48. Denmark - temperature trends STRONG WARMING 1.8°C

In total, Denmark has twenty-two sets of temperature data that exceed 480 months in length (see here). Of these, eight contain over 1200 months of data (long stations), with the longest being Copenhagen (Berkeley Earth ID: 154574) which has continuous data from 1798, and some data fragments that go as far back as 1768. This suggests that the country has a similar number of station temperature records as New Zealand (see Post 8), but surprisingly it is less than is found for the Danish autonomous territory of Greenland which has a population of less than 60,000 and which I will look at in detail at some point in the future.

Fig. 48.1: The locations of long stations (large squares) and medium stations (small diamonds) in Denmark. Those stations with a high warming trend are marked in red.

The distribution of these weather stations in Denmark is indicated in Fig. 48.1 above. It shows a fairly even spread that covers most of the country. It also shows that most of the station data exhibits some significant degree of warming, with only two stations exhibiting a cooling trend (defined as being a trend that is less than twice the uncertainty in the trend).

The data from Denmark is interesting in one other respect in that, of its fourteen medium length station temperature records (i.e. those with over 480 months of data but less than 1200), four have no data after 1970 but do have data going back to the 19th century, while six have no data before 1970. This means that these two groups of stations require different time periods for the calculation of the reference temperatures needed to find their monthly temperature anomalies. For an explanation of the rationale and process used to determine the temperature anomalies via the calculation of monthly reference temperatures (MRTs), please refer to my previous post.

 

Fig. 48.2: The maximum amount of temperature data available from Denmark each month for inclusion in the mean temperature trend.

This problem is illustrated in Fig. 48.2 above. The two peaks in the frequency distribution indicate the two different possibilities for the MRT period. As I pointed out in Post 47, ideally the MRT period needs to be about 30 years in length with at least 40% data coverage. One way to circumvent this problem is to calculate the temperature trend for two MRT time intervals (the data in Fig. 48.2 suggests that 1891-1920 and 1971-2000 should be optimal), compare the results, and if necessary take a weighted average. 

 

Fig. 48.3: The temperature trend for Denmark since 1750. The best fit is applied to the interval 1851-2000 and has a positive gradient of +1.18 ± 0.09 °C per century. The monthly temperature changes are defined relative to the 1971-2000 monthly averages.

If we choose 1971-2000 as our reference period for the MRTs, then the overall temperature trend is as shown in Fig. 48.3 above. The number of stations included each month in this overall trend is shown in Fig. 48.4 below. Overall, up to seventeen stations are included, but before 1970 that drops to less than ten with only one station with data before 1870. The result is that there appears to be a fairly continuous warming trend from 1851 to 2000 as indicated by the data in Fig. 48.3.

Fig. 48.4: The number of sets of station data included each month in the temperature trend for Denmark when the MRTs are calculated for the period 1971-2000.

Now consider what happens if we choose 1891-1920 as our reference period for the MRTs. The result is that there are more stations included before 1970, but less after (see Fig. 48.5 below). This also changes the form of the temperature trend in Fig. 48.6.

Fig. 48.5: The number of sets of station data included each month in the temperature trend for Denmark when the MRTs are calculated for the period 1891-1920.

What we now see in Fig. 48.6 is a much smaller warming trend before 1980 (less than 0.6 °C with possibly higher temperatures before 1800), but a more pronounced jump in temperatures after 1988. This similar to the trend seen for South Africa (see Post 37) and also for Europe as a whole (see Post 44). It is important to note, though, that all the data before 1860 in both Fig. 48.6 and Fig. 48.3 comes from just one station record: Copenhagen (Berkeley Earth ID: 154574). This means the accuracy and reliability of this data cannot be truly ascertained.

Fig. 48.6: The temperature trend for Denmark since 1750. The best fit is applied to the interval 1768-1980 and has a positive gradient of +0.30 ± 0.05 °C per century. The monthly temperature changes are defined relative to the 1891-1920 monthly averages.

The analysis outlined above means that we have two possible results for the temperature trend in Denmark. Both are fairly similar, and for once both are in general agreement with the trend published by Berkeley Earth (see Fig. 48.7 below). But can we combine them into a single result?

Fig. 48.7: The temperature trend for Denmark since 1750 according to Berkeley Earth.

The answer is yes. If we take the weighted average of each of the two trends in Fig. 48.3 and Fig. 48.6 we get the result shown in Fig. 48.8 below. The relative weightings of each month's data is determined by the number of stations included in the average for that month as indicated in Fig. 48.4 and Fig. 48.5 respectively. There is, though, one other factor we need to take into account: the different MRT intervals for the two original trends. Without a correction term this will distort the final data.

In order to allow for the differing MRTs, the trend curve in Fig. 48.3 needs to be first adjusted upwards so that the mean temperature anomaly for the period 1891-1920 is zero in order to be consistent with the data in Fig. 48.6. This requires an upward adjustment of 0.634 °C. Only after this adjustment has been made can the weighted average be determined.

Fig. 48.7: The weighted temperature trend for Denmark since 1750. The best fit is applied to the interval 1851-2000 and has a positive gradient of +1.02 ± 0.09 °C per century. The monthly temperature changes are defined relative to the 1891-1920 monthly averages.

Conclusions

The data from Denmark appears to show a warming trend of about 1.5 °C since 1850. This is by far the largest warming seen in any of the regional records that I have investigated so far. It is also one of the few that appears to agree with IPCC reported values. However, this is not as straightforward as it seems. For a start changing the MRT interval from 1971-2000 (as in Fig. 48.3) to 1891-1920 (as in Fig. 48.6) dramatically reduces the temperature trend before 1980. So which one is correct? 

Then there is the problem of the sudden jump in temperature in 1988 of 0.93 °C. A similar jump was seen in the temperature trend for the Europe data in Post 44 as well. The reason for this is still unclear (at least to me). In Post 45 I speculated that it could be the result of improved air quality in Europe due to EU legislation. Alternatively, it could be the consequence of a change in measurement method, such as a change from liquid-in-glass thermometers to electronic systems which occurred around that time. What is strange is the timing and suddenness of this increase. 

Whatever the true scale of the temperature rise in Denmark since 1750, it cannot be explained entirely by direct anthropogenic surface heating (DASH) or waste heat. The best estimate of the expected magnitude of DASH for Denmark (based on its population density) is about 0.35 °C since 1850. However, this could be greater if the source of the heating from human industrial activity is concentrated around the locations of the major weather stations. For example, a city like Greater London with an area 1569 km² consumes over 132 TWh of energy each year. This equates to a power density of 9.6 W/m², or an effective temperature rise of over 4 °C. Clearly something similar but less extreme could be occurring around the major cities in Denmark, but at the moment, without direct measurement data, that remains as speculation.