7. New Zealand - trend due to long stations SLIGHT WARMING

As I pointed out last time, New Zealand has some very good temperature data, or at least good in comparison to most other countries in the Southern Hemisphere. It has over 50 sets of station data, of which ten have over 1200 months of data, and about a dozen have more than 400 months of data, yet even that is not enough.

The stations with over 1200 months of data I shall denote as long stations, those with over 400 months I denote as medium stations. Their geographical locations are shown in Fig. 6.1 in the last post. To start with I shall analyse the long station records, (a) because there are a large number of them, and (b) because they are the records that will show the most discernible trend, yet even with many of these stations the trend can be ambiguous.

If we start with the individual stations, a typical temperature anomaly, i.e. the mean temperature each month minus the monthly reference temperature (MRT), is shown below together with its best fit line and a 5-year moving average. This is the data for Auckland (Albert Park). It dates back to 1853 and it is one of the oldest, longest and most complete records in New Zealand.

Fig. 7.1: Temperature anomaly for Albert Park, Auckland (1853-2013). The best fit line has a positive gradient of 0.18 ± 0.05 °C per century.

Two things are immediately apparent. The first is that the noise level on the anomaly data makes it difficult to discern the overall trend in the data even though the seasonal variation (the MRT) has been removed, and remember, the data represents the average temperature over a whole month not just a typical day. In fact the standard deviation of the data in Fig. 7.1 is 0.95 °C and still almost 25% of data lie outside this range.

The second is that, while the trend does become more discernible if a 5-year moving average is performed (the yellow curve), the trend is still not uniform, nor does it represent a single continuous rise or fall. In fact the temperature in 2013 appears to be no higher than in the 1850s and there is a clear sign of a longer term (150 year) oscillation.

Also shown in Fig. 7.1 is the best fit to the anomaly data. While this fit line has a positive slope of 0.18±0.05 °C per century it needs to be acknowledged that this is not due to a warming trend, but is purely as a consequence of the 150 year oscillation. If you look back at Fig. 4.7 you will recall that the best fit to a full sine wave always has a positive slope. What is clear, however, is that the trend shown in Fig. 7.1 bears no resemblance to the one expected for the region, as illustrated below and on the Berkeley Earth site, either in shape or magnitude.

Fig. 7.2: Berkeley Earth warming trend for New Zealand (1853-2013).

So what about the other nine stations? Well the next three longest are shown in Fig. 7.3. These are the records from Dunedin (ID 18603), Christchurch (157045) and Wellington (18625).

Fig. 7.3: Smoothed temperature anomalies and best fit lines for three stations.

Here the results are even more contradictory. In Fig. 7.3 each set of data is plotted as in the form of the 5-year moving average together with the best fit line. The legend on the graph indicates the slope of each best fit line in degrees Celsius per century, therefore the data for Dunedin-Musselburgh (Berkeley Earth ID = 18603) has a warming trend of 0.64°C per century, while that for Wellington-Kelburn (Berkeley Earth ID = 18625) has a cooling trend of -0.25 °C per century. That for Christchurch (Berkeley Earth ID = 157045) is slightly warming. Of the three datasets, those for Dunedin and Christchurch do appear to offer a passing resemblance to the Berkeley Earth trend in Fig. 7.2, although neither exhibits a temperature gradient as high as that in Fig. 7.2, while Christchurch and Wellington both appear to have a 150 year oscillation that is responsible for most of the slope in the best fit.

Fig. 7.4: Smoothed temperature anomalies and best fit lines for three stations.

If we consider the three next longest records we observe a common theme (see Fig. 7.4). After 1920 there is a distinct warming trend (as was seen in most of the previous data above) but before 1920 much of the important data is missing. Given what we have seen in previous data, it is reasonable to suppose that this data would be of a higher temperature than that which is present, and therefore that the warming trends indicated by the best fit lines in Fig. 7.4 are over-estimates.

Fig. 7.5: Smoothed temperature anomalies and best fit lines for three stations.

