As expected, the climatological statistics recently issued by Met Éireann show that this summer's rainfall was well above normal everywhere. Mean temperatures for the summer were a little higher than normal, by amounts varying in an irregular pattern between zero and 0.8 degrees Celsius across the country.

A first observation to be made is that the summer's rainfall pattern was the opposite of what is predicted by all climate models to result from the global warming associated with enhanced greenhouse gases. The model predictions are for warmer and drier Irish summers, with this trend being particularly marked in the east and southeast. Must we conclude from this lack of agreement between the predicted and observed rainfall that global warming isn't really occurring, or that the model predictions of the consequences of global warming are misleading?

I will argue here that neither of these conclusions should be drawn, but that the real cause of this summer's unusual rainfall was the natural variability of our Atlantic climate, which still overwhelms the emerging signal of global warming at our geographic location.

Global warming is now well authenticated. Its global average value is 0.7 degrees over the past hundred years. It is generally agreed by climate scientists, as evidenced by the 2007 Report of the Intergovernmental Panel on Climate Change (IPCC), that this warming is due to man-made greenhouse gas emissions and that, if emissions continue unabated, it will amount to several degrees by the end of the present century.

For well understood physical reasons, this warming is accentuated in the Arctic and in the interiors of continents, reaching twice its global average value in these regions.

Since Ireland is well away from the regions of accentuation of the man-made warming pattern, the extent of any warming occurring here that can be attributed to man-made effects is a small fraction of the 0.7-degree global figure. The remainder of any observed Irish warming is due to other causes.

The clearest signal of man-made global warming, indicating more than anything else that it is real and serious, is the rapid melting of the Arctic sea ice. At the end of summer 2007, the sea ice extent reached a dramatic minimum of 23 per cent below its previous minimum set in September 2005. This year's minimum, the second-lowest recorded, has reached close to last year's value, while polar scientists predict that the summer Arctic sea ice will melt all the way to the pole in coming decades. The rapidity of the melting is causing a degree of alarm in the international climate research community on a par with that caused by the discovery of the Antarctic Ozone Hole in the mid-1980s.

The models used for scientific predictions of the consequences of greenhouse gas increases are known to specialists as atmosphere-ocean general circulation models. Irish meteorologists have played their part in the development of such models. The leading Irish climate prediction group is the C4I Project (Community Climate Change Consortium for Ireland; www.c4i.ie), headed by Ray McGrath at Met Éireann and Prof Peter Lynch at UCD. The global models and the regional models embedded in them to provide greater local detail are consistent in predicting that, on average, warmer and drier Irish summers will accompany global warming.

So far, however, the models are unable to reproduce the full strength of regional patterns of natural low-frequency variability, such as the North Atlantic Oscillation.

This phenomenon, whose simplest measure is the difference in surface pressure between the Azores and Iceland, has a dominant influence in the present climate on the year-to-year variations in the seasonal weather of Northern Europe, including Ireland.

This summer's unusual Irish weather, caused by a succession of unstable low pressure centres moving slowly across the country, is linked with such modes of natural variability. The summer weather of 2008, though unusual, was far from unprecedented. Similar summers were experienced in 1986, 1985 and 1958. Going further back there were fears of famine in the 1870s due to three successive years of excessive summer rainfall.

An examination of the Atlantic temperatures for August, however, shows that there was a large pool of abnormally cold water stretching from the west coast of Ireland to the mid-Atlantic. This would have had a greater effect on the moisture content of depressions reaching Ireland than the distant warm area in the western Atlantic. Surface temperatures over Ireland were the coolest since 2002 in many places.

The evidence indicates that our unusual summer rainfall had little to do with man-made global warming and everything to do with natural variability. Speculation based on flimsy evidence that our bad weather was man-made is not aiding the vitally important cause of limiting greenhouse gas emissions. The necessity for emission controls can and must be based on solid scientific evidence.

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