150: Malta - temperature trends STABLE before 1980

The difficulty in determining the extent of climate change in Malta is the lack of data. According to Berkeley Earth (BE) there are only two stations with over 480 months of data and one of those (A. M. D.) has virtually no data after 1934. The other station is Luqa (BE ID:156721) which is situated near the main airport and actually has over 1800 months of data, so it could be a good indicator of the climate of Malta as a whole. Its monthly mean temperature anomaly (MTA) relative to the period 1981-2010 is shown in Fig. 150.1 below.

 

Fig. 150.1: The mean temperature change for Luqa since 1840 relative to the 1981-2010 monthly averages. The best fit is applied to the monthly mean data from 1881 to 1980 and has a slight positive gradient of +0.05 ± 0.11 °C per century.

 

The data in Fig. 150.1 indicates that there was virtually no climate change in Malta before 1980. Then after 1980 the local temperature appears to have risen by about 0.6°C, although this depends on how one interprets the data. If one uses the 5-year average (yellow line) as a guide the temperature appears to increase by over 1°C from 1880 to 2010. However, average temperatures in 2010 are only about 0.6°C above the 1881-1980 best fit line (in red) which is virtually horizontal. So this would indicate that temperatures in 2010 are only about 0.6°C above average.

 

Fig. 150.2: Temperature trends for Luqa based on Berkeley Earth adjusted data. The best fit linear trend line (in red) is for the period 1886-2010 and has a positive gradient of +0.56 ± 0.03°C/century.

 

A similar temperature trend for Luqa is seen in its Berkeley Earth (BE) adjusted data (see Fig. 150.2 above) which appears to have zero adjustments after 1870. However, the actual published BE trends for Malta shown in Fig. 150.3 below appear to exhibit more warming over the period from 1890 to 2000 (~1.6°C) than is seen in the Luqa data in Fig. 150.2 (~1.4°C). It is also interesting that the temperature trend in Fig. 150.3 extends back to 1750 even though there appears to be no temperature data for Malta before 1850 (for a list of stations in Malta see here).

 

 Fig. 150.3: The temperature trend for Malta since 1750 according to Berkeley Earth.

 

If we compare the raw data for Luqa with the BE adjusted version we find that there is virtually no difference between the two (see Fig. 150.4 below). It is actually very unusual for such a long dataset not to undergo at least one breakpoint adjustment so this result is quite surprising.

 

Fig. 150.4: The 5-year average of the monthly mean temperature change for Luqa since 1820 based on the original raw data from Fig. 150.1 (in blue) and the Berkeley Earth adjusted data from Fig. 150.2 (in red).

 

As there is no data from Malta to compare the Luqa data against we could instead compare it with temperature data from the nearby Italian islands of Lampedusa (BE ID: 155869) and Pantelleria (BE ID: 175525 ). The locations of these islands relative to Malta and Sicily is shown in the map in Fig. 150.5 below. Lampedusa is about 150 km from both Malta and Pantelleria.

Fig. 150.5: The (approximate) locations of the weather stations in Malta (Luqa), Lampedusa and Pantelleria. Those stations denoted with squares are long stations with over 1200 months of data, while diamonds denote medium stations with more than 480 months of data.

 

Unfortunately the temperature data from Lampedusa and Pantelleria only start around 1960 but both sets show temperature rises after 1980 that are similar to that seen in the Luqa data. There is also good correlation in the high frequency (less than 12 months) fluctuations, but the medium timescale fluctuations with peak widths of more than 12 months are not well correlated.

 

Fig. 150.6: A comparison of the 5-year average temperature change for the islands of Malta, Lampedusa and Pantelleria since 1960.

 

The other data that we could compare Luqa with is that of Italy. This is shown in Fig. 150.7 below and here the medium timescale fluctuations in the two datasets correlate much better.

 

Fig. 150.7: A comparison of the 5-year average temperature change for Malta and Italy.

 

 

Summary

The temperature data for Malta clearly shows that the climate warmed by over 0.6°C after 1980 (see Fig. 150.1).

Before 1980 there is a large amount of natural variation in the temperature data but no overall upward trend.

The temperature trend for Malta can only be determined using one set of station data: Luqa (BE ID:156721). This makes it very unreliable. Comparing it with the neighbouring islands of Lampedusa and Pantelleria only confirms the temperature trend after 1960 (see Fig. 150.6).

