the period 1901-2010. This in turn amounts to a contribution of over 0.4 °C between 1870 and 2010 as the same trend appears to extend back until at least 1870.

Concluding points

1) According to the raw data (shown in Fig. 26.1), temperatures in Australia may have risen by up to 0.8 °C over the last 60 years, but if so, they are still, at worst, no more than about 0.2 °C above some of the peak values seen in previous centuries. In all likelihood, the temperature rise is probably even less (the data in Fig. 26.1 suggests it may be less than 0.1 °C). Unfortunately a lack of data prior to 1853 precludes any more definitive conclusions than this.

2) The large changes in mean temperature seen over time in the instrumental temperature record for Australia appear to be natural and reversible, and therefore probably occur on a regular basis.

3) The adjustments made to individual temperature records through a combination of breakpoints and homogenization by Berkeley Earth do not appear to completely cancel when multiple station records are averaged. The analysis above suggests that such adjustments could actually add more than 0.4 °C to the warming trend for Australia, which would be more than double the amount that could be attributed to the raw data, as noted in Point 1 above.