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.

18. New South Wales - temperature trends PARABOLIC

In Post 7 and Post 8 I looked at the temperature trend in New Zealand since 1850. As I pointed out there, New Zealand has one of the longest and best temperature records in the Southern Hemisphere. However, in both respects the country is surpassed by Australia.

Overall, Australia has the best temperature records of any country in the Southern Hemisphere. Only Rio de Janeiro has a longer record than any of those found in Australia or New Zealand. And of the eight states that comprise Australia, the one with the most comprehensive records is New South Wales (NSW). Where New Zealand has 10 long station records with more than 1200 months of data, NSW has 33. It also has another 71 stations with more than 600 months of data, and another 31 with between 480 and 600 months of data. So, as the state with the best records, it is the obvious place to start when studying climate change in Australia.

Fig. 18.1: Location of NSW long stations (large squares) and medium stations (small diamonds). Those stations with a high warming trend since 1871 are marked in red.

i) The long station records

As already stated, these long stations have over 1200 months (or the equivalent of 100 years) of data. They are also fairly evenly distributed across NSW as shown in the map illustrated in Fig. 18.1 above. The stations in Fig. 18.1 are also differentiated according to the size of their warming trend. A high warming trend is defined as one where the trend is greater than twice the error or uncertainty in the trend (i.e. 95% confidence). Typically, the uncertainty for long stations is about ±0.1 °C per century, so the stations with a high warming trend will generally have a warming trend of over 0.2 °C per century. Only 11 of the 33 long stations fall into this category, while 14 have a negative trend (see Fig 18.2 below).

Fig. 18.2: Temperature trends of NSW weather stations for the period 1871-2010 plotted against the length of the temperature record in months.

The data in Fig. 18.2 shows that the stations with the largest warming trend are more likely to be those with the least data. This is mainly because they are also the stations that are least likely to have data that extends back before 1960. The long-term temperature trend in New South Wales is shown in Fig. 18.3 below and shows how the local climate has cooled for most of the 20th century before warming after 1960. As the shorter station records are concentrated in the post-1960 period, they tend to have warming trends, whereas the longer records that extend back to 1900 and beyond will be cooler because they also encompass the cooling period. The long station records will also be more important in determining the long-term trend of the region overall, again because they are the longest records and encompass the entire record, not just a small part of it.

Fig. 18.3: Temperature trend for NSW long stations since 1850. The best fit to the data has a gradient of +0.07 ± 0.08 °C per century.

The temperature trend in Fig. 18.3 was determined by calculating the mean of the anomalies of all 33 long stations. These stations are listed on Berkeley Earth here. The procedure for determining the temperature trend in Fig. 18.3 was as follows.

For each station record the monthly reference temperature (MRT) for each month was calculated by finding the mean temperature for that month for the period 1961-1990 (as explained here). These mean values were then subtracted from the raw temperature data to yield the anomaly for each month.  The mean deviation was then calculated for the anomaly and any data that was found to lie outside 6 deviations from the mean was labelled an outlier and excluded.

The factor of 6 was chosen because it relates to 6-sigma accuracy (99.9999998%), which is the standard tolerance level used in manufacturing and physics. As there could be up to 3000 data points per record potentially, an accuracy of less than 99.97% (3-sigma) would be highly likely to exclude a valid data point, even if it wasn't a genuine outlier. It would then require at least 4-sigma accuracy to differentiate an outlier and even then there would still be a 20% chance that the point was good.

Once the outliers have been excluded, the MRTs and the anomalies are recalculated. Finally, the mean of the anomalies of all 33 long stations is determined by adding the records and dividing the sum of the anomalies for each month by the number of temperature records for that month. This average trend is shown in Fig. 18.3.

The striking feature of the data in Fig. 18.3 is that there is no overall upward trend. The mean temperature in 2010 is no higher than it was in 1880, as indicated by the 5-year moving average. In between those dates the temperature trend declines by about 0.6 °C until the mid-1950s, and then rises again. The other main feature is the dip in the mean temperature before 1870. How long into the past this downward trend persists is impossible to determine though. The key question though is, what is the overall trend of the data? Are we seeing a rise or fall in temperature?

It may seem that the obvious way to answer this question is to perform a linear regression on all the available data. It is what Berkeley Earth do to their data, but this would be a mistake. The reason is that we do not have data that is randomly distributed around a straight line. There are oscillations in the data that we need to consider as well. The positions where these oscillations or peaks occur will affect the gradient of the best bit. That means our choice of time-frame for the fitting will affect our result as well.

As I pointed out in Post 4 (Fig. 4.7), the best fit to a sine wave is not a horizontal line of zero gradient, even if an integer number of periods are included in the fitting, and despite there always being equal amounts of data equally distributed above and below the x-axis. The key point is that the oscillatory component has to be symmetric about the centre of the fitting range, otherwise the asymmetry of the oscillations will bias the best fit line. For the data in Fig. 18.3 that means the fitting range needs to be from the first peak to the last peak, i.e. from 1884 to 2007. If we do this then we get a best fit trend of 0.07 ± 0.08 °C per century. In other words, there is a 19% probability that the trend is less than zero, and only a 5% probability it is more than +0.2 °C per century. Of course, if the data before 1884 is included, then the trend would be different. In fact it would be 0.23 ± 0.06 °C per century, but the data before 1884 is not a complete cycle.

Fig. 18.4: Temperature trend for NSW since 1840 according to Berkeley Earth.

Either way, the data in Fig. 18.3 bears no relation to that calculated by Berkeley Earth which is shown in Fig. 18.4 and is different is two major respects. Firstly, the Berkeley Earth trend is virtually zero from 1870 until 1960, and secondly it then exhibits a huge rise in temperature of more than 1 °C after 1960. The reason for this discrepancy will become apparent later in this post, but it is mainly to do with our dear old friend, the breakpoint adjustment.

