Floods have a variety of different causes. Some are born in spring, creatures of the melting snows of distant highlands, and triggered by the waxing warmth of a northward moving sun. Others, near the coast. may occur because a coincidence of moon and tide and wind conspires to raise the level of adjacent seas. Yet more, those short-lived incidents we call flash floods, may be the children of a single thunderstorm that provides, in one short, torrential downpour, more water than a parched landscape can cope with at the time.
The floods affecting central Europe recently, however, are of a kind not unfamiliar to us here in Ireland, They occur when sustained heavy and persistent rain over a wide area pours down on an already sodden countryside for weeks and weeks on end, drowning the landscape so that water steadily accumulates throughout an entire catchment area to a volume well beyond the carrying capacity of local rivers.
This happened, for example, in the case of the devastating Mississippi floods of 1993; it happened nearer home in Galway in March of 1995, and caused a mass evacuation in the Netherlands earlier that year; and it has been happening of late in the valleys of the Oder and the Neisse, in Germany and Poland.
The immediate cause of the floods in Germany is not difficult to find. Over several weeks a shallow but persistent area of low pressure stagnated over the area. Its frontal rainbelts, aligned for much of the time north-south to coincide with the Oder-Neisse valley,undulated to and fro over a relatively narrow zone, their activity enhanced by embedded thunderstorms that thrived in the moist sultry atmosphere of a continental summer. Such lows are not uncommon over continents in summertime; indeed, on a larger scale they are responsible for the monsoon regimes that bring life to many otherwise arid regions of the world. But over Europe such persistence and intensity is unusual, though not unknown.
But why did that low decide to settle over eastern Germany? Again, the immediate answer can be found with relative ease in a slightly anomalous pattern of the winds at upper levels in the atmosphere, but this is merely to invite another question...Any extreme occurrence nowadays raises the issue of whether we have begun at last to see the effects of global warming.
We know that over the last 15 years or thereabouts there has been a steady upward trend in the average global temperature, and some scientists feel that extreme events such as the floods in eastern Europe may be related to this rise.
Their case is plausible. A warmer atmosphere can accommodate the greater quantities of moisture that it acquires through enhanced evaporation, and this leaves more water available to fall as rain in places where conditions otherwise are right. Indeed, the consequences of global warming, the computer models tell us, are likely first to be noticeable as a greater frequency of extreme events throughout the world - more severe droughts in places prone to such occurrences, and a greater incidence of floods in others.
Could the havoc in the Oder-Neisse valley be a preview of what the future has in store?
The majority of scientists remain agnostic on these questions, so the considered post-mortem verdicts on the present crisis are unlikely to differ greatly from those delivered after the flooding of the Rhine valley in 1995 - a typical example would be that of the European Climate Support Network, a group of experts selected from the Meteorological Services of Europe, who said: "Our best estimate is that the factors that led to such abnormally high rainfalls are simply a natural feature of our highly variable climate. It is simply not possible at present to link, in a causal sense, specific weather-related events to any longer-term change in European climate".
The other prime suspect these days when any natural disaster comes along is the phenomenon we call El Nino. The term is used to describe a general warming of the surface waters of large areas of the Pacific Ocean which occurs naturally at irregular intervals of between three and seven years, and which is known to cause palpitations if the world's climate. At its peak, normally arid regions of sub-tropical South America are drenched with rain, other areas in the low latitudes in which rain is normally abundant experience droughts, and there are suspicions that it can at times affect weather in the higher latitudes as well.
The last El Nino episode concluded as recently as 1995. It was at its peak in 1993, and there is evidence to suggest its reverberations contributed to the Mississippi floods that year. Moreover, rising sea-surface temperatures off the coasts of Ecuador and Peru in recent months suggest that another El Nino has almost come upon us - far earlier then anyone expected.
Could it be that this latest El Nino is to blame for the misfortunes of the central Europeans? Most scientists would, again, respond that this is unlikely to be so; the current episode is in its infancy, and in any case, insofar as any El Nino influence is detectable in European rainfall patterns, it seems to be associated more with wet winters than with summer downpours.