For the remaining stations the lack of early data becomes an even bigger problem (see Fig. 7.5), and while a trend post-1940 can be discerned, the early data is highly fragmented. Nevertheless, the last two datasets in Fig. 7.5 both have a peak at about 1890 which is clearly evident on all the data in Fig. 7.1 and Fig. 7.3, and which is at a comparable height relative to the data around 1960 in each case. That suggests that most of this data is consistent and sound.

As I pointed out in post 5, if we wish to derive a regional trend such as that shown in Fig. 7.2, all we need to do is average the anomalies (provided all the data has been processed in a consistent manner and is reliable). When we do this for the ten stations described above, we get the dataset illustrated below in Fig. 7.6.

Fig. 7.6: The warming trend for New Zealand (1853-2013) based on the averaging of long station anomalies. The best fit line has a positive gradient of 0.29 ± 0.04 °C per century.

The mean temperature change or anomaly plotted in Fig. 7.6 has a slight upward trend and its best fit line has a gradient of 0.29±0.04 °C per century. However, this variation is clearly in two parts. From 1860 to 1940 the trend is clearly downwards, while from 1940 to 2000 it is upwards.

What is also clear is that the temperature trends in Fig. 7.6 are very similar to those shown for Auckland in Fig. 7.1 (and actually most of the other individual station datasets) but very dissimilar to that advanced by Berkeley Earth in Fig. 7.2. The question is why?

Well, there are two reasons, and they are both to do with how the temperature data is handled and processed. The graphs I have presented here in Fig. 7.1 and Figs. 7.3-7.6 all use the original temperature data as is. I first calculate the MRT following the method outlined here and subtract it from the original data to obtain the anomaly without the problem of the large seasonal variations. That is a standard procedure that all climate science groups should do. The time base I used for the calculation of the MRT was 1961-1990 for reasons outlined here. Again, this time frame appears to be fairly standard. However, despite this my MRT values differ slightly from those of Berkeley Earth. The reason for this I have not discovered yet, but it may be that old favourite of climate scientists, homogenization, or it may be a different choice of time frame. Whatever the reason, fortunately it makes little difference. If you sum the Berkeley Earth anomalies the result is very similar as shown in below in Fig. 7.7.

Fig. 7.7: The warming trend for New Zealand (1853-2013) based on the averaging of the Berkeley Earth anomalies for long stations.

That, however, is where the similarity ends because Berkeley Earth then play their joker: breakpoint alignment. This is a mathematical device that is supposed to account for imperfections in the data due to human measurement error, changes in instruments, location moves and changes in the time of day when the measurements were made. How this is implemented I will discuss at a later date. What is important here is the net result and that is shown in Fig. 7.8.

Fig. 7.8: The warming trend for New Zealand (1853-2013) based on the averaging of the adjusted Berkeley Earth anomalies for long stations after breakpoint alignment.

The differences between Fig. 7.6 (or Fig. 7.7) and Fig. 7.8 are subtle but clear when you finally see it. In Fig. 7.6 there is a discontinuity or kink in the gradient of the general trend around 1940 and the slope is shallow. In Fig. 7.8 the slope is more uniform and steeper. The gradient of the best fit line has now more than doubled to 0.60 ± 0.04 °C per century and the temperature rise from 1860 to 2010 has gone from virtually zero in Fig. 7.6 to an impressive 0.9 °C. Now, if we smooth the data in Fig. 7.8 using a 12-month and a 10-year moving average, we get the curves shown in Fig. 7.9 below which look very similar to the Berkeley Earth summary trend in Fig. 7.2.

Fig. 7.9: The smoothed warming trend for New Zealand (1853-2013) based on the averaging of the adjusted Berkeley Earth anomalies for long stations after breakpoint alignment.

This validates our averaging process, but the problem is that Fig. 7.9 bears very little resemblance to the original data in Fig. 7.6. This difference cannot be due to a difference in the averaging process, otherwise Fig. 7.9 would not resemble Fig. 7.2 so closely. That only leaves homogenization and breakpoint adjustments as the possible causes of the difference. The equally worrying question is, why are these adjustments being made? Those reasons may become apparent as we look at more of the global temperature data.