One final point to note is the "noise level" of this averaged regional data. The anomaly data in Fig. 18.3 has a standard deviation of 1.03 °C even though it is an average of data from 33 different datasets, each of which also has a standard deviation of about 1 °C for its anomalies, as was illustrated in the last post. If those datasets were independent we would expect the "noise" or fluctuations in data values about the mean to reduce to about 0.17 °C, yet it doesn't. This shows that averaging data from different stations does not reduce the noise level significantly. This is because, as I pointed out in Post 11, the temperature data between stations is strongly correlated over distances up to at least 500 km, and therefore it is not independent in a statistical sense. It also means that even regional temperature data for entire countries or continents is still very noisy, and is likely to be subject to the same fractal behaviour I have described here and here. I will demonstrate this again in part (iii) below.


ii) The long and medium station records

The data in Fig. 18.3 only includes data from stations with more than 100 years of data. If we also include stations with over 40 years or 480 months of data (which I denote as medium length stations) the results do not change significantly from that illustrated in Fig. 18.3. In this case the MRTs were calculated for the period 1981-2000.

Fig. 18.5: Temperature trend for NSW long and medium stations since 1850. The best fit to the data has a gradient of +0.02 ± 0.08 °C per century.

The mean anomaly for all long and medium stations in New South Wales (and Australian Capital Territory) is shown in Fig. 18.5 above. The trend of the best fit line is +0.02 ± 0.08 °C per century for the data between 1884 and 2006. Nevertheless, the data is still very different from that shown in Fig. 18.6 from Berkeley Earth. In fact, if we use the Berkeley Earth adjusted data to construct an equivalent trend we get a very different result.

Fig. 18.6: Temperature anomaly and 5-year moving average for Berkeley Earth adjusted data from NSW since 1840. The anomaly data was constructed by averaging the adjusted anomalies for each Berkeley Earth dataset. The best fit line (in red) is for the period 1861-1960 and has a gradient of -0.09 ± 0.11 °C per century.

The data shown in Fig 18.6 above was constructed in exactly the same manner as that described above for the construction of the regional trends in Fig. 18.3 and Fig. 18.5. It was formed simply by averaging the data from all the relevant station datasets. In this case, however, the data was not the raw temperature anomalies from all long and medium stations, but the Berkeley Earth adjusted anomalies from all long and medium stations.

What we find is that simply by adding the Berkeley Earth adjusted anomalies with no additional weighting, we obtain curves that are almost identical to those published by Berkeley Earth and shown in Fig. 18.4. Moreover, the Berkeley Earth adjusted data in virtually horizontal from 1861 until 1960, and then it exhibits a huge rise in temperature of more than 1 °C up to about 2010. The gradient of the trend before 1960 is -0.09 ± 0.11 °C per century (red line in Fig. 18.6), while after 1960 it is +2.2 ± 0.2 °C per century (red line in Fig. 18.7 below).

Fig. 18.7: Smoothed temperature trends (12-month and 10-year) for NSW since 1840 based on Berkeley Earth adjusted data. The best fit line (in red) is for the period 1961-2010 and has a gradient of +2.2 ± 0.2 °C per century.

The data in Fig. 18.7 is just the regional monthly anomaly data in Fig. 18.6 that has been smoothed, either with a 12-month moving average, or a 10-year moving average. This data demonstrates two things. Firstly, because it is virtually identical to the data in Fig. 18.4, it shows that the data in Fig. 18.4 is also effectively just the sum of the adjusted anomalies. This then also demonstrates that the effect of any weighting that Berkeley Earth may have applied to those different stations, to account for differences in the density of stations across the region, is negligible in terms of the overall result, and can therefore be ignored. More importantly, it demonstrates that the differences between the averaged anomaly I have presented in Fig. 18.5 and the Berkeley Earth adjusted data in Fig. 18.6 cannot be due to a lack of appropriate weighting of the individual stations in Fig. 18.5. Therefore, the only place that the difference in the two datasets can come from is in the breakpoint adjustments (and other minor adjustments) which were introduced by Berkeley Earth. The sum total of these adjustments is shown below in Fig. 18.8 together with the best fit for the range 1881-2010.

 Fig. 18.8: The Berkeley Earth breakpoint adjustment for NSW since 1840 together with the difference between the Berkeley Earth adjusted anomaly and the raw anomaly. The best fit to the breakpoint adjustment is +0.35 ± 0.02 °C per century.

The effect of the breakpoint adjustments in Fig. 18.8 is two-fold. On the one hand they flatten the data before 1920. On the other, they increase the warming trend after 1940. The net result is that a climate record with large temperature fluctuations but no overall warming trend becomes a "hockey-stick" with a catastrophic warming of over 1 °C since 1960.


iii) Noise and scaling

In the last post I analysed the scaling of the noise for a single temperature record in New South Wales - Newcastle Nobbys Signal Station (Berkeley Earth ID - 152044). This station showed a scaling behaviour with a power law of -0.266. If we repeat that analysis, but for the averaged New South Wales monthly anomaly data in Fig. 18.5, we get the data shown in Fig. 18.9 below.

Fig. 18.9: Scaling of the noise for the New South Wales mean anomaly. The gradient of the best fit line is -0.272 ± 0.005 and R² = 0.9985.

Here the standard deviation of the averaged data for NSW obeys a power law of the form N ^-a relative to the size of the smoothing average, N, where a = 0.272 ± 0.005. This is very similar to the values seen for individual temperature records, such as those of New Zealand that were discussed in Post 9 and Post 10, which implies that the averaging of multiple station records does not noticeably affect the overall scaling behaviour.In this case at least 33 station records are averaged. Normally we would expect such averaging to reduce the amplitude of the noise by at least a factor of 6. Yet here there is virtually no reduction in the noise level. This implies that the noise of each temperature record is not independent.

