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.

144: Evidence against temperature adjustments #4 (British Isles)

In the previous four posts I examined the temperature changes for Ireland (see Post 140), Scotland (see Post 142), England (see Post 143) and Great Britain (see Post 141). While all four sets of temperature data appeared similar from 1900 onwards, there were some differences, and these differences were most apparent in a comparison of the earlier data for Ireland and Great Britain. When the Great Britain data was separated into different trends for Scotland and England a similar degree of difference was observed with the Scotland data appearing to correlate more closely with Ireland, and England with Great Britain. In this post I will look to show this pictorially by comparing the various trends directly.

First, if we compare the data for Ireland, Scotland and England with Great Britain we see that England shows the closest agreement after 1900 but Scotland shows the better agreement before 1840 (see Fig. 144.1 below). The data depicted here are the 5-year moving averages of the mean temperature anomalies (MTAs) for each country as shown by the yellow curves in Fig. 140.2, Fig. 141.2, Fig. 142.2 and Fig. 143.2 in previous posts.

Fig. 144.1: The 5-year average temperature trends since 1760 for Ireland, Scotland and England each compared to that of Great Britain. For clarity the trends for Ireland and England are offset by +2°C and -1.5°C respectively.

What is striking about the trends in Fig. 144.1 is how similar they all are after 1860, while the greatest disparities occur before 1860. The reason for this is evident from Fig. 144.2 below which shows that the number of stations used to calculate each of the MTA for Ireland, Scotland and England drops below five before 1870. From this we can conclude two things. First, this suggests that if there are too few stations used in determining the MTA the accuracy decreases. Secondly we see that when there are sufficient stations used to determine the MTA the accuracy is so good that there is little difference between the MTA for different neighbouring countries. 

This is not the first time such conclusions have been drawn. The same effects were seen in Post 138 (Evidence against temperature adjustments #3) comparing trends in the different Scandinavian countries and Post 57 (The case against temperature data adjustments #1) comparing them in various central European countries. In all cases the conclusion is the same. If trends for neighbouring countries agree, then they are likely to all be correct, not all equally incorrect. Therefore no adjustments to the temperature data are needed or justified. A similar result is also encountered when comparing random samples of stations from the same region as was shown for the USA in Post 67 (More evidence against temperature data adjustments #2). The reason for this is that averaging a sufficiently large number of independent data sets results in a reduction in the size of the errors imported from each. This is known as regression towards the mean.

Fig. 144.2: The number of station records included each month in the averaging for the mean temperature trends in Fig. 144.1.

The second comparison I have performed is to compare data for Ireland, Scotland and England with each other. This is shown in Fig. 144.3 below. Now we see that the two countries that agree most closely are Scotland and Ireland while the data for England appears to exhibit more warming after 1980 and before 1900. This additional warming could be in excess of 0.5°C since 1840.

Fig. 144.3: Comparisons of the 5-year average temperature trends since 1760 for England and Scotland (two top curves, both offset by +2°C), Scotland and Ireland (two middle curves), and Ireland and England (two bottom curves, both offset by -2°C).

Conclusions

Once again a comparison of temperature data for neighbouring countries indicates that most adjustments to the data are unnecessary as the averaging process will correct for most errors via regression towards the mean.

The data for Scotland and Ireland are in closest agreement, probably because both have similar population densities and are more rural.

The data for England is in closest agreement with that of Great Britain, probably because England is the largest country in Great Britain and so its stations will always make the dominant contribution compared to other countries such as Scotland or Wales. 

The greater warming seen in England (of over 0.5°C) is further evidence that warming within countries is driven not just by carbon dioxide levels in the atmosphere and the greenhouse effect, but by local energy consumption as well. So net-zero will not be a panacea.

143: England - temperature trends WARMING

It is probably not surprising that England has more weather stations of note than Scotland. After all it has about ten times the population and almost twice the area. Yet the difference is not as great as one might imagine. For while Scotland has nine long stations with over 1200 months of data before 2014, England has only a slight advantage with ten stations. For medium stations with over 480 months of data the difference is greater with England having 55 compared to 13 in Scotland. There is, however, more clustering of stations in England as the map in Fig. 143.1 below shows.

Fig. 143.1: The (approximate) locations of the 65 longest weather station records in England. 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 England 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 England were calculated using the same 30-year period as for the UK in Post 141, namely from 1956-1985. 

