124: Arctic temperature trends - a comparison

In the previous five posts I examined the temperature changes for five different territories in the Arctic region (Greenland, Iceland, the Faroe Islands, Jan Mayen and Svalbard). All appeared to exhibit similar trends with a peak in temperatures in the 1930s followed by a dip, and then another rise after 1980. In this post I will compare and examine these five trends in more detail.

In Post 11 I demonstrated how the correlation between temperature trends from different stations depends on their separation: the further apart they are, the less well correlated they are. In fact if the distance between them exceeds 1500 km their correlation becomes very weak. On that basis we would not expect any great correlation between the the Faroes and Svalbard as they are over 2,000 km apart. In contrast, Greenland, the Faroes and Jan Mayen are all less than 600 km from Iceland, so the correlations of their temperature data with that from Iceland should be must stronger. The trends in Fig. 124.1 below attempts to do just that, compare the trends of Greenland, the Faroes and Jan Mayen with that of Iceland. For clarity the trends of Greenland and the Faroes are offset vertically by -3°C and +3°C respectively, as are their Icelandic comparator curves.

Fig. 124.1: The 5-year moving average temperature trends for Greenland, Jan Mayen and the Faroe Islands all compared against the equivalent trend for Iceland.

The data in Fig. 124.1 can be summarized as follows.

The trends of Greenland, Jan Mayen and the Faroe Islands all appear to follow the same broad pattern. Temperatures peak in the 1930s, then decline by about 2°C by the 1980s before peaking again after 2000. Only in Jan Mayen is the peak after 2000 higher (by about 0.5°C) than the one in the 1930s.

The trend from the Faroe Islands is most closely correlated with that of Iceland. Not only is the broad trend the same, but the smaller peaks and troughs also align well.

The smaller peaks for Greenland are not closely correlated with those of Iceland or the other two regions. This may be because the mean temperature anomaly (MTA) for Greenland is the result of averaging anomalies from stations over a much larger area than is the case for Iceland, Jan Mayen and the Faroe Islands. So some of the fine detail may be lost by the averaging of stations that are themselves not well correlated.

Jan Mayen shows better correlation with Iceland but its peaks and troughs are larger in size. This may be the result of it having a more extreme climate (due to being inside the Arctic Circle) where the temperature anomalies are naturally larger.

Fig. 124.2: The 5-year moving average temperature trends for Greenland, Iceland and Svalbard all compared against the equivalent trend for Jan Mayen.

If we repeat the process used for Fig. 124.1 but instead use the temperature trend of Jan Mayen as the reference comparator, then we get the trends shown in Fig. 124.2 above.

What we see from Fig. 124.2 is similar to what we saw in Fig. 124.1 with temperatures peaking in the 1930s, then declining by about 2°C by the 1980s before peaking again after 2000.

Once again the smaller peaks for the trend of Greenland are not closely correlated with those of the comparator (in this case Jan Mayen), and the reason is probably the same.

This time, though, the greatest correlation of the smaller peaks and troughs in each trend line is between those for Jan Mayen and Svalbard. This is perhaps not a surprise given that they are near(-ish) neighbours and both are well inside the Arctic Circle.

Summary and conclusions

The general long-term temperature trends of Greenland, Iceland, Jan Mayen and the Faroe Islands are well correlated over timescales of more than 20 years. This suggests that there is no need to adjust the temperature data because the data is correct.

Correlations on shorter timescales (5-10 years) are generally weaker. The two notable exceptions are firstly Jan Mayen and Svalbard, and secondly Iceland and the Faroe Islands.

120: Iceland - temperature trends COOLING before 2000

It is tempting to think of Iceland as being Greenland's little brother. This is not just because Iceland is smaller and close to Greenland, but also because their changes in climate over the last two hundred years are very similar. Like in Greenland, the climate of Iceland cooled from 1930 to 1990 before the mean temperatures rebounded. And like in Greenland, the mean temperatures today are no higher than they were in the 1930s (see Fig. 120.1 below).

Fig. 120.1: The mean temperature change for Iceland since 1920 relative to the 1971-2000 monthly averages. The best fit is applied to the monthly mean data from 1931 to 2000 and has a negative gradient of -1.43 ± 0.24 °C per century.

In order to quantify the changes to the climate of Iceland the temperature anomalies for each of the 21 stations with the most data (over 300 months) were determined and averaged. This was done using the usual method as outlined in Post 47 and involved first calculating the temperature anomaly each month for each station, and then averaging those anomalies to determine the mean temperature anomaly (MTA) for the region. The MTA since 1920 is shown as a time series in Fig. 120.1 above and clearly shows that temperatures declined continuously from 1930 to 1990 before levelling off and then rebounding.

The process of determining the MTA in Fig. 120.1 involved first determining the monthly reference temperatures (MRTs) for each station using a set reference period, in this case from 1971 to 2000, and then subtracting the MRTs from the raw temperature data to deliver the anomalies. If a station had at least twelve valid temperatures per month within the MRT interval (1971-2000) then its anomalies were included in the calculation of the mean temperature anomaly (MTA). The total number of stations included in the MTA in Fig. 120.1 each month is indicated in Fig. 120.2 below. The peak in the frequency after 1980 suggests that the 1971-2000 interval was indeed the most appropriate to use for the MRTs, although 1981-2010 would have been equally appropriate.

