Last year I posted an entry calling attention to the weak evidence of an upward trend in the temperature data for Dublin. As we learned from the debates about climate data before the Copenhagen Climate Change Conference, it is important for scientists to explore data that do not neatly fit their models. So I thought it would be worth adding another year to the material I posted last year. All but the most recent data are taken from the CSO database. For recent months I have used the Met Éireann data.
In 2009 the average temperature recorded at Dublin Airport was 9.5 degrees C, the same as that reported for 1958, the first year in the CSO’s database. The average for the thirty-year period 1961-1990 – used by Met Éireann to represent the long-run – was 9.6 degrees C. The year 2009 was the second in a row when the average temperature was at or below the long run average.
There was no very warm weather in Dublin during the last two years. The highest temperatures recorded in both years were in June – only 22.3 degrees in 2008 and a somewhat better 24.8 in 2009. Back in August 1990 a torrid 28.7 was recorded.
The warmest month in both 2008 and 2009 was August, with the same average temperature of 15.3 in both years. These values are low compared with July 1989, the warmest month since 1958, when the average was 17.9 degrees.
Last winter (December 2008, January and February 2009) was cold in Dublin, with an average temperature of 4.6 compared to the long-run average of 5.9. As we are well aware, December 2009 was very cold, with an average of 3.9, but the month was not as exceptionally cold in Dublin as in other parts of Ireland. A lower monthly temperature (3.7) was recorded in Dublin as recently as February 1996. However, record lows were recorded around Christmas and the New Year and into January 2010, so if things don’t warm up soon this winter (December 2009, January and February 2010) is set to break the record lows for Dublin too.
In December 2009 many newspapers carried headlines to the effect that the first decade of the twenty-first century was the warmest on record in many countries. Not so in Dublin, where the 1990s were fractionally warmer than 2000s. However, the variation between decades is very small – the standard deviation for the averages of the five decades since 1960 is only 0.21 degrees, compared with 0.42 for the fifty years of this period. The 1960s were relatively cold – average temperature of 9.4 – but since then the decade averages have only varied between 9.7 in the 1970s and 10.0 in the 1990s. The relatively low temperatures recorded in the 1960s prompted a lot of discussion of global cooling.
As shown in my previous post, the evidence of an upward trend in Dublin’s temperature is weak. The graph below shows the annual averages over the 52-year period 1958-2009. As noted above, the series actually ends up in 2009 where it starts in 1958, at 9.5. In most statistical analyses it would be hard to maintain the existence of a significant trend if over fifty years the series reverted back to where it started from!
In fact, for the whole 52-year period there is weak evidence of a statistically significant positive trend:
TEMP = 0.0097 (YEAR-1958) + 9.5163 R² = 0.1189
However, the relationship is unstable between sample sub-periods. This is illustrated simply by splitting the 52-year period into two halves. The result for the first sub-period (1958-1983) is
TEMP = 0.0144 (YEAR – 1958) + 9.4553 R² = 0.0796
The result for the second sub=period (1984-2009) is:
TEMP = 0.0062 (YEAR-1958) + 9.8118 R² = 0.0121
which is not significant at any of the usual levels.
There seems to have been a shift in emphasis from temperature to rainfall statistics in commentaries on Irish weather patterns. This is natural given the wet summers of 2008 and 2009 and the flooding during last November. But the rainfall at Dublin Airport last year was only 25% above the long-run average and well below the levels recorded in 2002, 1966, 1960, and 1958. There has been no significant trend in yearlt rainfall over the past fifty-two years.
Observations for fifty two years at one weather station may not have much significance for global weather/climate trends, but the lack of evidence for warming in Dublin surely merits more attention than it receives in Irish discussions of climate change.