The resulting MTA is shown as a time series in Fig. 143.2 below and clearly shows that temperatures rose slightly over 150 years up until 1975 before increasing more rapidly thereafter. In this respect the MTA data for England more resembles that of Great Britain (see Fig. 141.2 in Post 141) than it does that of Scotland (see Fig. 142.2 in Post 142) or Ireland (see Fig. 140.2 in Post 140).

Fig. 143.2: The mean temperature change for England since 1760 relative to the 1956-1985 monthly averages. The best fit is applied to the monthly mean data from 1826 to 1975 and has a positive gradient of +0.35 ± 0.08 °C per century.

The temperature trend for England was calculated 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 graph in Fig. 143.3 below indicates how many stations were available each month in order to contribute to that month's MTA.

The MRTs for England were calculated using the same 30-year period as for the UK in Post 141, namely from 1956-1985. The resulting MTA is shown as a time series in Fig. 143.2 above and clearly shows that temperatures were slowly increasing for over 150 years up until 1975. Then at some point in the 1980s (probably in 1988) the mean temperature appears to increase abruptly by about 1°C. This is a phenomenon that has been seen in many other temperature trends across Europe.

Fig. 143.3: The number of station records included each month in the mean temperature anomaly (MTA) trend for England in Fig. 143.2.

If we next consider the change in temperature based on Berkeley Earth (BE) adjusted data we get the MTA data in Fig. 143.4 below. This again was determined by averaging each month the anomalies from the 65 longest stations and also suggests that the climate was warming slowly before 1980 but then warmed more strongly by over 1°C thereafter.

Fig. 143.4: Temperature trends for England based on Berkeley Earth adjusted data. The best fit linear trend line (in red) is for the period 1826-1975 and has a positive gradient of +0.27 ± 0.03°C/century.

The difference between the MTA based on raw unadjusted data (from Fig. 143.2) and the MTA based on BE adjusted data (from Fig. 143.4) is shown in Fig. 143.5 below. The blue curve in Fig. 143.5 is the difference in MTA values between the adjusted data (Fig. 143.4) and the unadjusted data (Fig. 143.2) and represents the total of all the data adjustments made including those from homogenization, gridding, Kriging and most significantly breakpoint adjustments. The orange curve is the contribution to those adjustments arising solely from breakpoint adjustments.

Fig. 143.5: The contribution of Berkeley Earth (BE) adjustments to the anomaly data in Fig. 143.4 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 1871-2010 has a small positive gradient of +0.003 ± 0.003 °C per century. The orange curve shows the contribution just from breakpoint adjustments.

The overall impact of any adjustments can perhaps be seen more clearly if we compare the 5-year averages for the raw unadjusted data and the BE adjusted data as is shown in Fig. 143.6 below. This shows that the two datasets agree almost perfectly from 1870 onwards while before 1870 the adjusted data implies the climate is more stable. It should be noted though that the MTA trends before 1870 are based on data from less than five stations and this drops to less than two before 1850 (see Fig. 143.3). This is one reason why the MTA exhibits more natural variability over the earlier period before 1870.

Fig. 143.6: The 5-year mean temperature change for England since 1760 based on the original raw data from Fig. 143.2 (in blue) and the Berkeley Earth adjusted data from Fig. 143.4 (in red).

Summary

What the raw data for England shows is that the climate was warming slowly for most of the period up to 1975 (see Fig. 143.2). This is similar to the trend seen previously for Great Britain (see Fig. 141.2 in Post 141), but is different from both Scotland (see Fig. 142.2 in Post 142) and Ireland (see Fig. 140.2 in Post 140) where little warming was seen in this period. This suggests that England is the dominant country in determining the overall climate of the UK but is also the outlier. But why?

The obvious answer is that England has a much greater population density and so experiences much more urban or surface heating from human activities (see Post 14, Post 29, Post 127 and Post 134). As I pointed out in Section (iv) of Post 127, the energy consumption of Greater London is sufficient to raise the local temperature by over 4°C.


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.

142: Scotland - temperature trends STABLE before 1980

In my previous post I looked at the temperature trends for Great Britain, i.e. the United Kingdom (UK) minus Northern Ireland. These exhibited a large amount of warming (over 1°C), most of which has occurred after 1980. This is not surprising as it is in agreement with other temperature trends that I have analysed, most of which also appear to exhibit some warming after 1980. However, in Great Britain there was still significant warming before 1980, albeit at a much slower rate compared to the post-1980 period. This is more unusual and is also slightly different to the situation found in Ireland (see Post 140) where any warming before 1980 was negligible. So why the difference? Is Ireland the outlier, or is it Great Britain? 

