Over the next four posts I will look at the temperature data of Scandinavia starting with Norway. Like the rest of Scandinavia, Norway has some of the best temperature data in Western Europe despite having large areas with low population densities and harsh Arctic climates. Some of the temperature data extends back to the mid-eighteenth century. What this data shows is that for two hundred years up to 1980 the climate was more or less stable and exhibited no significant temperature increase. Then around 1988 the mean temperature appears to jump abruptly by about 1°C. This pattern is also seen in much of the rest of Europe as I first demonstrated in Post 44.
In total Norway has nineteen long stations with over 1200 months of data before 2014 and 78 medium stations with over 480 months of data (for a full list of stations see here). Their approximate locations are shown on the map in Fig. 135.1 below. The stations are fairly evenly distributed across the country with thirty of the stations lying within the Arctic Circle, but there does appear to be a greater concentration in the south and a relatively low number of stations between Trondheim and Tromsø in the middle of the country. It is also apparent that over 60% of stations are located on or near the coast.
Fig. 135.1: The (approximate) locations of the 97 longest weather station records in Norway. 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.
In order to quantify the changes to the climate of Norway the temperature anomalies for all stations with over 480 months of data before 2014 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 country. This MTA is shown as a time series in Fig. 135.2 and clearly shows that temperatures were fairly stable up until 1980. However at some point in the 1980s (probably in 1988) the mean temperature appears to increase abruptly by about 1°C.
Fig. 135.2: The mean temperature change for Norway since 1880 relative to the 1961-1990 monthly averages. The best fit is applied to the monthly mean data from 1881 to 1980 and has a positive gradient of +0.23 ± 0.16 °C per century.
The process of determining the MTA in Fig. 135.2 involved first determining the monthly reference temperatures (MRTs) for each station using a set reference period, in this case from 1961 to 1990, 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. 135.2 each month is indicated in Fig. 135.3 below. The peak in the frequency between 1960 and 1990 suggests that the 1961-1990 interval was indeed the most appropriate to use for the MRTs.
Fig. 135.3: The number of station records included each month in the mean temperature anomaly (MTA) trend for Norway in Fig. 135.2.
The data in Fig. 135.3 indicates that the greatest coverage of the country for temperature data is after 1950 with up to 93 long and medium stations in operation at any one time. This drops to about twenty in 1930 and to less than five before 1850. This means that the MTA for Norway before 1890 will be less reliable than its values after 1950. Note that a reliable MTA generally needs data from at least sixteen stations (see Post 57 for evidence). However if we calculate the MTA for Norway back to 1760 we obtain the trends shown in Fig. 135.4 below. These show that the MTA remains stable for much of the two hundred years before 1980 but the noise level increases before 1800 when the MTA is dependent on only one station: Trondheim.
Fig. 135.4: The mean temperature change for Norway since 1760 relative to the 1961-1990 monthly averages. The best fit is applied to the monthly mean data from 1781 to 1980 and has a slight positive gradient of +0.06 ± 0.06 °C per century.
If we next consider the change in temperature based on Berkeley Earth (BE) adjusted data we get the MTA data in Fig. 135.5 below. This again was determined by averaging each monthly anomaly from the 97 longest stations and suggests that the climate was fairly stable before 1880 but then warmed by over 1°C thereafter. In fact the 10-year average suggests a warming of over 1.5°C. Not only that but the warming is more continuous in nature than the raw data in Fig. 135.4 actually shows.
Fig. 135.5: Temperature trends for Norway 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 +0.86 ± 0.04°C/century.
Comparing the curves in Fig. 135.5 with the published Berkeley Earth (BE) version for Norway in Fig. 135.6 below we see 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. 135.5 using adjusted data is as effective and accurate as the more complex gridding method used by Berkeley Earth in Fig. 135.6. In which case simple averaging should be just as effective and accurate in generating the MTA using raw unadjusted data in Fig. 135.4 and Fig. 135.2. In other words, the discrepancy between the adjusted data in Fig. 135.5 and the unadjusted data in Fig. 135.4 cannot be due to the averaging process. Any form of weighted averaging would also not affect the results.
Fig. 135.6: The temperature trend for Norway since 1750 according to Berkeley Earth.
Most of the differences between the MTA in Fig. 135.4 and the BE versions using adjusted data in Fig. 135.6 are instead 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. 135.4 and Fig. 135.5.
The magnitudes of these adjustments are shown graphically in Fig. 135.7 below. The blue curve is the difference in MTA values between adjusted (Fig. 135.5) and unadjusted data (Fig. 135.4), while the orange curve is the contribution to those adjustments arising solely from breakpoint adjustments. The overall adjustment from 1880 to 2013 is small, less than +0.2°C. The main impact is to change the shape of the long term trend from a step-like jump in Fig. 135.4 to a more continuous increase in Fig. 135.5. This involves raising temperatures between 1920 and 1980 by 0.2°C while lowering slightly temperatures before 1920.
Fig. 135.7: The contribution of Berkeley Earth (BE) adjustments to the anomaly data in Fig. 135.5 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 1876-2010 has a positive gradient of +0.143 ± 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 Norway remained stable for over two hundred years up until the 1980s (see Fig. 135.4). Then it suddenly increased in temperature by 1°C. Why?
In contrast, adjusted temperature data from Berkeley Earth claims to show that the climate of Norway has warmed more or less continuously since 1860 by over 1.5°C (see Fig. 135.4 and Fig. 135.5).
Acronyms
BE = Berkeley Earth.
MRT = monthly reference temperature (see Post 47).
MTA = mean temperature anomaly.
List of all stations in Norway with links to their raw data files.
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