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.
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