The climate change seen in Sweden over the last 200 years is similar to that seen in Norway over the same period. This is perhaps not surprising given that they are nearest neighbours. The only major difference is the temperature change before 1980. In the case of Norway this was negligible (see Fig. 135.4 in Post 135) and was less than a tenth of the natural variation in the 5-year average. For Sweden there is a modest but distinct upward warming trend from 1860 to 1980 that amounts to about 0.6°C in total. That said this warming is still only comparable with the natural variation in the 5-year average of the mean temperature anomaly (MTA). Then after 1980 the MTA jumps abruptly by about 1°C, just as it did in Norway. The net result is that the temperature trend of Sweden over the last 150 years is one of the few regional trends that actually resembles the global trends published by NOAA, NASA-GISS, Hadley-CRU and Berkeley Earth, but this may not be quite what it seems.
As Sweden has a larger area and larger population than Norway it is not surprising that it has more weather stations. In total Sweden has 25 long stations with over 1200 months of data before 2014 and another 126 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. 136.1 below. The stations are fairly evenly distributed across most of the country, but there does appear to be a greater concentration in the south and only twelve stations (i.e. 8%) are within the Arctic Circle. It does have two of the longest and earliest temperature records though: Uppsala and Stockholm. Both have over 3000 months of more or less continuous data that extend back to before 1760.
Fig. 136.1: The (approximate) locations of the 151 longest weather station records in Sweden. 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 Sweden 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. 136.2 and clearly shows that temperatures were rising slowly up until 1980. Then at some point in the 1980s (probably in 1988) the mean temperature appears to increase abruptly by about 1°C.
Fig. 136.2: The mean temperature change for Sweden since 1880 relative to the 1971-2000 monthly averages. The best fit is applied to the monthly mean data from 1881 to 1980 and has a positive gradient of +0.46 ± 0.19 °C per century.
The process of determining the MTA in Fig. 136.2 involved first determining the monthly reference temperatures (MRTs) for each station using a common reference period, in this case from 1971 to 2000, 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. 136.2 each month is indicated in Fig. 136.3 below. The peak in the frequency between 1960 and 2010 suggests that the 1971-2000 interval was indeed the most appropriate to use for the MRTs.
Fig. 136.3: The number of station records included each month in the mean temperature anomaly (MTA) trend for Sweden in Fig. 136.2.
The data in Fig. 136.3 indicates that the period of greatest coverage of the country for temperature data is after 1960 with up to 150 long and medium stations in operation at any one time. This drops to about 25 between 1890 and 1960 and to four or less before 1860. This means that the MTA for Sweden before 1860 will be less reliable than its values after 1890. Note that a reliable MTA generally needs data from at least sixteen stations (see Post 57 for evidence).
Nevertheless, we can calculate an MTA for Sweden back to 1720 and if we do so we obtain the trend shown in Fig. 136.4 below. This appears to show that the MTA exhibited a slow and gentle warming for much of the two hundred years before 1980 before the abrupt jump around 1988, but it can be interpreted differently.
Fig. 136.4: The mean temperature change for Sweden since 1720 relative to the 1971-2000 monthly averages. The best fit is applied to the monthly mean data from 1731 to 1980 and has a slight positive gradient of +0.30 ± 0.05 °C per century.
If the best fit is performed for the interval 1761-1860 then the gradient is -0.06 ± 0.22°C while the best fit to data from 1861 to 2010 yields a strong positive gradient of +0.99 ± 0.11°C (see Fig. 136.5 below). So changing the fitting interval can change the result; but remember, as Fig. 136.3 shows, the MTA data before 1860 is based on only four station records. Similarly, changing the fitting interval to 1861-2010 also changes the interpretation of the data after 1980. The abrupt jump around 1988 now looks more like a temporary plateauing of the MTA data between 1940 and 1980. So which is correct? The problem is it is difficult to know due to the large natural fluctuations in the MTA data: both could be true or neither could be true. What we do know is that the 1988 jump is seen in the MTA for other countries and for Europe as a whole (see Post 44).
Fig. 136.5: The mean temperature change for Sweden since 1820 relative to the 1971-2000 monthly averages. The best fit is applied to the monthly mean data from 1861 to 2010 and has a positive gradient of +0.99 ± 0.11 °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. 136.6 below. This again was determined by averaging each monthly anomaly from the 151 longest stations and it suggests that the climate was fairly stable before 1870 but then warmed by over 1.5°C thereafter. In fact the 10-year average suggests a warming of almost 2°C. Moreover the data in Fig. 136.6 is in good agreement with the MTA based on unadjusted data in Fig. 136.4.
Fig. 136.6: Temperature trends for Sweden 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 +1.00 ± 0.06°C/century.
Comparing the curves in Fig. 136.6 with the published Berkeley Earth (BE) version for Sweden in Fig. 136.7 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. 136.6 using adjusted data is as effective and accurate as the more complex gridding method used by Berkeley Earth in Fig. 136.7. In which case simple averaging should be just as effective and accurate in generating the MTA using raw unadjusted data in Fig. 136.4. In other words, any discrepancy between the adjusted data in Fig. 136.6 and the unadjusted data in Fig. 136.4 cannot be due to the averaging process.
Fig. 136.7: The temperature trend for Sweden since 1750 according to Berkeley Earth.
Most of the (slight) differences between the MTA in Fig. 136.4 and the BE versions using adjusted data in Fig. 136.7 are instead mainly due to the data processing procedures used by Berkeley Earth. These include homogenization, gridding, Kriging and most significantly breakpoint adjustments. These can lead to changes to the original temperature data, the magnitude of these adjustments being the difference in the MTA values seen in Fig. 136.4 and Fig. 136.6.
The magnitudes of these adjustments are shown graphically in Fig. 136.8 below. The blue curve is the difference in MTA values between adjusted (Fig. 136.6) and unadjusted data (Fig. 136.4), while the orange curve is the contribution to those adjustments arising solely from breakpoint adjustments. The change in the overall adjustment from 1876 to 2010 is small, less than +0.06°C while the much larger 0.4°C offset is due to the difference in MRT intervals used to determine the anomalies in Fig. 136.4 and Fig. 136.6 and can be ignored. What this shows is that the BE adjustments in this case are small which is why the data in Fig. 136.4 and Fig. 136.6 agree so well.
Fig. 136.8: The contribution of Berkeley Earth (BE) adjustments to the anomaly data in Fig. 136.6 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 1876-1980 has a positive gradient of +0.046 ± 0.002 °C per century. The orange curve shows the contribution just from breakpoint adjustments.
Summary
According to the raw unadjusted temperature data, the climate of Sweden has warmed gradually for over 150 years by about 1.5°C in total (see Fig. 136.4). There is some evidence of an abrupt jump of 1°C around 1988 but this depends on how one interprets the data and the fitting process. The temperature before 1860 appear to have been stable for over one hundred years.
In this instance the adjusted temperature data from Berkeley Earth closely follows that of the unadjusted data (see Fig. 136.6).
Acronyms
BE = Berkeley Earth.
MRT = monthly reference temperature (see Post 47).
MTA = mean temperature anomaly.
List of all stations in Sweden with links to their raw data files.
No comments:
Post a Comment