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.
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