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.

122: Jan Mayen - temperature trends VARIABLE

The Norwegian island of Jan Mayen lies in the Arctic Circle approximately 600 km northeast of Iceland, 500 km east of Greenland and almost 1000 km from the coast of Norway. It is just over 50 km long and is inhabited only by a few Norwegian military and meteorological personnel on six month deployments. It does, though, have two weather stations (BE ID: 16234 and BE ID: 157312 with 735 and 1113 months of data respectively) that provide the only temperature data for over 500 km in all directions (see Fig. 122.1 below).

 

Fig. 122.1: The (approximate) locations of the two longest weather station records in Jan Mayen. 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.

 

The mean temperature anomaly (MTA) for Jan Mayen is shown in Fig. 122.1 below. From 1920 to 1990 the temperature trend is downward with temperatures falling by about 0.7°C according to the best fit line (red line) or 1.5°C according to the 5-year moving average (yellow line). After 1990 the temperature increases by up to 2°C. Both of these changes are less than the natural variation seen in the 5-year average (yellow curve) so it is impossible to definitively attribute them to climate change.

 

Fig. 122.2: The mean temperature change for Jan Mayen since 1920 relative to the 1971-2000 monthly averages. The best fit is applied to the monthly mean data from 1921 to 2000 and has a negative gradient of -0.93 ± 0.25 °C per century.

The MTA in Fig. 122.2 is the average of anomaly data from two stations (see Fig. 122.3 below) and was determined using the usual method as outlined in Post 47. The anomalies were determined relative to monthly reference temperatures (MRT) with the MRTs calculated using data from 1971-2000.

Fig. 122.3: The number of station records included each month in the mean temperature anomaly (MTA) trend for Jan Mayen in Fig. 122.2.

 

Repeating the averaging process using data that has been adjusted by Berkeley Earth (BE) yields the temperature curve shown in Fig. 122.4 below. In this case the 10-year average (orange curve) is broadly similar in shape to the yellow curve in Fig. 122.2, but the temperature fall before 1990 and the rise after are both slightly larger. In both cases, however, the temperatures in 2010 are less than 0.5°C higher than in 1930.

 

Fig. 122.4: Temperature trends for Jan Mayen based on Berkeley Earth adjusted data. The best fit linear trend line (in red) is for the period 1921-2000 and has a negative gradient of -1.04 ± 0.11°C/century.

 

The similarity in the two sets of data (Fig. 122.2 and Fig. 122.4) is reflected in the scale of the adjustments made to the original data by Berkeley Earth. The magnitudes of these adjustments are shown graphically in Fig. 122.5 below. The blue curve is the difference in MTA values between adjusted (Fig. 122.4) and unadjusted data (Fig. 122.2), while the orange curve is the contribution to those adjustments arising solely from breakpoint adjustments. In this case neither are particularly large, with data after 1950 being adjusted down by about 0.18°C. The only significant adjustment is for data between 1960 and 1976 which is adjusted upwards by about 0.9°C. The effect of this is to reduce the size of the temperature dip before 1970 seen in Fig. 122.2.

 

Fig. 122.5: The contribution of Berkeley Earth (BE) adjustments to the anomaly data in Fig. 122.4 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 1921-2000 has a slight negative gradient of -0.07 ± 0.04 °C per century. The orange curve shows the contribution just from breakpoint adjustments.

 

Summary

According to the raw unadjusted temperature data, the climate of Jan Mayen has remained fairly stable since 1930 (see Fig. 122.2).

Over the same period adjusted temperature data from Berkeley Earth appears to show similar climate variations (see Fig. 122.5).

The patterns seen in the temperature data for Jan Mayen in Fig. 122.2 (decline before 1990 and warming after) are similar to those seen previously for nearby islands of Greenland (Fig. 119.1 in Post 119), Iceland (Fig. 120.1 in Post 120) and the Faroes (Fig. 121.1 in Post 121).



Acronyms

BE = Berkeley Earth.