One way to find out is to look separately at the constituent parts of Great Britain: England, Scotland and Wales. If some of these are more similar to Ireland, then that may suggest Ireland is not the outlier but some other parts of the UK may be. Unfortunately there are only about eight stations in Wales of any note, of which only five are medium stations with over 480 months of data, and none have more than a thousand months of data. This means that it is only possible to determine an accurate temperature trend for Wales since 1970. As the most significant differences in the temperature data of Ireland and Great Britain occur well before 1970, the data from Wales is unlikely to be of much use is determining the cause. So for this analysis I will concentrate on England and first Scotland where the quality of the data is far greater.

Fig. 142.1: The (approximate) locations of the 22 longest weather station records in Scotland. 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.

Scotland has nine long stations with over 1200 months of data before 2014 and a further thirteen medium stations with over 480 months of data. These stations are well distributed across the region as the map in Fig. 142.1 above illustrates. This means a simple average of their monthly temperature anomalies should yield a reasonably accurate temperature trend for the country as a whole. This trend is shown in Fig. 142.2 below.

Fig. 142.2: The mean temperature change for Scotland since 1760 relative to the 1956-1985 monthly averages. The best fit is applied to the monthly mean data from 1826 to 1975 and has a slight positive gradient of +0.18 ± 0.07 °C per century.

In order to quantify the changes to the climate of Scotland 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 Scotland were calculated using the same 30-year period as for the UK in Post 141, namely from 1956-1985. The resulting MTA is shown as a time series in Fig. 142.2 and clearly shows that temperatures were fairly stable for over 150 years up until 1975 with only a slight increase being detectable. However, this increase is less than the natural variation in the 5-year average (see the yellow curve in Fig142.2).

Then at some point in the 1980s (probably in 1988) the mean temperature appears to increase abruptly by about 1°C. This is a phenomenon that has been seen in many other temperature trends across Europe. There is also some evidence of additional warming before 1840 which results in an average trend of +0.28°C per century from 1781 to 1980, a 50% increase on the trend for 1826-1975 in Fig. 142.2. However, as the trend before 1850 is based on data from only two stations (see Fig. 142.3 below) it cannot be relied upon.

Fig. 142.3: The number of station records included each month in the mean temperature anomaly (MTA) trend for Scotland in Fig. 142.2.

If we next consider the change in temperature based on Berkeley Earth (BE) adjusted data we get the MTA data in Fig. 142.4 below. This again was determined by averaging each month the anomalies from the 22 longest stations and suggests that the climate was fairly stable before 1880 but then warmed by over 1°C thereafter. In fact the 10-year average suggests there was no warming from 1781 to 1920 but the trend from 1901 to 2020 shows a warming of over 0.75°C. Not only that but the warming is more continuous in nature than the raw data in Fig. 142.2 indicates.

Fig. 142.4: Temperature trends for Scotland based on Berkeley Earth adjusted data. The best fit linear trend line (in red) is for the period 1826-1975 and has a positive gradient of +0.33 ± 0.03°C/century.

What is also apparent is that the trend in Fig. 142.4 for data from 1826 to 1975 is almost double the equivalent trend in Fig. 142.2. The reason for this is the adjustments made to the data by Berkeley Earth (BE). These adjustments 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. 142.4 and the raw data in Fig. 142.2. The magnitudes of these adjustments are shown graphically in Fig. 142.5 below. 

Fig. 142.5: The contribution of Berkeley Earth (BE) adjustments to the anomaly data in Fig. 142.4 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 1851-2010 has a positive gradient of +0.158 ± 0.002 °C per century. The orange curve shows the contribution just from breakpoint adjustments.

The blue curve in Fig. 142.5 is the difference in MTA values between the adjusted data (Fig. 142.4) and the unadjusted data (Fig. 142.2), while the orange curve is the contribution to those adjustments arising solely from breakpoint adjustments. Overall these adjustments appear to add almost 0.3°C of warming to the trend between 1840 and 2010. Before 1840 the adjustments reduce the warming. The overall impact can be seen more clearly if we compare the 5-year averages for the raw data and the BE adjusted data as is shown in Fig. 142.6 below.

Fig. 142.6: The 5-year mean temperature change for Scotland since 1760 based on the original raw data from Fig. 142.2 (in blue) and the Berkeley Earth adjusted data from Fig. 142.4 (in red).

