i) Weather station distribution
Hobart (Ellerslie Road) is the station with the longest temperature record in Australia, dating back to 1841. Unfortunately this record is not continuous as can be seen in Fig. 20.1 below (taken from Berkeley Earth).
Fig. 20.1: Temperature anomaly for Hobart - Ellerslie Road (Berkeley Earth ID - 151745).
This is indicative of the overall state of weather station data in Tasmania. It is good, but not good enough. The state of Tasmania in Australia has only 28 sets of weather station data
that are longer than 480 months (the equivalent of 40 years), of which only 2 are long stations with over 1200 months of data. Compared to Victoria and New South Wales these numbers are very low. Moreover, most of those stations are located on the north and east coasts, with only 10 situated inland (see Fig. 20.2 below). This means that the stations in Tasmania are not evenly distributed across the
state, nor are they fully representative of the overall climate of the state.
Fig. 20.2: The locations of long stations (large squares) and medium
stations (small diamonds) in Tasmania. Those stations with a high warming trend
since 1841 are marked in red.
The station locations shown in Fig. 20.2 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. 20.2, it appears that most of the
warming in Tasmania is found in largest city, Hobart, and around the coast. Similar trends
were seen in New South Wales and Victoria. It can be seen in Fig. 20.2 that only 5 of the 28 stations have a stable or cooling trend. However, as was the case in Victoria, it is the shorter length stations with more recent data where the
warming is more pronounced.
ii) The trend in mean temperature
Fig. 20.3: Temperature trend for long stations in Tasmania since 1840. The best fit linear trend line (in red) is for the period 1841-2010 and has a gradient of +0.135 ± 0.049 °C/century.
Adding the temperature anomalies from all stations in Tasmania with more than 480
months of data yields the trend shown in Fig. 20.3 above. In this case
the monthly reference temperatures (MRTs) were calculated for the period
1961-1990 as this period enabled the greatest number of stations to be included in the average.
The trend in Fig. 20.3 similar to that for New South Wales shown in
Fig. 18.3 previously. There is evidence of a decline in temperatures from 1880 to 1940 followed by a slow rise. In this case the overall temperature trend from
1890 to 2007 (as indicated by the red line) is +0.135 ± 0.049 °C per
century. In other words, only a very low or moderate warming is taking place over the entire period. However, the trend for the period 1941-2010 is +1.29 ± 0.14 °C per
century, which equates to a temperature rise of over 0.9 °C since 1941. This is more than balanced though by a large (but incomplete) peak in temperature before 1900.
iii) The Berkeley Earth (BE) mean temperature trend
Fig. 20.4: Temperature trend for long stations in Victoria since 1840 derived using the Berkeley Earth adjusted data. The best fit linear trend line (in red) is for the period 1941-2010 and has a gradient of +1.62 ± 0.07 °C/century.
If we repeat the temperature averaging process for the Berkeley Earth adjusted data
we get the trend shown above in Fig. 20.4. This also shows an initial
slight downward trend, but only before 1940, after which there is a
strong positive trend of +1.62 ± 0.07 °C/century that raises the overall
temperature by over 1.1 °C before 2010. The trend in Fig. 20.4 is
almost identical to
the plot shown on the Berkeley Earth site (see Fig. 20.5 below), and also resembles both the
IPCC "hockey stick" and the
instrumental temperature record since 1850, at least qualitatively.
Fig. 20.5: Temperature trend for Tasmania since 1840 according to Berkeley Earth.
The data in Fig. 20.4 and Fig. 20.5 is the 12-month moving average, but the same pattern of data is also seen in the monthly averages of the Berkeley adjusted data (see Fig. 20.6 below). The best fit to this data over the period 1841-2010 is a modest 0.496 ± 0.049 °C per century, but this still equates to an overall temperature rise of more than 0.84 °C since 1841. That is similar to the temperature rise seen after 1941 in Fig. 20.3. The obvious question is which graph is correct: the one in Fig. 20.3 or the one in Fig. 20.6 below?
Fig. 20.6: Temperature trend for all long and medium stations in Tasmania since 1840 based on Berkeley Earth adjusted monthly data. The best fit linear trend line (in red) is for the period 1841-2010 and has a gradient of +0.496 ± 0.056 °C/century.
iv) Comparison of unadjusted and BE adjusted temperature data
The difference between the data in Fig. 20.6 and that in
Fig. 20.3 is almost entirely due to the adjustments made to the data by
Berkeley Earth (BE). These adjustments are shown in Fig. 20.7 below.