Fig. 120.2: The number of station records included each month in the mean temperature anomaly (MTA) trend for Iceland in Fig. 120.1.

The data in Fig. 120.2 above indicates that after 1940 there were up to 21 active stations, but before 1940 there were less than about six with only one station being operational before 1870. As six is generally too low a number to produce a reliable trend, the MTA data in Fig. 120.1 was truncated with only data post-1920 being shown. However, if all the data is considered, the MTA trend will have data extending back to 1823 as shown in Fig. 120.3 below. Note also that the low number of stations before 1900 results in a much higher variance of points in Fig. 120.3 about the mean (yellow line). This is more evidence of the greater unreliability of this earlier data, which is why the plot shown in Fig. 120.1 is more statistically reliable.

Fig. 120.3: The mean temperature change for Iceland since 1820 relative to the 1971-2000 monthly averages. The best fit is applied to the monthly mean data from 1841 to 2000 and has a positive gradient of +0.71 ± 0.08 °C per century.

The locations of the 21 stations used to determine the MTA in Fig. 120.3 are shown in the map in Fig. 120.4 below. Of these 21 stations, six are long stations with over 1200 months of data before 2014, and a further five are medium stations with over 480 months of data. The stations are evenly distributed across the island with most on, or near, the coast.

Fig. 120.4: The (approximate) locations of the 21 longest weather station records in Iceland. Those stations with a high warming trend between 1911 and 2010 are marked in red while those with a cooling or stable trend are marked in blue. Those denoted with squares are long stations with over 1200 months of data, while diamonds denote stations with more than 300 months of data.

If we next consider the change in temperature based on Berkeley Earth (BE) adjusted data we get the MTA data shown in Fig. 120.5 below. This again was determined by averaging each monthly anomaly from the 21 longest stations in Iceland. The mean temperature follows a similar trajectory to that of the unadjusted data in Fig. 120.3 with temperatures fluctuating by over 1°C and a large peak occurring around 1930. However the BE adjustments appear to have lowered this peak slightly relative to temperatures in 2010 when compared to the raw data in Fig. 120.3.

Fig. 120.5: Temperature trends for Iceland based on Berkeley Earth adjusted data. The best fit linear trend line (in red) is for the period 1841-2010 and has a positive gradient of +0.51 ± 0.03°C/century.

 

Comparing the curves in Fig. 120.5 with the published Berkeley Earth (BE) version for Iceland in Fig. 120.6 below shows that there is good agreement between the two sets of data. This indicates that the simple averaging of anomalies used to generate the BE MTA in Fig. 120.5 is as effective and accurate as the more complex gridding method used by Berkeley Earth in Fig. 120.6. In which case simple averaging should be just as effective and accurate in generating the MTA using raw unadjusted data in Fig. 120.1.

Fig. 120.6: The temperature trend for Iceland since 1750 according to Berkeley Earth.

Most of the differences between the MTA in Fig. 120.3 and the BE versions using adjusted data in Fig. 120.6  are mainly due to the data processing procedures used by Berkeley Earth. These include homogenization, gridding, Kriging and most significantly breakpoint adjustments. These lead to changes to the original temperature data, the magnitude of these adjustments being the difference in the MTA values seen in Fig. 120.3 and Fig. 120.5.

The magnitudes of these adjustments are shown graphically in Fig. 120.7 below. The blue curve is the difference in MTA values between adjusted (Fig. 120.5) and unadjusted data (Fig. 120.1), while the orange curve is the contribution to those adjustments arising solely from breakpoint adjustments. Both result in a consistent upward trend after 1920 with the former leading to an additional warming since 1930 of up to 0.25°C. These adjustments are, however, much smaller in total than the natural variation seen in the raw data in Fig. 120.3, so while they change the overall magnitude of the climate changes slightly, the general form of the temperature trends in Fig. 120.5 and Fig. 120.3 look broadly similar.

Fig. 120.7: The contribution of Berkeley Earth (BE) adjustments to the anomaly data in Fig. 120.5 after smoothing with a 12-month moving average. The blue curve represents the total BE adjustments including those from homogenization. The linear best fit (red line) to these adjustments for the period 1921-2010 has a positive gradient of +0.234 ± 0.007 °C per century. The orange curve shows the contribution just from breakpoint adjustments.

Summary

According to the raw unadjusted temperature data, the climate of Iceland has cooled from 1930 to 1990 by about 1°C. It then warmed by a similar but slightly smaller amount until 2005 (see Fig. 120.1).

Over the same period adjusted temperature data from Berkeley Earth appears to show that the climate of Iceland has warmed only fractionally since 1930, but by up to 2°C since the 1800s (see Fig. 120.5).

The reliability of the temperature data before 1930 is debatable due to the low number of stations and the large jumps in temperature that occur repeatedly. The origin of these jumps is uncertain but cannot solely be the result of greenhouse gas emissions when those emissions increased the atmospheric carbon dioxide concentration by so little compared to today.


Acronyms

BE = Berkeley Earth.

MRT = monthly reference temperature (see Post 47).

MTA = mean temperature anomaly.

Link to list of all stations in Iceland and their raw data files.