The consistency of this scaling behaviour also allows us to extrapolate in order to predict the value of the standard deviation of temperature records with much longer timeframes or moving averages. For example, from Fig. 18.9 we can infer that a 100-year moving average of the NSW temperature record would still have a standard deviation of 0.15 °C. That means that even over very long timescales there will be fluctuations of up to 0.6 °C in the mean temperature in NSW (to a 95% confidence level).


iv) Final thoughts

I pointed out earlier in this post that the trend I constructed in Fig. 18.5 would be largely dictated by data from the 33 long stations. It is worth noting that for most of these 33 records the temperature trend is very similar to the average shown in Fig. 18.5. Only one record closely resembles the Berkeley Earth trend featured in Fig. 18.4. That station is Sydney - Observatory Hill (Berkeley Earth ID - 151986), which just happens to be bang in the middle of the biggest city in the entire country.


v) Conclusions

1) The average temperatures in New South Wales (NSW) today are no greater than they were in the 1880s, 1920s and 1940s (see Fig. 18.5). This suggests that global warming trends were negligible during the 20th century, at least in NSW.

2) The NSW long-term temperature trend exhibits fluctuations of more than 0.5 °C over timescales of more than 100 years (see Fig. 18.5). So what climate scientists think is anthropogenic is, in reality, possibly (or probably?) just noise.

3) Breakpoint adjustments can completely change the form and shape of the long-term temperature trend (see Fig. 18.7). They are not neutral and they are probably not necessary, particularly if the data from multiple stations is going to be averaged anyway.

4) Breakpoint adjustments can add at least 0.35 °C per century to the long-term temperature trend for NSW (see Fig. 18.8). When combined with the changes to the shape of the trend, their effective contribution can be even greater.

5) The noise present in the NSW regional average data is no less than that seen in data from individual station records (i.e. both have a standard deviation of at least 1 °C) despite the regional average data being the result an averaging process of more than 33 records. This also suggests that the unadjusted monthly temperature fluctuations of most stations are highly correlated, otherwise the averaging process would reduce the noise level.

6) The noise in the NSW regional temperature average scales in a similar way to that seen in data for individual temperature records except that the power law is N ^-0.27, where N is the size of the sliding window in the moving average (see Fig. 18.9). The index of this power law is similar to the values seen in individual station records which appear to be centred around N ^-0.25.

7) As the standard deviation for the 60-month smoothed data is 0.35 °C, this means that there is a 50% probability of a temperature rise of more than 0.65 °C occurring over the course of a century in New South Wales purely by random chance, as I explained here. It also implies that chaotic effects are dominant, even over long (i.e. more than 100 years) timescales.

 

Addendum (21/03/2021)

The trend in Fig. 18.5 above was calculated by averaging the monthly anomalies from all stations in NSW with over 480 months of data. The anomalies were calculated relative to the MRT interval of 1981-2000, and for each station to qualify for inclusion in the average it needed to have at least 12 years of data in the 1981-2000 interval for each of the 12 months of the year.

Fig. 18.10: The number of station records included each month in the mean temperature trend for NSW in Fig. 18.5 when the MRT interval is 1981-2000.

The result is that almost 100 station records are included in the trend between 1970 and 1990 (see Fig. 18.10 above). Even between 1907 and 1957 there are over 50 station records included in the mean trend, and about 20 are included between 1880 and 1907. 

Fig. 18.11: Temperature trend for NSW since 1910 according to the Australian Government Bureau of Meteorology (BoM) in 2014.

In Fig. 18.4 I showed the temperature trend for NSW as reported by Berkeley Earth in 2014. It turns out this is very similar to the trend for NSW reported by the Australian Government's Bureau of Meteorology (BoM). The BoM version for 2014 is shown in Fig.18.11 above. It clearly shows that there is a temperature rise (in the 11-year moving average) of about 1.2 °C from 1950 to 2008, and a rise of about 0.7 °C from 1918 to 2008. Yet curiously the picture seven years later is a bit different, as is shown in Fig. 18.12 below.

Fig. 18.12: Temperature trend for NSW since 1910 according to the Australian Government Bureau of Meteorology (BoM) in 2021.

Note how many of the temperatures before 1970 have decreased compared to the equivalent data in Fig. 18.11, some by up to 0.2 °C, while many of those from 1980-2013 have increased, even though the reference period of 1961-1990 remains the same. This is clearly shown in the 11-year moving average where the net result is that the temperature rise from 1950 to 2008 has increased to about 1.35 °C, while the rise from 1918 to 2008 has increased to 0.9 °C. And yet the raw data for these periods haven't changed: only the interpretation and data analysis have.

17. Noise, fractals and scaling revisited

In Post 9 (Fooled by randomness) I presented evidence suggesting that the temperature records used to construct the global temperature trends might exhibit behaviour that could be fractal or quasi-fractal in nature. As a prelude to discussing the temperature trends in Australia, I thought I would first look at this quasi-fractal behaviour in more detail. To illustrate this phenomenon I am going to examine the data from the Newcastle Nobbys Signal Station in New South Wales (Berkeley Earth ID - 152044). I have chosen this station data because it is a fairly long temperature record (almost 150 years), and it is typical of the data seen in New South Wales.

Fig. 17.1: Temperature anomaly for Newcastle Nobbys Signal Station since 1860. Also shown are the standard deviation of the data (σ) and the predicted 95% (±2σ) and 99.7% (±3σ) confidence levels.

i) The noise spectrum

The temperature anomaly for the Newcastle station is shown in Fig. 17.1 above. The monthly reference temperature (MRT) for each month was calculated for the period 1961-1990, and these 12 values were subtracted from the raw data for the respective month to yield the anomalies shown in Fig. 17.1. The dataset has a standard deviation (σ) of 0.907 °C, which is indicated by the red lines on the graph. Also shown are the ±2σ (in green) and ±3σ boundaries (in orange).

Fig. 17.2: The predicted 68.27% (±σ), 95% (±2σ) and 99.7% (±3σ) confidence levels for data with a Gaussian probability density function and a standard deviation of σ.

If the data in Fig. 17.1 has a Gaussian frequency spectrum, you would expect to see a distribution of the data similar to that illustrated in Fig. 17.2 above, with 68.27% of data located within ±σ of the mean, 95.45% within ±2σ, and 99.73% within ±3σ. The data in Fig. 17.1 appears to be distributed roughly along these lines. In Fig. 17.3 below I have converted the anomaly data in Fig. 17.1 to a frequency distribution (with a certain amount of smoothing added in order to reduce the discrete data to a continuum) and then compared this distribution (in blue) to the expected Gaussian distribution with the same standard deviation (σ) of 0.907 °C (red curve). As can be readily seen, the degree of agreement is good. This suggests that this anomaly data, and probably most other anomaly data, does indeed have a probability distribution that is Gaussian.