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 MRT is calculated. As I wrote last time, 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 the MRT being determined by just averaging the data
from within that dataset as I have done for Fig. 20.3. In the case of the Tasmania data, the MRT adjustments, although large in amplitude, do not seem to have a great impact on the trend. It is the breakpoint adjustments that affect the trend, as illustrated in Fig. 20.7 below.
Fig. 20.7: The Berkeley Earth breakpoint adjustment (in yellow) for Tasmania 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 breakpoint adjustment data
is +0.25 ± 0.12 °C per century.
What we can see in Fig. 20.7 is that the total effect of all the
adjustments is to introduce a positive warming of +0.25 ± 0.12
°C/century for the period 1941-2010. This equates to a temperature rise
of at least 0.18 °C being added to the original data between 1941 and
2010 by the Berkeley Earth data processing, but this is much less than the adjustments seen for the data from Victoria and NSW. These adjustments
are not neutral, though, and they significantly alter the temperature trend. This is because they do not just add to the positive trend post-1941 (thus enhancing the blade of the
hockey stick), they also help to erase the peaks in the anomaly data before 1990 (thereby smoothing the handle of the
hockey stick). What we need to ascertain is whether these alterations are justified.
Normally, we would be extremely cautious about the data in Fig. 20.3 before 1900 as it has a number of significant deficiencies.
Firstly, it is based on only one temperature record,
Hobart (Ellerslie Road). That means the statistical error will be high.
Secondly, it is not continuous. That means there is a high probability of there being defective data in this time interval.
Thirdly, the data after 1900 appears qualitatively different from that before 1900.
However, the data before 1900 has one major virtue. It is consistent with data in NSW and Victoria over the same time-frame. Both NSW and Victoria exhibit peaks in their temperature records at 1880, that then decline significantly as you go back further in time to 1860. That is the evidence that corroborates the Tasmania data. It is also the evidence that negates the need for the breakpoint adjustments. You do not need to eradicate the peaks and troughs in the temperature trend before 1900 because these peaks and troughs are real, not erroneous artefacts that need to be expunged.
One final point to note is how the data in Fig. 20.3 compares well with similar data for Victoria and New South Wales despite the distribution of weather stations in Tasmania being less than ideal. This again suggests that, while the distribution may be sub-optimal and may not reflect the whole of Tasmania geographically, there resulting temperature trend may be much more representative.
v) Noise and its scaling behaviour
Fig. 20.8: The change in standard deviation of the Tasmania mean anomaly after smoothing with a moving average of size N. The gradient of the best fit line is -0.217 ± 0.007 and R2 = 0.9946.
Finally, if we look at the effect of data smoothing on the noise level,
we again see strong evidence of scaling behaviour (see Fig. 20.8 above). Once again the noise (as defined by the standard deviation) scales
as
N -a with the exponent
a = 0.217 ± 0.007. Again, this is similar to the scaling seen previously for NSW (
a = 0.272 ± 0.005) and Victoria (
a = 0.257 ± 0.015). It indicates that when smoothed with a 5-year moving average the data will still have a standard deviation of 0.38 °C. This implies that there is a 50% probability of fluctuations in the 5-year moving temperature average exceeding 0.7 °C over any 100 year interval. In addition the standard deviation for a 100-year moving average, determined by interpolation, will be 0.19 °C, again suggesting that the long-term fluctuations in temperature will be significant.
vi) Conclusions
1) Based on the original station data, there is no evidence of any rise
in overall temperatures in Tasmania since 1841 (see Fig. 20.3).
2) The overall temperature trend for Tasmania since 1850 is broadly similar to that seen for both New South Wales and Victoria.
3) The long-term temperature trend for Tasmania exhibits fluctuations of
more than ±2 °C over timescales of more than 100 years (see Fig. 20.3).
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. 20.4). They are not neutral.
5) Breakpoint adjustments added at least 0.25 °C to the
long-term temperature trend for Tasmania since 1941 (see Fig. 20.7).
6) The noise in the regional temperature average for Tasmania scales in a similar
way to that seen for New South Wales and Victoria except that
the power law is
N -0.22, where
N is the size of the sliding window in the moving average (see Fig. 20.8).
7) As the standard deviation for the 60-month smoothed temperature anomalies is 0.38 °C,
this means that there is still a 50% probability of a temperature rise
of more than 0.70 °C occurring over the course of a century in Tasmania purely by random chance, as I explained
here.
8) The statistical results presented here, and for New South Wales and Victoria previously,
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.