There does appear to be a good correlation between the temperature trend of Luqa in Malta and that of Italy in Fig. 148.2 of Post 148 extending back as far as 1890 (see Fig. 150.7). This would suggest that the temperature trends of Malta and Italy have been very similar over at least the last 150 years.



Acronyms

BE = Berkeley Earth.

MRT = monthly reference temperature (see Post 47).

MTA = mean temperature anomaly.

Long station = a station with over 1200 months (100 years) of data before 2014.

Medium station = a station with over 480 months (40 years) of data before 2014.

List of all stations in Malta with links to their raw data files.

149: Portugal, Spain and France - a comparison

In Post 138 I compared the temperature trends of the Scandinavian countries to see it there were any similarities. There were. In fact there was almost perfect agreement between the 5-year average trends of Norway, Sweden and Finland as far back as 1900 (see Fig. 138.3). As both Norway and Sweden had about twenty sets of station data that went back to 1900 and Finland had about ten, this demonstrated that averaging over a large number of independent data sets eliminates most measurements errors: a consequence of regression towards the mean.

In Post 144 I repeated this procedure for trends from Ireland, Scotland and England and obtained a similar result (see Fig. 144.3), although the trend for England differed slightly from the other two due to its greater urbanization. This demonstrates that neighbouring regions should have similar climates, or at least they should experience similar changes to their climates. So is this also the case for Portugal, Spain and France? I ask this because the results from Post 146 suggest that before 1980 the climate of Spain was cooling while Post 145 suggests that that of Portugal was warming. Well, the results in Fig. 149.1 below show that in fact the temperature trends of Spain and Portugal are very well correlated as far back as 1940, then they diverge. France, on the other hand, is only very weakly correlated to both Spain and Portugal.

Fig. 149.1: A comparison of the 5-year average temperature trends since 1800 for Portugal (green), Spain (red) and France (blue). The two upper trends are offset by +2°C for clarity and the bottom two trends are offset by -2°C.

This discrepancy can be explained in part by the number of stations contributing to the mean temperature anomaly (MTA) of each country per month (see Fig. 149.2 below). Before 1940 there are only two stations contributing to the Portugal MTA, which is probably why it diverges from the Spain MTA which consistently has over ten contributing stations. However, this cannot fully explain the poor correlation of the French data to that of either Spain or Portugal, even though France also has a low number of stations before 1940. The issue here is that the France MTA has a high number of contributing stations after 1960, as do Spain and Portugal, and yet its correlation to both of their MTAs is still poor after 1960. That said, its overall trend since 1860 does follow that of Portugal quite closely.

Fig. 149.2: The number of station records included each month in the averaging for the mean temperature trends of each country in Fig. 149.1.

It should be remembered, though, that the MTAs of both Portugal and France before 1940 are strongly dependent on only two or three sets of station data, and in both cases most of these stations are located in the biggest cities: Paris, Marseille, Lisbon and Porto. These four stations also all appear to exhibit severe continuous warming since 1900 consistent with the effect of urban heat islands. In which case the similarity between the MTA trends of Portugal and France before 1940 may simply be a consequence of parallel economic development in their largest cities.

Instead these comparisons suggest that France may actually have a completely different climate to the Iberian Peninsula even though it is its closest neighbour. The reason for this may be down to geography and the influence of the Pyrenees mountain range at the border that effectively insulates one region from the other.

148: Italy - temperature trends STABLE before 1980

Whereas France only has four long stations with over 1200 months of data, Italy has at least twelve long stations, most with data stretching back over two hundred years. This means it is possible to determine  with a high degree of certainty the extent of climate change in Italy as far back as 1820. What this climate data shows is that the climate of Italy was stable for almost two hundred years up until 1980. Then over the last forty years it has warmed by about 1°C.

In addition to its twelve long stations, Italy also has 81 medium stations with over 480 months of data (for a full list see here). The locations of these 93 stations are shown on the map in Fig. 148.1 below. While the stations are generally spread evenly, there is a higher concentration of long stations in the north of the country compared to the south, and some clustering around Milan, Venice and Rome.

Fig. 148.1: The (approximate) locations of the 93 longest weather station records in Italy. 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 Italy 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 relative to its monthly reference temperature (MRT), and then averaging those anomalies to determine the mean temperature anomaly (MTA) for the whole country for each month. The MRTs for each station in Italy were calculated using the same 30-year period, namely from 1961 to 1990. 