Fig. 17.3: The calculated probability distribution of temperature 68.27% (±σ), 95% (±2σ) and 99.7% (±3σ) confidence levels for data with a Gaussian probability density function and a standard deviation of σ.

The real benefit of knowing the nature of the probability density function is that it allows us to calculate the likelihood that an extreme temperature change is the the result of a random process, or if not, then whether it may then be anthropogenic in origin. However, the data in Fig. 17.1 is not the temperature change. It represents the difference of each month's temperate from the mean, not the difference from a previous time. We can however, use the data for the former to simulate the latter. This is demonstrated in Fig. 17.4 below.

Fig. 17.4: Year on year change in the monthly temperature anomaly for Newcastle Nobbys Signal Station since 1860. Also shown are the ±σ, ±2σ and ±3σ confidence levels for the monthly anomalies in Fig. 17.1 for comparison.

The data in Fig. 17.4 was derived by calculating the temperature change between identical months one year apart using the anomaly data in Fig. 17.1. What is noticeable is that the spread of the data is greater in Fig. 17.4 than in Fig. 17.1 as indicated by the ±σ, ±2σ and ±3σ lines that refer back to the Fig. 17.1 data. In fact the standard deviation of the data in Fig. 17.4 should be a factor of √2 greater than that from which it is derived. This is because the error (or standard deviation) in the difference of two independent data readings, σ₁₂, depends on the squares of the errors of the original two readings.

(17.1)

As the two readings being subtracted are from the same dataset, they will have the same standard deviation. Therefore, it follows that if σ₁ = σ₂ = σ, then σ₁₂ = σ.√2 (so the standard deviation of the temperature difference is 41% larger than that of the original data). This means that extreme changes in temperature are more frequent than the data in Fig. 17.1 might suggest if one were to look only at the proportion of data that lies outside the ±σ, ±2σ or ±3σ boundaries.



ii) The scaling of the noise with data averaging or smoothing

A key point of note in Post 9 was the scaling behaviour of the noise. It has become apparent that some readers of this blog do not fully understand the methodology I used in Post 9, so I thought it might be useful to explain it again in a bit more detail here.

The central issue is how the data behaves as you smooth it. The smoothing process involves a moving average with a sliding window of data centred on the point that is being averaged. The aim is usually to reduce or eliminate the high frequency noise so that only the underlying trend remains. This is important in analysing climate data because the data in often very noisy, as is illustrated in Fig. 17.1. There is clearly an upward trend in that data between 1960 and 2010, but it is difficult to discern precisely what is happening before 1960. Smoothing can help identify this. However, smoothing also has some negative consequences. It inevitably removes any genuine structure in the data that has a similar frequency to the noise. It will also reduce the heights of maxima and raise the levels of minima in the data. The degree to which this happens will depend upon the sharpness of the turning points relative to the width of the sliding window.

The smoothing process is as follows. First you choose the number, N, of adjacent data points that you wish to average. The general rule is that the bigger the number N, the smaller in value the noise will become, and so the clearer the underlying trend will be.

Suppose you choose N = 5. That means for a given data value for the temperature in Fig. 17.1 (say for March 1966), you add the five temperature anomaly values centred on March 1966 and find the average or mean. That means adding the values for January, February, March, April and May anomalies together and dividing the result by five. Then you repeat this for every data point in Fig. 17.1 by moving along the dataset point by point. As you do so the set of five points to be averaged moves, hence the term "moving average". The width of the data being averaged (in this case N = 5) is the width of your "sliding window". The end result is a completely new dataset that follows the original data but has less noise. This is shown in Fig. 17.5 below where the 4 month smoothed data (N = 4) in green clearly has less noise than the original data in Fig. 17.1. However, smoothing the data by using 16 points (black curve in Fig. 17.5) reduces the noise level even further. What is interesting is why?

Fig. 17.5: The 4-month and 16-month smoothed monthly temperature anomalies for Newcastle Nobbys Signal Station.

The noise reduces because it is random, and if you add enough random numbers together eventually they will average to zero as the approximately equal number of negative and positive values you are adding will gradually cancel. If the noise is white noise, you would expect the mean amplitude of the noise, as represented by the standard deviation, σ, to decrease by a factor of √N. So, smoothing with a 4-month moving average (N = 4) should halve the noise amplitude or standard deviation, while smoothing with a 16-month moving average (N = 16) should reduce the standard deviation of the noise by a factor of 4. Except that does not happen with most temperature data.

Instead what we find is that a 4-month moving average (N = 4) only reduces the standard deviation of the noise by a factor of approximately √2, while a 16-month moving average (N = 16) only reduces it by a factor of about 2. In fact while the standard deviation, σ, for the anomaly data in Fig. 17.1 is 0.907 °C, we find that σ = 0.635 °C for N = 4, and σ = 0.429 °C for N = 16. So, rather than the noise decreasing by a factor of √N, the smoothing appears to reduce it much more slowly. How much more slowly is illustrated in Fig. 17.6 below. 

Fig. 17.6: The scaling behaviour of the smoothed temperature anomaly data for Newcastle Nobbys Signal Station. The graph shows the standard deviations (S.D.) for data smoothed with different moving averages (N). The gradient of the best fit line is -0.266 ± 0.006 and R² = 0.9978.

The data in Fig. 17.6 shows the results for the standard deviation of the original data in Fig. 17.1 (N = 1), together with the standard deviation of same data after it has been smoothed with six different values for N (3, 6, 9, 12, 24, 60). The linear regression or best fit to the log-log plot of the data indicates that the smoothing reduces the standard deviation each time by a factor close to N ^0.266 rather than the expected √N.

Because the trend line in Fig. 17.6 fits the data so well, it means we can use it to extrapolate with a high degree of confidence. That allows us to predict the standard deviation of the smoothed data for any level of smoothing, N, we might choose. For example, if N = 120, then each data point in the smoothed dataset would represent the mean temperature over a decade. Likewise, if N = 1200, each data point would represent the mean temperature for the century of data that was centred on that point in time.