What the data in Fig. 142.6 shows is the amount of warming that has been added by the BE adjustments. While it is less than the natural warming it is still significant and adds over 0.2°C of warming to the period from 1876 to 2010. The result is a trend of 0.74°C per century after 1875 (as shown in Fig. 142.7 below) compared to only 0.57°C per century for the raw data in Fig. 142.2 for the same period. The main impact of the adjustments before 1900 appears to be to flatten the curve and thus eliminate any

Fig. 142.7: Temperature trends for Scotland 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 +0.74 ± 0.03°C/century.

Summary

What the raw data for Scotland shows is that the climate was stable for 150 years up to 1975 with warming of less than 0.18°C per century. This is similar to that seen in Ireland of 0.14°C per century (see Fig. 140.2 in Post 140) and significantly less than the value of 0.46°C per century for Great Britain (see Fig. 141.2 in Post 141). This suggests that Ireland and Scotland are not the outliers. So is England, and why?


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.

141: United Kingdom - temperature trends WARMING

In this post I will consider the temperature data of the United Kingdom (UK), or more specifically Great Britain as the data for Northern Ireland was included in the analysis of the temperature change for Ireland in the last post. Overall the UK has 21 long stations with over 1200 months of data before 2014, of which two are in Northern Ireland and are thus excluded from this analysis. There are also another 73 medium stations with over 480 months of data all of which are within Great Britain or the Isle of Man (for a full list see here). The locations of these 92 long and medium stations are shown on the map in Fig. 141.1 below. What the data from these stations appear to show is that the climate of the UK has warmed gradually by about 0.6°C over the the two hundred years before 1980 but has since warmed further by a similar amount in under forty years.

Fig. 141.1: The (approximate) locations of the 92 longest weather station records in the United Kingdom (excluding Northern Ireland). 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 the UK 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. 141.2 below and clearly shows that temperatures have increased by about 1°C since 1760.

Fig. 141.2: The mean temperature change for the United Kingdom since 1760 relative to the 1956-1985 monthly averages. The best fit is applied to the monthly mean data from 1871 to 1980 and has a positive gradient of +0.46 ± 0.10 °C per century.

The process of determining the MTA in Fig. 141.2 involved first determining the monthly reference temperatures (MRTs) for each station using a common 30-year reference period, in this case from 1956 to 1985, 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. 141.2 each month is indicated in Fig. 141.3 below. This graph shows that there was a sudden increase in stations in 1973 while some existing station were moved or discontinued at about the same time. In order to include as many of these stations as possible in the MTA the MRT interval was set as 1956-1985 so that it overlapped both periods before and after 1973.

Fig. 141.3: The number of station records included each month in the averaging for the mean temperature trends in Fig. 141.2.

The data in Fig. 141.3 indicates that the greatest coverage of the country for temperature data is after 1973 with up to 76 long and medium stations in operation at any one time. This drops to about 28 in 1930 and to less than five before 1850. This means that the MTA for the UK before 1890 will be less reliable than its values after 1950. Note that a reliable MTA generally needs data from at least sixteen stations (see Post 57 for evidence) otherwise errors in the data from individual stations become significant.

Fig. 141.4: The mean temperature change for the United Kingdom since 1760 relative to the 1956-1985 monthly averages. The best fit is applied to the monthly mean data from 1781 to 1980 and has a moderate positive gradient of +0.29 ± 0.05 °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. 141.5 below. This again was determined by averaging each monthly anomaly from the 92 longest stations and also suggests that the UK climate has warmed by over 1°C. In this case, though, the warming appears to occur almost exclusively after 1875 with the climate being stable before this date and gradually warming (with some significant variation) thereafter.

Fig. 141.5: Temperature trends for the United Kingdom 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 +0.80 ± 0.03°C/century.

Comparing the curves in Fig. 141.5 with the published Berkeley Earth (BE) version for the UK in Fig. 141.6 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. 141.5 using adjusted data is as effective and accurate as the more complex gridding method used by Berkeley Earth in Fig. 141.6. This is a conclusion that is not unique to this case. In fact it is true of virtually all the country and regional data I have examined for this blog so far.

This means that the simple averaging process used for the data in Fig. 141.5 should be just as effective and accurate in generating the MTA using raw unadjusted data in Fig. 141.2 and Fig. 141.4. Consequently, any major discrepancy between the adjusted data in Fig. 141.5 and the unadjusted data in Fig. 141.4 cannot be due to the different averaging processes used, but must instead be the result of the Berkeley Earth adjustments.

Fig. 141.6: The temperature trend for the United Kingdom since 1750 according to Berkeley Earth.

