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.