Thus, the best fit line in Fig. 17.6 allows us to predict that σ = 0.252 °C for N = 120, and σ = 0.137 °C for N = 1200 for the smoothed temperature anomaly at this location. Given the rate of change in global temperatures that is claimed for global warming (i.e. 0.7-1.0 °C), these numbers are not insignificant and suggest that low frequency noise may be a significant component of any temperature rise climate scientists claim they are detecting.


iii) The implications for 100-year temperature trends

To illustrate the potential implications of noise in the temperature record, consider the following example. The data in Fig. 17.1 has a standard deviation of 0.907 °C. When smoothed with a 5-year moving average this reduces to 0.31 °C. That implies that 95.45% of the data will be within ±0.62 °C of the mean and 2.275 % will have values that are more than 0.62 °C above the mean. The final 2.275% will have values that are more than 0.62 °C below the mean.

But as we saw in Fig. 17.4, when we look at changes in temperature between years, the standard deviation increases by about a factor of √2. For the data in Fig. 17.4 that means a value of 0.43 °C, and now the fraction of data that sees a rise of 0.86 °C will be p = 2.275%.

But what we really want to know is how likely any temperature rise of this magnitude will be over a 100 year period if it occurs purely by chance. The answer to that will (approximately) be that the overall probability P₁₀₀ will be determined by

P₁₀₀ = 1 - (1 - p)²⁰ 

(17.2)

As p = 0.02275, then P₁₀₀ = 0.37. The power of 20 in Eq. 17.2 reflects the fact that in a 100 year interval there will be an average of 20 possible attempts for the 5-year average to jump by the desired amount or more. In other words, there is a 37% probability of a 0.86°C temperature rise occurring purely by random chance. That is not insignificant. To find the probability of other temperature rises occurring, the maths is more difficult, but not impossible. We just need to find p.

We now know that the anomaly fluctuations obey a Gaussian probability distribution of the form

(17.3)

where σ is the standard deviation and ∆T is the change in temperature from the mean. The probability p that ∆T exceeds some critical value ∆T₀ will be given by

(17.4)

where x = ∆T₀/σ√2 and erfc(x) is the complementary error function. Using these equations we can now find the probability of other temperature rises occurring naturally via random processes.

As I have already demonstrated, there is a 37% probability of a 0.86°C temperature rise in 100 years due to random chance. In other words, if ∆T₀ = 2σ, and σ = 0.43 °C, then p = 0.02275 and P₁₀₀ = 0.37. Similarly, the above analysis means that this probability (P₁₀₀) will increase to 89% for a ∆T₀ = 0.7 °C temperature rise and fall to 18% for a 1.0 °C rise for the same value of σ. In addition, the data implies that there will be a 50% probability of a random temperature rise of more than 0.78 °C at some time over the century. It should also be remembered that all these probabilities are based on the initial standard deviation of 0.907 °C. If the standard deviation increases (as is seen, for example, in other datasets), then so too will the probabilities.


iv) The scaling of the noise frequency

So far I have only considered the scaling behaviour of the anomaly amplitude. But it is clear from the data in Fig. 17.1 and Fig. 17.5 that smoothing the data changes the underlying frequency of the fluctuations as well. One might expect this frequency to decrease linearly with the smoothing factor N, but it does not.

The easiest way to quantify the frequency of the fluctuations is to count the number of times the data crosses the mean value of the anomaly in a given time interval. For example, in Fig. 17.1 the data crosses the mean value 317 times over the course of 140 years. This decreases to 155 for a 3-month moving average, and 54 for a 12-month moving average.

Fig. 17.7: The scaling behaviour of the mean period of the smoothed temperature anomalies for Newcastle Nobbys Signal Station for different moving averages (N). The gradient of the best fit line is 0.756 ± 0.031 and R² = 0.9917.

Plotting this data on a log-log plot again yields a linear trend indicating that the period of the fluctuations increases as N ^a. In this case, though, a = 0.756 ± 0.031 with a residual that is 9% of the standard deviation of y-values(see Fig. 17.7 above).

In Post 9 I showed theoretical plots (see Figs. 9.7 - 9.10) to highlight how the scaling could be used to predict the qualitative behaviour of longer time series. However, these plots did not include the appropriate scaling of the time axis. It assumed the scaling of the time base was proportional to N. If the correct scaling is now included, the results will be similar, but the time axis will expand slightly.

Fig. 17.8: Simulated 2000 year temperature anomalies with 50-year mean for Newcastle Nobbys Signal Station.

As an example of the power of predictive scaling, consider the scaled data shown in Fig. 17.8 above. This data aims to simulate the behaviour of the Newcastle Nobbys Signal Station (Berkeley Earth ID - 152044) time series over an arbitrary 2000 year time period where the data will typically have been smoothed to a resolution of 50 years. That means N = 600 and the standard deviation of the fluctuations will be 0.183 °C. The time base and the mean period of the fluctuations, however, will increase by a factor of about 120. Finally the data is filtered so that only every 6th point is plotted. This corresponds to one point per decade. What Fig. 17.8 shows is that large swings in the mean decadal temperature of more up to 0.5 °C within a century are expected to be fairly common. The big question is, is it realistic? And can we find any corroboration for it?

Fig. 17.9: A 2000 year proxy temperature record for China.

Of course we do not have temperature records that are 2000 years long, but we do have some proxy records, including this one from China (see Fig. 17.9 above). Intriguingly, the China data in Fig. 17.9 shows many features that are not that dissimilar to the simulated data in Fig. 17.8. The standard deviations of the fluctuations in both appear to be around to 0.2 °C and the period of the fluctuations appear similar as well. So, is the China data evidence of the validity of our approach? Quite possibly, but then I might be biased.


v) Conclusions

Wherever I look in climate science data, all I can see is noise. It appears that much of this noise is more persistent than that seen with typical white noise, and this becomes significant in temperature trends that are several decades or centuries in length. It leads to long timescale (low frequency) fluctuations that are comparable in amplitude to those currently being attributed to global warming. This represents a serious challenge to current climate science orthodoxy, because I cannot see how you can definitively distinguish one effect from the other, based on the available data.