Most of the differences between the MTA in Fig. 141.4 and the BE versions using adjusted data in Fig. 141.6 are 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. 141.4 and Fig. 141.5.

Fig. 141.7: The contribution of Berkeley Earth (BE) adjustments to the anomaly data in Fig. 141.5 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 1921-2000 has a positive gradient of +0.097 ± 0.004 °C per century. The orange curve shows the contribution just from breakpoint adjustments.

The magnitudes of these adjustments are shown graphically in Fig. 141.7 above. The blue curve is the difference in MTA values between adjusted (Fig. 141.5) and unadjusted data (Fig. 141.4), while the orange curve is the contribution to those adjustments arising solely from breakpoint adjustments. The overall adjustment from 1900 to 2013 is small, less than +0.2°C. A greater impact is seen before 1880. This appears to change the shape of the long term trend before 1900 from a gradual warming in Fig. 141.4 to a more stable climate in Fig. 141.5. This can be seen more clearly in the comparison curves in Fig. 141.8 below. These also show that the adjustments made after 1900 add slightly to the observed warming. In this case, however, both these corrections are smaller than those seen in other posts, particularly for countries in the Southern Hemisphere.

Fig. 141.8: The 5-year mean temperature change for the United Kingdom since 1760 based on the original raw data from Fig. 141.2 (in blue) and the Berkeley Earth adjusted data from Fig. 141.5 (in red).

Summary

The temperature data from UK stations appears to indicate that the climate of the UK has warmed by about 1°C since 1760, and most of this warming has occurred since 1900 (see Fig. 141.2). In fact over half the warming has occurred since 1980.

The pattern of warming is broadly the same for both the MTA calculated using raw data (See Fig. 141.2) and that based on Berkeley Earth adjusted data (see Fig. 141.5).

The MTA data of the UK before 1900 appears to show more warming than is seen in similar data for Ireland (see Post 140) even though the two territories are near neighbours and their data are more similar after 1900. One reason for this could be the greater population density and industrialization of the UK compared to Ireland. One way to test this hypothesis would be to analyse the temperature for England and Scotland separately and compare these with Ireland. If the England data is the exception then that would support the hypothesis.


Acronyms

BE = Berkeley Earth.

MRT = monthly reference temperature (see Post 47).

MTA = mean temperature anomaly.

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

140: Ireland - temperature trends STABLE before 1980

The island of Ireland has nineteen weather stations with over 480 months of data before the end of 2013. All but two of these are in the Republic of Ireland. The two stations in Northern Ireland (Armagh and Belfast Airport) are though both long stations with over 1200 months of data. In addition there are a further six long stations in the Republic of Ireland together with eleven medium stations (for a full list see here). The locations of these nineteen stations are shown on the map in Fig. 140.1 below. Other than a small cluster around Dublin, the stations are evenly spread across the island. This means that any average of the temperature anomalies from these nineteen stations should approximate well to the true relative temperature change for Ireland. What this averaging shows is that the climate of Ireland was fairly stable until 1980 but with medium term fluctuations of up to 1°C in the mean temperature. After 1980 the mean temperature has probably risen by about 1°C. In other words, the rise since 1980 is comparable to the fluctuations.

Fig. 140.1: The (approximate) locations of the 19 longest weather station records in Ireland. 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 Ireland 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 region. This MTA is shown as a time series in Fig. 140.2 and clearly shows that temperatures were fairly stable up until 1980. However at some point in the 1980s (probably in 1988) the mean temperature appears to increase abruptly by almost 1°C.

Fig. 140.2: The mean temperature change for Ireland since 1820 relative to the 1961-1990 monthly averages. The best fit is applied to the monthly mean data from 1846 to 1975 and has a positive gradient of +0.14 ± 0.08 °C per century.

The process of determining the MTA in Fig. 140.2 involved first determining the monthly reference temperatures (MRTs) for each station using a common reference period, in this case from 1961 to 1990, 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. 140.2 each month is indicated in Fig. 140.3 below. The peak in the frequency between 1960 and 2010 suggests that the 1961-1990 interval for the MRTs was a good choice.

Fig. 140.3: The number of station records included each month in the mean temperature anomaly (MTA) trend for Ireland in Fig. 140.2.

If we next consider the change in temperature based on Berkeley Earth (BE) adjusted data we get the MTA data in Fig. 140.4 below. This again was determined by averaging each monthly anomaly from the nineteen longest stations and suggests that the climate was fairly stable before 1920 but then warmed thereafter. In fact the 10-year average suggests a warming of almost 1.5°C from 1840 to 2000. Not only that but the warming is more continuous in nature than the raw data in Fig. 140.2 actually shows.

