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.

145: Portugal - temperature trends WARMING

The biggest problem in quantifying the extent of climate change in mainland Portugal is the lack of data before 1960. There are only two long stations with over 1200 months of data before 2014, the most significant of which is located in the capital, Lisbon (see here). This station appears to have recorded a temperature increase of over 2°C since 1850, yet the only other station with a comparable length of data in Coimbra shows a more modest increase with significant natural variability (see here) while data from Porto appears to suggest temperatures in the 19th century were warmer than today (see here). As Lisbon is very likely subject to some degree of urban heat island effect due to its size, this makes any accurate determination of the climate changes in Portugal before 1960 problematic.

In addition to the two long stations at Lisbon and Coimbra, there are also fifteen medium stations with over 480 months of data (for a full list of stations see here). The locations of all seventeen of these stations are shown on the map in Fig. 145.1 below. This suggests that they are fairly evenly distributed across the country with no part of the country being more than 70 km from a weather station. This is consistent with the station density of seventeen in 92,212 km² (the area of Portugal), or one in every 5424 km².

Fig. 145.1: The (approximate) locations of the 17 longest weather station records in Portugal. Those stations with a high warming trend 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 Portugal 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 Portugal were calculated using the 30-year period from 1961 to 1990. The resulting MTA is shown as a time series in Fig. 145.2 below.

Fig. 145.2: The mean temperature change for Portugal since 1850 relative to the 1961-1990 monthly averages. The best fit is applied to the monthly mean data from 1871 to 1980 and has a positive gradient of +0.96 ± 0.08 °C per century.

The MTA in Fig. 145.2 clearly shows temperatures rising continuously from 1850 to 2013. However, as has already been pointed out, much of the increase before 1940 is due to the influence of the station in Lisbon. This is demonstrated by the graph below in Fig. 145.3 which shows only two or three stations contributing to the MTA before 1940. After 1960 the MTA is much more reliable as it is the result of averaging data from up to fifteen stations each month.

Fig. 145.3: The number of station records included each month in the mean temperature anomaly (MTA) trend for Portugal in Fig. 145.2.

If we next consider the change in temperature based on Berkeley Earth (BE) adjusted data we get the MTA data in Fig. 145.4 below. This again was determined by averaging each month the anomalies from the seventeen longest stations and suggests that the climate warmed slowly by 0.4°C in the one hundred years prior to 1980 and then warmed by another 0.7°C in the next thirty years.

Fig. 145.4: Temperature trends for Portugal based on Berkeley Earth adjusted data. The best fit linear trend line (in red) is for the period 1871-1980 and has a positive gradient of +0.41 ± 0.04°C/century.

Comparing the curves in Fig. 145.4 with those in the published Berkeley Earth (BE) version for Portugal shown in Fig. 145.5 below indicates that there is good agreement between the two sets of data. This demonstrates that the simple averaging of anomalies used to generate the BE MTA in Fig. 145.4 is as effective and accurate as the more complex gridding method used by Berkeley Earth in Fig. 145.5. In which case simple averaging should be just as effective and accurate in generating the MTA using raw unadjusted data in Fig. 145.2 even though the geographical distribution of stations is not completely homogeneous, as was shown in Fig. 145.1.

Fig. 145.5: The temperature trend for Portugal since 1750 according to Berkeley Earth.

While the temperature trends for the raw unadjusted data in Fig. 145.2 and the BE adjusted data in Fig. 145.4 look very similar, there are some significant differences. These can be seen more clearly by comparing the 5-year average of each dataset as shown in Fig. 145.6 below. This shows how the adjustments in Fig. 145.4 have altered the shape of the trend between 1900 and 2010 but not the overall total temperature change. The impact is to reduce the warming between 1900 and 1980 and to increase it thereafter, so making it look more like the classical 'hockey stick'.

Fig. 145.6: The 5-year mean temperature change for Portugal since 1850 based on the original raw data from Fig. 145.2 (in blue) and the Berkeley Earth adjusted data from Fig. 145.4 (in red).

The adjustments themselves can be calculated by subtracting the MTA values of the raw data in Fig. 145.2 from the adjusted values used in Fig. 145.4. The result is shown in Fig. 145.7 below.

Fig. 145.7: The contribution of Berkeley Earth (BE) adjustments to the anomaly data in Fig. 145.4 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 1871-1980 has a negative gradient of -0.238 ± 0.017 °C per century. The orange curve shows the contribution just from breakpoint adjustments.

The blue curve in Fig. 145.7 is the difference in MTA values between the adjusted data (Fig. 145.4) and the unadjusted data (Fig. 145.2), while the orange curve is the contribution to those adjustments arising solely from breakpoint adjustments. Additional contributions to the blue curve come from other adjustments based on techniques such as homogenization, gridding, Kriging and also any difference in MRT interval. Overall these adjustments appear to reduce the warming between 1900 and 1980 by about 0.3°C and then increase it by a similar amount from 1980 to 2010.

Summary

Both the raw data and the BE adjusted data appear to show that the climate of Portugal has warmed by about 1°C since 1890 (see Fig. 145.6). Most of this warming has occurred since 1980.

The effect of BE adjustments is to modify the shape of the trend from 1900 onwards rather than to increase or decrease the amount of overall warming (see Fig. 145.7).

While the trend since 1960 is incontrovertible as it is based on data from about fifteen different stations each month (see Fig. 145.3), the trend for earlier data is less so as it is based on only three sets of station data that profoundly contradict each other (Lisbon, Coimbra and Porto). However, data from Portugal's neighbour Spain in the next post may resolve this issue.

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 Portugal with links to their raw data files.