In physics we normally aim for a 3-sigma level of confidence before we even begin to make claims of proof regarding any theory or hypothesis. Normally it requires 5-sigma levels of confidence. The Higgs boson was confirmed with a 5-sigma level of confidence that has now risen to nearly 7-sigma. Yet most climate data seems incapable of exhibiting even a 1-sigma level of confidence when compared to any available theory or model. But the claims made by many climate scientists about said data would seem to imply otherwise. None of their pronouncements ever seem to be qualified. It is this lack of doubt and critical thinking among climate scientists that I find most troubling.

Fig. 17.10: A 10,000 year proxy temperature record for Greenland based on the GISP2 ice core isotope data.

Take the data in Fig. 17.10  above, for example. This is a plot of the inland temperature of the Greenland ice sheet over the last 10,000 years based on ice core data. It shows large fluctuations in temperature that are comparable to those seen in modern temperature records, but the mean period of the fluctuations is much larger. That is probably because the timescale is much larger. But the question is: are these fluctuations real in the sense that there is an immediate identifiable cause for them? Or are they just noise? The fact is we don't know. Climate scientists have proposed countless theories to account for the fluctuations, but none are watertight. Yet the possibility that it might all be just noise resulting from the afterglow of a much bigger physical event (such as Milankovitch oscillations in this case), in the same way that the cosmic microwave background is the afterglow of the Big Bang, still doesn't appear to register with most of them.

16. The story so far

 

The main purpose of this blog has been to analyse the physics behind climate change, and then to compare what the basic physics and raw data are indicating with what the climate scientists are saying. These are the results so far.

Post 7 looked at the temperature trend in New Zealand and found that the overall mean temperature actually declined until 1940 before increasing slightly up to the present day. The overall temperature change was a slight rise, but only amounting to about 0.25 °C since the mid 19th century. This is much less than the 1 °C rise climate scientists claim.

Post 8 examined the temperature trend in New Zealand in more detail and found that the breakpoint adjustments made to the data by Berkeley Earth, that were intended to correct for data flaws, actually added more warming to the trend than was in the original data.

Post 9 looked at the noise spectrum of the New Zealand data and found evidence of self-similarity and scaling behaviour with a fractal dimension of about 0.25. This implies that long-term temperature records over several thousands of years should still see fluctuations between the average temperature each century of over 0.5 °C, even without human intervention. In other words, a temperature rise (or fall) of at least up to 1 °C over a century is likely to be fairly common over time, and perfectly natural.

Post 10 looked at the impact of Berkeley Earth's breakpoint adjustments on the scaling behaviour of the temperature records and found that they had a negative impact. In other words the integrity of the data appeared to decline rather than improve after the adjustments were made.

Post 11 looked at the degree of correlation between pairs of temperature records in New Zealand as a function of their distance apart. For the original data a strong linear negative trend was observed for the maximum possible correlation between station pairs over distances up to 3000 km. But again the effect of Berkeley Earth's breakpoint adjustments to the data was a negative one. This trend became less detectable after the adjustments had been made. The one-year and five-year moving average smoothed data did become more highly correlated though.

After analysing the physics that dictate how the Sun and the Earth's atmosphere interact to set the Earth's surface temperature in Post 13, I then explored the implications of direct heating or energy liberation by humans at the Earth's surface in Post 14. Calculations of this direct anthropogenic surface heating (DASH) showed that while human energy use only contributed an average increase of 0.013 °C to the current overall global temperature, this energy use was highly concentrated. It is practically zero over the oceans and the poles, but in the USA it leads to an average increase of almost 0.2 °C. This rises to 0.3 °C in Texas and 0.5 °C in Pennsylvania. Yet in Europe the increases are typically even greater. In England the increase is almost 0.7 °C, and in the Benelux countries almost 1.0 °C. Perhaps more significantly for our understanding of retreating glaciers, the mean temperature rise from this effect for all the alpine countries is at least 0.3 °C.

Finally in Post 15 I looked at the energy requirements for sea level rise (SLR). Recent papers have claimed that sea levels are rising by up to 3.5 mm per year while NOAA/NASA satellite data puts the rise at 3.1 mm per year. These values are non-trivial but are still a long way short of the rate needed to cause serious environmental problems over the next 100 years.

In upcoming posts I will examine more of the global temperature data. But given what I have discovered so far, it would be a surprise if the results were found to be as clear cut as climate scientists claim. Contrary to what many claim, the science is not settled, and the data is open to many interpretations. That is not to say that everything is hunky dory though. Far from it.

15. The truth about sea level rise

One of the most emotive and alarmist claims made by climate scientists is that global warming will lead to a catastrophic sea level rise (SLR) that will submerge major cities and lead to an unprecedented humanitarian crisis and global extinction event.

Fig. 15.1: Britain's favourite polar bear - Peppy.

On the face of it this seems quite plausible, even likely. We see images of collapsing ice shelves and retreating glaciers on an almost daily basis. We see icebergs the size of cities being calved off from Antarctica and then polar bears looking forlorn on icebergs the size of a lifeboat, like something from a Fox’s glacier mint advert. So what is the reality?

Fig. 15.2: Life imitating art.

There are two principal ways that sea levels might change: either the amount of water in the sea changes, or the existing sea water changes its density. The former can happen if ice caps on Greenland or Antarctica melt. It cannot happen through the melting of sea ice because of Archimedes' Principle as I explained in Post 2, nor for the same reason can an increase in sea ice change the sea level. One alternatively mechanism is through increased evaporation of condensation, but that requires the humidity of the atmosphere to change. As for density changes, these are governed mainly by changes in the temperature of the sea water.