Fig. 140.4: Temperature trends for Ireland based on Berkeley Earth adjusted data. The best fit linear trend line (in red) is for the period 1896-2005 and has a positive gradient of +0.63 ± 0.04°C/century.

If we compare the curves in Fig. 140.4 with the published Berkeley Earth (BE) version for Ireland in Fig. 140.5 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. 140.4 using adjusted data is as effective and accurate as the more complex gridding method used by Berkeley Earth in Fig. 140.5. In which case simple averaging should be just as effective and accurate in generating the MTA using raw unadjusted data in Fig. 140.2.

Fig. 140.5: The temperature trend for Ireland since 1750 according to Berkeley Earth.

Any differences between the MTA based on raw unadjusted data in Fig. 140.2 and the BE version using adjusted data in Fig. 140.4 are 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. 140.2 and Fig. 140.4. 

Fig. 140.6: The contribution of Berkeley Earth (BE) adjustments to the anomaly data in Fig. 140.4 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 1881-2010 has a positive gradient of +0.028 ± 0.003 °C per century. The orange curve shows the contribution just from breakpoint adjustments.

The magnitudes of these adjustments are shown graphically in Fig. 140.6 above. The blue curve is the difference in MTA values between adjusted (Fig. 140.4) and unadjusted data (Fig. 140.2), while the orange curve is the contribution to those adjustments arising solely from breakpoint adjustments. The overall adjustment from 1880 to 1990 is small, less than ±0.1°C. The largest adjustments to the data occur after 1990 and before 1880. These adjustments add almost 0.2°C of warming to the data after 1990 and add almost 0.3°C of cooling to the data before 1880. While these changes are small in themselves, cumulatively they add almost 0.5°C to the warming trend. The full impact of these adjustments can be seen most clearly by comparing the the 5-year moving averages of the data in Fig. 140.2 and Fig. 140.4 as shown in Fig. 140.7 below.

Fig. 140.7: The 5-year mean temperature change for Ireland since 1820 based on the original raw data from Fig. 140.2 (in blue) and the Berkeley Earth adjusted data from Fig. 140.4 (in red).

Summary

According to the raw unadjusted temperature data, the climate of Ireland remained stable for 150 years up until the 1980s (see Fig. 140.2). Then it suddenly increased in temperature by almost 1°C. Why?

In contrast, adjusted temperature data from Berkeley Earth claims to show that the climate of Ireland has warmed more or less continuously since 1900. This warming is a bit more than 1°C (see Fig. 140.4).

Comparing the adjusted MTA data (see Fig. 140.4) with the unadjusted MTA data (see Fig. 140.2) suggests that the adjustments may have added up to 0.5°C to the overall warming since 1850 (see Fig. 140.7).


Acronyms

BE = Berkeley Earth.

MRT = monthly reference temperature (see Post 47).

MTA = mean temperature anomaly.

List of all stations in the Republic of Ireland with links to their raw data files.

139: Alaska - temperature trends WARMING (probably)

The US state most often linked to climate change is Alaska. This is probably because it is seen as having an Arctic climate even though only about a third of the state actually lies within the Arctic Circle. In fact Alaska is no more northerly than Norway and its Aleutian Island chain stretches further south than London and Berlin. It has an area three times that of France but its population is less than that of Marseille, yet it has an extensive network of weather stations that is greater in data quality than that seen in many industrialized countries. Ordinarily this should be sufficient to determine the temperature change for Alaska to a high level of precision but it isn't. In fact the data is so inconclusive it is difficult to determine whether Alaska has warmed at all over the last one hundred years let alone quantify that warming and discern when exactly it occurred. This is because the natural variation in the long term temperature averages is far greater than the likely warming.

There are one hundred stations in Alaska with over 480 months of data before 2014 including seven long stations with over 1200 months of data. Of the 93 medium stations with over 480 months of data twenty have over 1000 months of data (for a full list of stations see here). The locations of these stations are shown in Fig. 139.1 below.

Fig. 139.1: The (approximate) locations of the 100 longest weather station records in Alaska. 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.

