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.

121: Faroe Islands - temperature trends STABLE

The longest temperature record in the Faroe Islands is from a station at Thorshavn. It has nearly 1800 months of data, but no other temperature record in the islands has more than 435 months of data. In fact only another two (at Vagar and Akraberg) have more than 120 months of data (see here for a list). This means that the temperature record for the Faroe Islands is heavily dependent on the Thorshavn data with the other two stations only affecting the mean temperatures after 1980. 

The result is the time series for the mean temperature anomaly (MTA) shown in Fig. 121.1 below. What is striking is that the general form of the five-year moving average (yellow curve) is very similar to those for both Greenland (see Fig. 119.1 in Post 119) and Iceland (see Post 120.1 in Post 120). Temperatures declined from 1930 until 1990 and then rebounded. In all three cases the temperatures today are no higher than in the 1930s but the amount of cooling from 1930-1990 is different, being highest for Greenland and least for the Faroe Islands. This is not a surprise, though, as it is well known that climates that are more extreme (i.e. Greenland) experience larger temperature fluctuations than are seen in more temperate climes (i.e. the Faroe Islands).

Fig. 121.1: The mean temperature change for the Faroe Islands since 1920 relative to the 1981-2010 monthly averages. The best fit is applied to the monthly mean data from 1931 to 2010 and has a negative gradient of -0.40 ± 0.15 °C per century.

In order to quantify the changes to the climate of the Faroe Islands the temperature anomalies for the three longest station records were 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 process of determining the MTA in Fig. 121.1 involved first determining the monthly reference temperatures (MRTs) for each station using a set reference period, in this case from 1981 to 2010, and then subtracting the MRTs from the raw temperature data to deliver the anomalies. The total number of stations included in the MTA in Fig. 121.1 each month is indicated in Fig. 121.2 below. The peak in the frequency after 1980 illustrates why the 1981-2010 interval was indeed the most appropriate to use for the MRTs.

Fig. 121.2: The number of station records included each month in the mean temperature anomaly (MTA) trend for the Faroe Islands in Fig. 121.1.

If all the data is considered, the MTA trend for the Faroe Islands will have data extending back to 1867 as shown in Fig. 121.3 below. Much of the data before 1930 indicates that temperatures then, at least for Thorshavn, were cooler than in the 1930s and to day. Again this is similar to the results from Greenland and Iceland, but it is unclear how much of this temperature change is genuine warming of the climate and how much is just the result of natural fluctuations. But the fact that these features are repeated in multiple regions means that they cannot be discounted as being isolated results that are the result of poor measurements.

Fig. 121.3: The mean temperature change for the Faroe Islands since 1860 relative to the 1981-2010 monthly averages. The best fit is applied to the monthly mean data from 1871 to 2010 and has a positive gradient of +0.27 ± 0.07 °C per century.

The locations of the three stations used to determine the MTA in Fig. 121.3 are shown in the map in Fig. 11214 below. The temperature data from all three appear to exhibit modest warming trends, but this is mainly due to the fitting intervals used in each case. Vagar and Akraberg only have data after 1970 where the MTA in Fig. 121.3 is warming. The trend for Thorshavn was for data from 1911-2010, and is therefore biased by the fact that the fitting interval does not extend from peak to peak. This is akin to fitting to a sine wave as I explained with Fig. 4.7 in Post 4. So the positive trend is not real.

Fig. 121.4: The (approximate) locations of the three longest weather station records in the Faroe Islands. 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 240 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. 121.5 below. This again was determined by averaging each monthly anomaly from the three longest stations in the Faroe Islands. The mean temperature follows a similar trajectory to that of the unadjusted data in Fig. 121.3 with temperatures fluctuating by over 1°C and a large peak occurring around 1930. However the BE adjustments appear to have lowered this peak relative to temperatures in 2010 by over 0.25°C compared to the raw data in Fig. 121.3.

Fig. 121.5: Temperature trends for the Faroe Islands based on Berkeley Earth adjusted data. The best fit linear trend line (in red) is for the period 1871-2010 and has a positive gradient of +0.61 ± 0.03°C/century.

Comparing the curves in Fig. 121.5 with the published Berkeley Earth (BE) version for the Faroe Islands in Fig. 121.6 below shows that there is good agreement between the two sets of data. However, Berkeley Earth appear to think they can determine the temperature back to 1760 even though there is no data before 1860. Who says climate scientists are pessimists?

Fig. 121.6: The temperature trend for the Faroe Islands since 1750 according to Berkeley Earth.

The differences between the MTA in Fig. 121.3 and the BE versions using adjusted data in Fig. 121.5  can be determined by calculating the difference in the MTA values seen in Fig. 121.3 and Fig. 121.5. The result is shown graphically in Fig. 121.7 below. The blue curve is the difference in MTA values between adjusted (Fig. 121.5) and unadjusted data (Fig. 121.3), while the orange curve is the contribution to those adjustments arising solely from breakpoint adjustments. What is clear is that there is only one significant adjustment. That is a breakpoint adjustment of over 0.25°C made to the Thorshavn data in 1951. The difference between the blue and orange curves in Fig. 121.7 is due to the difference in MRT interval used in Fig. 121.3 and by Berkeley Earth in Fig. 121.5.

Fig. 121.7: The contribution of Berkeley Earth (BE) adjustments to the anomaly data in Fig. 121.5 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 1871-2010 has a positive gradient of +0.330 ± 0.005 °C per century. The orange curve shows the contribution just from breakpoint adjustments.

Summary

According to the raw unadjusted temperature data, the climate of the Faroe Islands has cooled from 1930 to 1980 by about 1°C. It then warmed by a similar but slightly smaller amount until 2005 (see Fig. 121.1).

Over the same period adjusted temperature data from Berkeley Earth appears to show that the climate of the Faroe Islands has warmed by over 0.25°C since 1930 and over 1°C since the 1800s (see Fig. 121.5).

The difference in the raw unadjusted data (Fig. 121.3) and the adjusted data (Fig. 121.5) is mainly due to a single breakpoint adjustment of 0.26°C in 1951.

Any warming trend since 1870 is small (~0.3°C) compared to what look like natural temperature variations (~1°C). The origin of these variations (shown in Fig. 121.3) is uncertain but cannot be the result of greenhouse gas emissions when those emissions were so low compared to today. However, similar patterns are seen in the temperature data of nearby islands of Greenland (Post 119), Iceland (Post 120). and Jan Mayen (from 1920 only), so these features seen in the data before 1930 may be real and representative of synchronous climate variations occurring across the North Atlantic/Arctic region.



Acronyms

BE = Berkeley Earth.

MRT = monthly reference temperature (see Post 47).

MTA = mean temperature anomaly.

Link to list of all stations in the Faroe Islands and their raw data files.