Scenario 1: Thermal Expansion

In the case of rising sea temperatures it is not the current global temperature that is important per se, but the temperature history and the thermal budget of the Earth. Global temperature rises can only raise sea levels directly (excluding from ice melting) by thermal expansion of the sea water. As I pointed out in Post 2, the coefficient of thermal expansion by volume for water is 0.000207 per degree Celsius or 207 ppm/°C. So a column of water 1000 metres or 1 km high will increase in height by only 20.7 cm if its temperature increases by 1 °C (or 1 K where K is the unit of absolute thermodynamic temperature - the kelvin).

But there is another factor we need to consider - the total heat or thermal energy required to do this. This is because this energy needs to come from somewhere, and once used it remains trapped in the water. It is, therefore, energy that has been sequestrated, the effect of which is to create an imbalance between the amount of energy the Earth receives from the Sun, and the amount it emits back out into space. This difference can only come from the net energy imbalance of the Earth’s energy budget.

In the last post we saw that this imbalance is currently estimated to be as much as (and no-one is saying it is more than) 0.9 W/m². If this figure is true (and as I pointed out, there is enormous uncertainty over its accuracy), and if it had been constant over the last 100 years (which is very unlikely), it would imply that the Earth has absorbed a total of 4.591x10¹⁴ joules of energy every second in that time period, or 1.45 x 10²⁴ J in total. Of course that is the upper limit of what is likely. No-one seriously thinks the Earth’s energy imbalance has always been 0.9 W/m². So the average over the last 100 years must be considerably less, and probably less than half.

Whatever the value, though, this heat will increase the temperature of the oceans. The question is, by how much, or to what depth?

As the specific heat capacity of water is 4200 Jkg^-1K^-1, the amount of energy required to heat 1 kg of water by 1 K (or 1 °C as these temperature changes are the same) will be 4200 J. If our 1 km high water column has a cross-section of 1 m², then it will contain 1000 tonnes of waters. Therefore, the total energy required to raise the temperature of the entire column by 1 °C will be 1,000,000 times greater than 4200 J, in other words 4.2 x 10⁹ J. As 70.8% of the Earth’s surface is covered by the oceans, the total volume of water down to a depth of 1 km will be 3.61 x 10¹⁷ m³. The total mass will be 3.61 x 10²⁰ kg, and the total heat capacity will be 4200 times higher still at 1.52 x 10²⁴ J/°C. So the mean temperature rise of the oceans down to a depth of 1 km over then last 100 years will be (at the absolute maximum) 1.45 x 10²⁴ ÷ (1.52 x 10²⁴) = 0.95 °C.

So, if we assume that the Earth’s energy imbalance over the last 100 years has been 0.9 W/m² everywhere and at all times, and if we assume that this heat has all ended up in the ocean, and it has heated the top 1000 m only, then the temperature rise of that top layer of water will be 0.95 °C. Some may find this number suspiciously close to the value claimed for global warming of about 1 °C per century. In other words, that climate scientists have worked backwards. They have assumed that the oceans must heat up by the same amount as the land over the same period, and to a depth of up to 1000 m, and worked out the amount of heat required to do this. From this they have inferred an imbalance in the Earth’s thermal budget rather than measured it.

Whatever the sequence of events, the resulting sea level rise (SLR) will be 197 mm (i.e. 0.95 x 207). If the warming layer of the ocean is thinner (say 500 m) but the surface energy imbalance is still 0.9 W/m², its mean temperature rise will be greater (an unlikely 1.90 °C for a 500 m thick layer), but the SLR will be the same, in other words a massive 1.97 mm per year. So what is clear is that the maximum sea level rise that can occur depends on the energy imbalance and not the water depth. In reality, the surface energy imbalance may be less than 0.9 W/m² (G. L. Stephens et al., Nature Geoscience 5, 691–696 (2012) suggest 0.6 W/m² as I pointed out in Post 13) and has in all likelihood got worse over time. So while it may be 0.9 W/m² now, it has probably averaged less than half of 0.9 W/m² over the last 100 years as global temperatures have risen. In which case the SLR has probably been less than 100 mm over the last century (or less than 66 mm if Stephens et al. are correct). But what about the future?

Assuming that the surface energy imbalance remains at 0.9 W/m² for the foreseeable future, and if we now assume that only that portion of the surface energy imbalance over the oceans is actually absorbed by the oceans, then the heat absorbed by the oceans each year will be 28.4 MJ/m². If this were absorbed by a column of water 1000 m deep it would result in a temperature rise of 6.76 mK. If it were instead absorbed by a column of water only 100 m deep it would result in a temperature rise of 67.6 mK. Either way, the resulting thermal expansion would be 1.40 mm per year. Even over 100 years this is a long way short of the 10 m rise some doom-mongers are projecting, and on its own is unlikely to pose a major threat to human civilization or the planet.

But thermal expansion is just one component of the overall problem, because not all the 0.9 W/m² (or 0.6 W/m²) need end up in the oceans. Some may instead end up melting the ice caps.


Scenario 2: Melting Ice Caps

In this scenario there are a number of factors that we need to identify and address. Firstly, where is the ice? If it is floating on the ocean surface then it cannot add to the sea level increase when it melts, despite 9% of the ice being above the water line. This is because of Archimedes’ principle as I explained in Post 2. It can, though, sequestrate energy and actually reduce warming elsewhere. The only ice that can increase sea levels when it melts is ice that is on land. This is found mainly on Greenland (3 million cubic kilometres) and Antarctica (30 million cubic kilometres).

Then there is the question of the ice temperature. Before it can melt it needs to be heated to 0 °C, yet the mean temperature of the ice on Antarctica is about -50 °C. In order to raise the ice temperature to melting point would require 3.20 x 10²⁴ J of energy (the specific heat capacity of ice is 2108 Jkg^-1K^-1 and its relative density is 0.92).

Next, you need to melt the ice. This will require an energy input of 1.01 x 10²⁵ J (the specific latent heat of ice is 332 kJ/kg); and then you need to heat it to the ambient temperature of the Earth, about 15 °C, otherwise it will cool the oceans and your global temperatures will go down. This requires another 1.91 x 10²⁴ J of energy. So the total energy required is 1.52 x 10²⁵ J. Given that the amount of power available to achieve this is at most only 0.9 W/m², that means it would take at least 1100 years to occur. But even that assumes that all the power from the Earth’s energy imbalance could be channelled somehow into melting the ice caps and nothing else.