 

The map in Fig. 139.1 shows that most of the temperature data for Alaska come from stations that are outside the Arctic Circle. In fact of the one hundred longest stations in Alaska only eight are actually inside the Arctic Circle. And while the remainder are fairly evenly distributed geographically, there are significant clusters of stations around Anchorage, Fairbanks and the panhandle along the coast in the southeast between the the border of Canada and the Alexander Archipelago. As usual for simplicity I will disregard this clustering and assume it makes very little difference to the measured temperature change as it only affects the contribution or weighting of about 15% of stations.

In order to quantify the changes to the climate of Alaska 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 temperatures (MRT), 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. 139.2 below with the MRTs for each station calculated using data between 1961 and 1990,  (again using the methodology outlined in Post 47).

Fig. 139.2: The mean temperature change for Alaska since 1900 relative to the 1961-1990 monthly averages. The best fit is applied to the monthly mean data from 1921 to 2000 and has a positive gradient of +0.31 ± 0.31 °C per century.

The data in Fig. 139.2 above illustrates the difficulty of determining a definitive temperature trend when the data is subject to significant variability over time. In this case choosing to fit to the data from 1921 to 2000 leads to a small positive gradient of 0.31°C per century, but this is no bigger than the uncertainty and so is not statistically significant. If other fitting intervals are chosen then the gradient can be significantly different. For example, an interval of 1921-1995 results in a gradient of 0.18°C per century while 1926-2005 produces 0.96°C per century. All of which poses the awkward question, which result is correct?

In my opinion there is no obvious answer, but there are two factors that we could consider that may shed some additional light on the problem. The first of these is to choose an appropriate fitting interval based on the cycle of the natural variations (i.e. fitting from peak to peak), while the second is to concentrate on data that is the result of averaging the greatest number of station records. 

In Post 4 I explained how the best fit line to a single period of a sine wave gives a non-zero gradient (see Fig. 4.7) whereas fitting to a cosine wave does not. This is because a cosine wave is symmetric about the y-axis while the sine wave is anti-symmetric. As most temperature data tends to oscillate over time due to natural variations it therefore follows that the gradient of any fit to that data will depend on the interval chosen relative to the peaks of those natural oscillations. 

In order to avoid biasing the gradient due to asymmetry in the fitting range, the range should be symmetric relative to the natural oscillations. These natural oscillations are seen most clearly in the 5-year moving average (see the yellow curve in Fig. 139.2). So the fitting range should be chosen so that it starts and ends on a peak in the 5-year average, or alternatively starts and ends on a trough. The best fit in Fig. 139.2 does not do this. It starts near a trough at 1921 and ends on a plateau in 2000. But if we change the fitting interval from 1914 to 2003 then the interval starts and ends on a peak in the 5-year average. The result is the best fit shown in Fig. 139.3 below.

Fig. 139.3: The mean temperature change for Alaska since 1900 relative to the 1961-1990 monthly averages. The best fit is applied to the monthly mean data from 1914 to 2003 and has a positive gradient of +0.71 ± 0.26 °C per century.

The gradient of the best fit in Fig. 139.3 is more than twice that in Fig. 139.2 even though the data hasn't changed. This is simply a result of changing the fitting interval. Of course the underlying reason why a change of fitting interval makes such a big difference in this case is that the natural fluctuations in the 5-year average are so large. These changes in temperature can exceed 2°C in less than five years. So we could ask, is the temperature rise of about 0.7°C indicated by the best fit in Fig. 139.3 really that significant in comparison?

Fig. 139.4: The number of station records included each month in the mean temperature anomaly (MTA) trend for Alaska in Fig. 139.2 and Fig. 139.3.

The second factor in determining any choice of fitting range is the quantity of data available. The graph in Fig. 139.4 above shows the number of stations included in the MTA in Fig. 139.2 and Fig. 139.3. From 1920 onwards there are over twenty stations each month. In the previous post and in Post 57 I argued that at least ten, and possibly over twenty-five stations are needed in order for the MTA to be reliable, so this condition is satisfied for all months after January 1920. The data before 1920 will therefore be much less reliable, but there is still enough data to allow us to calculate an approximate MTA as far back as the 1820s. This is shown in Fig. 139.5 below.

Fig. 139.5: The mean temperature change for Alaska since 1820 relative to the 1961-1990 monthly averages. The best fit is applied to the monthly mean data from 1911 to 2010 and has a positive gradient of +0.73 ± 0.22 °C per century.

The data in Fig. 139.5 indicates that it is possible to calculate and MTA as far back as 1829, but before 1900 there are gaps in the data and most of the MTA data for this period is based on an average of anomaly data from less than three different stations. So that raises questions over its reliability.

