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.

119: Greenland - temperature trends COOLING before 1990

Climate catastrophe is generally thought of as occurring in two ways: rising temperatures and rising sea levels. Greenland is fairly unique in that it is claimed by climate science to be indicative of both. The only problem is that in reality things aren't that simple. In fact Greenland is currently colder than it was 100 years ago.

The significance of Greenland is two-fold. Firstly it is the largest landmass in the Arctic Circle. As such it is one of the best indicators of climate change near the North Pole given that, as I showed in the previous post, there is no reliable temperature data within 840 km of the North Pole. But secondly, Greenland, like Antarctica, is a large store of frozen fresh water. Its ice sheet is second only in size to that of Antarctica, and has an average thickness of 1,500 m, rising to over 3,700 m above sea level at some points. If it were to melt completely it would raise global sea levels by more than seven metres. Yet between 1930 and 1990 the climate of Greenland actually cooled by almost 2°C, and while the mean temperature has risen quite sharply by a similar amount since 1990, mean temperatures are still below their 1930 levels (see Fig. 119.1 below).

Fig. 119.1: The mean temperature change for Greenland since 1920 relative to the 1976-2005 monthly averages. The best fit is applied to the monthly mean data from 1931 to 1990 and has a negative gradient of -2.81 ± 0.35 °C per century.

In order to quantify the changes to the climate of Greenland the temperature anomalies for each of the 39 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. 119.1 above and clearly shows that temperatures declined continuously from 1930 to 1990 before rebounding.

The process of determining the MTA in Fig. 119.1 involved first determining the monthly reference temperatures (MRTs) for each station using a set reference period, in this case from 1976 to 2005, 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 (1976-2005) 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. 119.1 each month is indicated in Fig. 119.2 below. The peak in the frequency after 1980 suggests that the 1976-2005 interval was indeed the most appropriate to use for the MRTs.

Fig. 119.2: The number of station records included each month in the mean temperature anomaly (MTA) trend for Greenland in Fig. 119.1.

The data in Fig. 119.2 above indicates that after 1960 there were up to 39 active stations, but before 1890 there were less than about five. As five is generally too low a number to produce a reliable trend, particularly over a large region like Greenland, the MTA data in Fig. 119.1 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 1863 as shown in Fig. 119.3 below. Note also that the low number of stations before 1940 results in a much higher variance of points in Fig. 119.3 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. 119.1 is more statistically reliable.

Fig. 119.3: The mean temperature change for Greenland since 1860 relative to the 1976-2005 monthly averages. The best fit is applied to the monthly mean data from 1871 to 2010 and has a positive gradient of +0.93 ± 0.12 °C per century.

The locations of the 39 stations used to determine the MTA in Fig. 119.3 are shown in the map in Fig. 119.4 below. This appears to show that the stations are all located on the coast, with none in the interior or at altitude, and a majority on the south-west coast. Of these 39 stations, five are long stations with over 1200 months of data before 2014, and a further eighteen are medium stations with over 480 months of data. What is more remarkable is how many stations Greenland has despite its low population. This may be because it has historically been part of the Kingdom of Denmark. The result is it has a similar amount of temperature data as Denmark (see Fig. 48.4 and Fig. 48.5 in Post 48) yet its population is only 1% of that of Denmark.

Fig. 119.4: The (approximate) locations of the 32 longest weather station records in Greenland. 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. 119.5 below. This again was determined by averaging each monthly anomaly from the 39 longest stations in Greenland. The mean temperature follows a similar trajectory to that of the unadjusted data in Fig. 119.3 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 0.5°C compared to the raw data in Fig. 119.3.

Fig. 119.5: Temperature trends for Greenland 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 +1.43 ± 0.07°C/century.

Comparing the curves in Fig. 119.5 with the published Berkeley Earth (BE) version for Greenland in Fig. 119.6 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. 119.5 is as effective and accurate as the more complex gridding method used by Berkeley Earth in Fig. 119.6. In which case simple averaging should be just as effective and accurate in generating the MTA using raw unadjusted data in Fig. 119.1 even though the geographical distribution of stations is far from homogeneous, as was shown in Fig. 119.4.

Fig. 119.6: The temperature trend for Greenland since 1820 according to Berkeley Earth.

Most of the differences between the MTA in Fig. 119.3 and the BE versions using adjusted data in Fig. 119.6  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. 119.3 and Fig. 119.5. 

The magnitudes of these adjustments are shown graphically in Fig. 119.7 below. The blue curve is the difference in MTA values between adjusted (Fig. 119.5) and unadjusted data (Fig. 119.1), while the orange curve is the contribution to those adjustments arising solely from breakpoint adjustments. Both are considerable after 1920 with the former leading to an additional warming since 1930 of up to 0.5°C. These adjustments are, however, much smaller in total than the natural variation of 2°C seen in the raw data in Fig. 119.3, so while they change the overall magnitude of the climate changes slightly, the general form of the temperature trends in Fig. 119.5 and Fig. 119.3 look broadly similar.

Fig. 119.7: The contribution of Berkeley Earth (BE) adjustments to the anomaly data in Fig. 119.5 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 1921-2000 has a positive gradient of +0.56 ± 0.02 °C per century. The orange curve shows the contribution just from breakpoint adjustments.

Summary

According to the raw unadjusted temperature data, the climate of Greenland has cooled from 1930 to 1990 by about 2°C. It then warmed by a similar but slightly smaller amount until 2005 (see Fig. 119.1).

Over the same period adjusted temperature data from Berkeley Earth appears to show that the climate of Greenland has warmed by over 0.5°C since 1930 and up to 3.5°C since the 1880s (see Fig. 119.5).

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. However, similar patterns are seen in the temperature data of nearby islands of Iceland and Jan Mayen (from 1920 only), so these features seen in the data before 1930 may be real changes to the climate and not localized data errors.

Acronyms

BE = Berkeley Earth.

MRT = monthly reference temperature (see Post 47).

MTA = mean temperature anomaly.

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

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.