In reality most will initially go into the oceans. As Antarctica and Greenland comprise only 3% of the Earth’s surface area, a more realistic estimate is that it would take up to 30,000 years, by which time we would be in the next ice age. Recent studies of Greenland appear to confirm this as they show Greenland has lost less that 0.05% of its ice in the last 10 years, but this may just be cyclical. 

One final point of note: while the total volume of ice on Antarctica is 30 million cubic kilometres, almost a third of this is below sea level. The net result is that if all the ice on Antarctica and Greenland were to melt, sea levels would rise by 65 metres not 91 metres. Yet over 30,000 years this will amount to a rise of only about 2 mm per year.


Scenario 3: Evaporation

The final possibility is that the sea water might just evaporate into the atmosphere leading to a loss of volume in the sea rather than a gain. Approximately 0.4% of the atmosphere by volume is water vapour. But if all this water were to suddenly condense out of the atmosphere it would only add 3.6 cm to the depth of the oceans. Given that the ambient temperature at the surface of the Earth is 15 °C, while the humidity at 0 °C would be expected to drop to near zero, this suggests that an increase of 1 °C in the surface temperature of the air would lead to a decrease in sea levels of around 2.4 mm. That is a pretty crude estimate, though, and assumes that the vapour pressure of water increases proportionately with temperature from its freezing point. A more accurate estimate can be made using the Clausius-Clapeyron equation.

Fig. 15.3: Schematic of phase boundaries on a P-T diagram.

This equation (see Eq. 15.1) relates the the slope of a phase boundary in a pressure-temperature diagram to the thermodynamic temperature, T, the molar latent heat for the phase change, L, and the change in molar volume across the boundary ∆V. An example of a phase diagram is shown in Fig. 15.3 above.

(15.1)

For a change from liquid to gas (as in evaporation) the term ∆V should be the difference in molar volumes between the water in the liquid phase and the vapour in the gas phase. However, as the volume of the vapour at the relevant pressures we are likely to encounter (i.e. around atmospheric pressure) is so much greater than it is for water (in fact by more than a factor of 1000) we can use the ideal gas law in Eq. 15.2 to substitute the molar volume of water vapour V for ∆V on the basis that the molar volume of the liquid is negligible.

(15.2)

In Eq. 15.2 the term V is the volume of one mole of water vapour at a pressure P and a temperature T. The term R is the molar gas constant where R = 8.314 Jmol^-1K^-1 and n is the molar density in mol/m³. The approximation of V for ∆V allows us to make a substitution from Eq. 15.2 into Eq. 15.1 to generate Eq. 15.3 which is now a function of only two variables, P and T.

(15.3)

This allows us to relate fractional changes in the pressure of the gaseous phase to fractional changes in temperature along the phase boundary. The differential in Eq. 15.3 implies that for small changes in P and T the following relation holds

(15.4)

while Eq. 15.2 yields the following relation between P, T and n.

(15.5)

Equating Eq. 15.4 with Eq. 15.5 gives the result for the fractional change in molar concentration of the vapour that will occur due to  evaporation across the phase boundary for a temperature change ∆T.

(15.6)

The relation in Eq. 15.6 allows us to estimate the change in water vapour concentration n as the temperature T changes. So for example, if the temperature T = 288 K and the latent heat of evaporation of water is 40.8 kJmol^-1K^-1, then a temperature rise of ∆T = 1 K will yield a fractional change of water vapour concentration of 0.0557. That in turn implies a total fall in sea level over the period of the temperature rise (which is about 100 years) of 2.0 mm (= 0.0557x36). Reassuringly, this is not that dissimilar to our original estimate of 2.4 mm, thus demonstrating two important points. Firstly, that the result is robust and consistent. Secondly, that our original back-of-the-envelope approximation, much loved by physicists everywhere, did not let us down.

The advantage of calculating the fractional change in n using Eq. 15.6 rather than calculating ∆n directly is that it means that we can avoid the complication of working out the relative humidity. The Clausius-Clapeyron equation, strictly speaking, only applies to closed systems in equilibrium, i.e. at 100% relative humidity. In open systems, such as the Earth's atmosphere above large oceans, the humidity is always less than the maximum. But Eq. 15.6 effectively removes the issue of relative humidity as it just introduces an additional scaling term that applies more or less equally to n and ∆n. Therefore it cancels out in Eq. 15.6. All of this may be somewhat pedantic, however, as the sea level fall due to evaporation is a full two orders of magnitude less than the previous two effects considered.


So the conclusion is this: prophesies of apocalyptic rising sea levels and submerging cities are still just alarmist nonsense. The physics proves that there is currently not enough energy available to achieve this on the timescale that some climate scientists predict, at least not yet. Thermal expansion and melting ice caps will each add not much more than 2 mm per year to sea levels. A recent paper by Anny Cazenave et al. (Advances in Space Research 62(7) 1639-1653 (2018) ) puts the sea level rise (SLR) from all sources at about 3.5 mm per year for the period 2005-2015  (see Fig. 15.4 below). These numbers do appear to be more consistent with the surface energy imbalance of 0.9 W/m² reported by Trenberth and co-workers rather than the 0.6 W/m² of Stephens et al., particularly the thermal expansion component.

Fig. 15.4: Possible breakdown of different contributions to sea level rise (1993-2015) from Cazenave et al.

One of the striking features of Fig. 15.4 in my view is the low contribution to sea level rise from ice melt in Antarctica compared to that from glaciers. Three explanations spring to mind. Firstly, there is probably more warming in the Northern Hemisphere because that is where the heat is being generated. Secondly, the glaciers in Europe are very close to the source of that heating. And thirdly, the ice in Antarctica is much colder than that in alpine glaciers, and so requires more heat to melt it. So, as I pointed out in the last post, this could mean that alpine glaciers will continue to recede, not because of CO₂ emissions, but because of local human industrial activity that leads to surface heating of the local environment, and thus a temperature rise of more than 0.3 °C above pre-industrial levels.