So how should we interpret this data? The station frequency data in Fig. 139.4 suggests only data after 1900 or even 1920 is sufficiently reliable. As for the data after 1900, there are many ways to interpret it. For example, if we just look at data from 1901 to 1975 the best fit (as determined from trough to trough) is strongly negative (see Fig. 139.6 below). But after 1975 the temperature appears to increase abruptly by about 1°C. So is this interpretation of the temperature trend any more believable than those shown in Fig. 139.2 or Fig. 139.3? It is hard to tell, again because of the high level of natural variability in the data which could be varying on multiple timescales. Such multi-frequency variability is potentially indicative of chaotic or fractal behaviour as I discussed in Post 9, Post 17 and Post 42.

Fig. 139.6: The mean temperature change for Alaska since 1900 relative to the 1961-1990 monthly averages. The best fit is applied to the monthly mean data from 1901 to 1975 and has a negative gradient of -0.52 ± 0.33 °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. 139.7 below. This again was determined by averaging the anomalies for each month from the one hundred longest stations in Alaska and suggests that the climate of Alaska has warmed by over 1°C since 1870, but with large natural variations of up to 1.5°C in the 10-year average.

Fig. 139.7: Temperature trends for Alaska 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. 139.7 with the published Berkeley Earth (BE) version for Alaska in Fig. 139.8 below we see that there is good agreement between the two sets of data as far back as 1880. This indicates that the simple averaging of anomalies used to generate the BE MTA in Fig. 139.7 using adjusted data is as effective and accurate as the more complex gridding method used by Berkeley Earth in Fig. 139.8. In which case simple averaging should be just as effective and accurate in generating the MTA using raw unadjusted data in Fig. 139.2 and Fig. 139.5. In other words, any discrepancy between the adjusted data in Fig. 139.7 and the unadjusted data in Fig. 139.5 cannot be due to the averaging process. Any form of weighted averaging would also not affect the results.

Fig. 139.8: The temperature trend for Alaska since 1820 according to Berkeley Earth.

Most of the differences between the MTA in Fig. 139.6 and the BE versions using adjusted data in Fig. 139.7 are instead mainly due to the data processing procedures used by Berkeley Earth. These include homogenization, gridding, Kriging and most significantly breakpoint adjustments. These lead to changes to the original temperature data, the magnitude of these adjustments being the difference in the MTA values seen in Fig. 139.5 and Fig. 139.7.

Fig. 139.9: The contribution of Berkeley Earth (BE) adjustments to the anomaly data in Fig. 139.7 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 1921-2000 has a positive gradient of +0.262 ± 0.009 °C per century. The orange curve shows the contribution just from breakpoint adjustments.

The magnitudes of these adjustments are shown graphically in Fig. 139.9 above. The blue curve is the difference in MTA values between adjusted (Fig. 139.7) and unadjusted data (Fig. 139.5), while the orange curve is the contribution to those adjustments arising solely from breakpoint adjustments. The overall adjustment from 1920 to 2000 is small, about +0.2°C. Nevertheless, it can be seen in the difference in the 5-year means (see Fig. 139.10 below) for the unadjusted data (blue curve) and the adjusted data (red curve). The difference, though, is about ten times less than the variability in the two MTAs over time. The data in Fig. 139.10 also highlights the difficulty in interpreting the data. If the data between 1940 and 1980 were missing or ignored, then one could postulate that Alaska has seen fairly consistent warming since 1900 amounting to about 1°C in total. But if the 1940-1980 data is included the data all looks very random.

Fig. 139.10: The 5-year mean temperature change for Alaska since 1900 based on the original raw data (in blue) and the Berkeley Earth adjusted data (in red).

Summary

The temperature data for Alaska demonstrates the difficulty in determining an accurate temperature trend for a region when the climate is subject to a high degree of variability.

It is possible that the climate has warmed by almost 1°C since 1900 (see Fig. 139.3), or it might not have warmed at all (see Fig. 139.2).

If the climate has warmed, this warming may have been fairly continuous (see Fig. 139.3), or it could have been fairly recent, occurring mainly after 1980 (see Fig. 139.6).

The one thing we can say is that the difference between the temperature rise based on Berkeley Earth adjusted data (see Fig. 139.7) and that based on the raw unadjusted data (see Fig. 139.5) is small (less than 0.3°C) and much less that the 5-year natural variability of the data (about 2°C).


Acronyms

BE = Berkeley Earth.

MRT = monthly reference temperature (see Post 47).

MTA = mean temperature anomaly.

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