MRT = monthly reference temperature (see Post 47).

MTA = mean temperature anomaly.

118: Arctic Circle - temperature trends STABLE before 2000

If the Antarctic is the region of our planet that is most often associated with climate change, then the Arctic is the region that is often considered to be experiencing the most extreme climate change. Yet the reality is rather different. Since 1970 the mean temperature has indeed risen by about 2.5°C, yet between 1920 and 2000 the mean temperature (as indicated by the 5-year average) fluctuated by over 2°C (see Fig. 118.1 below). The largest temperature change since 1920 is +2.8°C between 1964 and 2010, yet the peak in 2010 is only 1°C higher than the previous peak in 1938. That increase may still sound like a lot, but when put in context against the size of the natural variability it is relatively modest, and is not as statistically significant as a 0.5°C or 1°C temperature rise seen in regions where the historic temperatures were previously stable.

The other point to bear in mind is the greater variability in temperatures that is seen in cold climates. It is not uncommon for the monthly anomalies for stations in both the Arctic and Antarctic to exceed ±10°C, or even ±20°C in some cases. In more temperate climes ±5°C would be considered extreme. The fact is that when the climate is more extreme, it is less stable. This is also seen around much of the equator as well.

Fig. 118.1: The mean temperature change since 1920 for the Arctic above 68.5°N (excluding Scandinavia) relative to the 1981-2010 monthly averages. The best fit is applied to the monthly mean data from 1921 to 2000 and has a negative gradient of -0.57 ± 0.18 °C per century.

The trend for the mean temperature anomaly (MTA) shown in Fig. 118.1 was determined by averaging the monthly anomalies from all the station records in the Arctic within 2,400 km of the North Pole that had at least 480 months of data before 2014 (for a list see here). This amounted to 88 datasets in total, of which five were long stations with 1200 months of data and the remaining 83 were medium stations with over 480 months of data. Of these medium stations 30 had over 900 months of data. It should be noted that an additional 34 stations from Scandinavia (Norway, Sweden and Finland) were excluded as their number and density would (in my opinion) have distorted the analysis. These countries will be analysed separately in the near future, and the discussion at the end of this post will compare the results shown in Fig. 118.1 with the equivalent result if the Scandinavian stations were included (see Fig. 118.7).

Fig. 118.2: The number of station records included each month in the mean temperature anomaly (MTA) trend for the Arctic in Fig. 118.1.

The process of determining the MTA in Fig. 118.1 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. If a station had at least twelve valid temperatures per month within the MRT interval 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. 118.1 each month is indicated in Fig. 118.2 above. This indicates that after 1950 there were at least fifty active stations, but before 1920 there were less than about ten. As ten is generally too low a number to produce a reliable trend, particularly over a large region like the Arctic, the data in Fig. 118.1 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 1866 as shown in Fig. 118.3 below. Note also that the low number of stations before 1920 results in a much higher variance of points 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. 118.1 is a more statistically reliable.

Fig. 118.3: The mean temperature change since 1860 for the Arctic above 68.5°N (excluding Scandinavia) relative to the 1981-2010 monthly averages. The best fit is applied to the monthly mean data from 1921 to 2000 and has a negative gradient of -0.57 ± 0.18 °C per century.

The locations of the 88 stations used to determine the MTA in Fig. 118.3 are shown in the map in Fig. 118.4 below. This appears to show that the geographical spread is fairly uniform, although there does appear to be more stations in the eastern hemisphere than in the west. The distribution of those 35 stations with over 900 months of data is fairly even as well, which suggests that the simple average of the anomalies from all stations used to construct the MTA in Fig. 118.1 should still yield a fairly accurate temperature trend for the region as a whole. The one noticeable deficiency is the absence of stations within the 80th parallel. In fact the closest medium station to the North Pole is Alert in Canada (Berkeley Earth ID: 153879) which is over 840 km from the North Pole. The result is that we do not know what is happening to temperatures at the North Pole, and we have never known.