The resulting MTA is shown as a time series in Fig. 148.2 below and clearly shows that temperatures were stable for over two hundred years up until 1980. Then they appear to increase rapidly by about 1.0°C over thirty years. That said, the change is comparable in size and speed to natural variations seen in earlier parts of the trend such as in 1940. On the other hand the MTA after 1980 is based on data from many more stations (up to ninety) and so is likely to be more accurate.

Fig. 148.2: The mean temperature change for Italy since 1740 relative to the 1961-1990 monthly averages. The best fit is applied to the monthly mean data from 1881 to 1980 and has a slight positive gradient of +0.03 ± 0.12 °C per century.

The total number of stations included in the MTA in Fig. 148.2 each month is shown in Fig. 148.3 below. The peak in the frequency around 1970 suggests that the 1961-1990 interval was indeed the most appropriate to use for the MRTs. It also indicates that data from about ten stations were used to calculate the MTA for almost every month back to 1820. As fifteen stations appears to the minimum number needed to provide an accurate MTA, this suggests that the trend in Fig. 148.2 is reliable at least as far back as 1820.

Fig. 148.3: The number of station records included each month in the mean temperature anomaly (MTA) trend for Italy in Fig. 148.2.

If we next consider the change in temperature based on Berkeley Earth (BE) adjusted data we get the MTA data in Fig. 148.4 below. This again was determined by averaging each month the anomalies from the 93 longest stations but here the picture is slightly different from that depicted in Fig. 148.2. Instead of a stable MTA we see a positive trend of over 0.5°C per century suggesting that the climate was warming before 1980. This clearly contradicts the raw data in Fig. 148.2.

Fig. 148.4: Temperature trends for Italy based on Berkeley Earth adjusted data. The best fit linear trend line (in red) is for the period 1881-1980 and has a positive gradient of +0.53 ± 0.04°C/century.

But if we next compare the curves in Fig. 148.4 with those from the published Berkeley Earth (BE) version for Italy shown in Fig. 148.5 below, we see that there is excellent agreement between the two sets of data at least as far back as 1770. This indicates that the simple averaging of adjusted anomalies used to generate the BE MTA in Fig. 148.4 is as effective and accurate as the more complex gridding method used by Berkeley Earth in Fig. 148.5. In which case simple averaging should be just as effective and accurate in generating the MTA using raw unadjusted data in Fig. 148.2.

Fig. 148.5: The temperature trend for Italy since 1750 according to Berkeley Earth.

The differences between the MTA in Fig. 148.2 and the BE versions using adjusted data in Fig. 148.4  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. 148.2 and Fig. 148.4. The magnitudes of these adjustments are shown graphically in Fig. 148.6 below. The blue curve is the difference in MTA values between adjusted (Fig. 148.4) and unadjusted data (Fig. 148.2), while the orange curve is the contribution to those adjustments arising solely from breakpoint adjustments. Between 1880 and 1980 both are considerable with the former leading to an additional warming since 1880 of over 0.5°C.

Fig. 148.6: The contribution of Berkeley Earth (BE) adjustments to the anomaly data in Fig. 148.4 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 1881-1980 has a positive gradient of +0.510 ± 0.009 °C per century. The orange curve shows the contribution just from breakpoint adjustments.

The overall impact of the BE adjustments can be seen more clearly if we compare the 5-year averages for the raw data (from Fig. 148.2) and the BE adjusted data (from Fig. 148.4). This comparison is shown in Fig. 148.7 below. It clearly shows that the trend based on adjusted data (red curve) exhibits considerably more warming since 1840 but slightly less since 1990.

Fig. 148.7: The 5-year mean temperature change for Italy since 1740 based on the original raw data from Fig. 148.2 (in blue) and the Berkeley Earth adjusted data from Fig. 148.4 (in red).

Summary

The raw unadjusted temperature data for Italy clearly shows that the climate was stable from 1780 to 1980 (see Fig. 148.2).

In contrast, the BE adjusted data claims that the climate first cooled and then warmed by 0.5°C from 1880 to 1980 (see Fig. 148.4).

After 1980 the climate has clearly warmed by about 1°C but this is still only of the same magnitude as the natural variations in the climate, so it may be too early to state definitively how much of the warming is permanent and how much more is still to come.



Acronyms

BE = Berkeley Earth.

MRT = monthly reference temperature (see Post 47).

MTA = mean temperature anomaly.

Long station = a station with over 1200 months (100 years) of data before 2014.

Medium station = a station with over 480 months (40 years) of data before 2014.

List of all stations in Italy with links to their raw data files.