Fig. 118.4: The (approximate) locations of the 88 longest weather station records in the Arctic within 2400 km of the North Pole (excluding those in Norway, Sweden and Finland). 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 medium stations with more than 480 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. 118.5 below. This again was determined by averaging each monthly anomaly from the 88 longest stations in the Arctic. The mean temperature follows a similar trajectory to that of the unadjusted data in Fig. 118.3 with temperatures fluctuating by over 1°C throughout the 20th century but rising to record highs after 2000.

Fig. 118.5: Temperature trends for the Arctic above 68.5°N based on Berkeley Earth adjusted data. The best fit linear trend line (in red) is for the period 1876-2010 and has a positive gradient of +1.80 ± 0.05°C/century.

Any differences between the MTA in Fig. 118.3 and the BE versions using adjusted data in Fig. 118.5  are mainly due to the data processing procedures used by Berkeley Earth. These include homogenization, gridding, Kriging and most significantly breakpoint adjustments. 

The magnitude of these adjustments can be determined by calculating the difference in the MTA values seen in Fig. 118.3 and Fig. 118.5. The result is shown graphically in Fig. 118.6 below. The blue curve is the difference in MTA values between adjusted (Fig. 118.5) and unadjusted data (Fig. 118.3), while the orange curve is the contribution to those adjustments arising solely from breakpoint adjustments. Both are relatively small for most of the 20th century, varying by about 0.2°C about their midpoints. The large offset between the blue and orange curves is mainly due to a difference in MRT interval used. I used 1981-2010 for the data in Fig. 118.3 whereas Berkeley Earth (BE) tend to use 1961-1990. 

The main conclusion to be drawn from the data in Fig. 118.6 is that the BE adjustments have two effects. Firstly, they reduce the amplitude of the data oscillations before 2000, and secondly, they reduce slightly the temperature rise after 2000.

Fig. 118.6: The contribution of Berkeley Earth (BE) adjustments to the anomaly data in Fig. 116.4 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 1921-2000 has a positive gradient of +0.179 ± 0.013 °C per century. The orange curve shows the contribution just from breakpoint adjustments.

Finally, there is the issue of the omitted Scandinavia data that was mentioned at the start of this post. This amounts to five long stations (all in Norway) and a further 29 medium stations (one in Sweden, six in Finland, and the rest in Norway). Including these stations in the averaging process yields the MTA time-series shown in Fig. 118.7 below.

Fig. 118.7: The mean temperature change since 1860 for the Arctic above 68.5°N including Scandinavia relative to the 1981-2010 monthly averages. The best fit is applied to the monthly mean data from 1936 to 2005 and has a positive gradient of +0.09 ± 0.21 °C per century.

Comparing Fig. 118.7 with Fig. 118.3 indicates that the inclusion of the Scandinavia data reduces the temperature variability between 1920 and 2000, and also reduces the temperature rise from 1964 to 2010 by about 0.5°C (from 2.8°C to 2.3°C). What it certainly does not do is make the climate change worse. Nor does the BE adjusted data. In fact the most extreme temperature changes are seen in Fig. 118.1.

Summary

According to the raw unadjusted temperature data, the climate of the Arctic experienced large temperature variations of up to 1.7°C (or ±0.85°C about the midpoint) throughout the 20th century (see Fig. 118.1). 

Since 1964 temperatures have increased by 2.8°C from their minimum value.

Since 2000 the Arctic has warmed by up to 1.5°C but it is unclear how much of this is permanent climate change and how much is just more natural variability. Comparing the data in 2010 with the previous peak in 1938 suggests that 1.0°C of the rise may be permanent.

Over the same period adjusted temperature data from Berkeley Earth appears to show similar results (see Fig. 118.5) but with about 0.5°C less warming since 1970 and a variation of ±0.5°C before.

 


Acronyms

BE = Berkeley Earth.

MRT = monthly reference temperature (see Post 47).

MTA = mean temperature anomaly.

Link to list of all stations within 2,400 km of the North Pole and their raw data files.