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Friday 2 December 2016

History offers dire warnings on freeze

Published 10/01/2010 | 08:54

WINTERS in Ireland over the last 15 years have been characterised by being mild and wet, relatively speaking. Yet out of the blue and after horrific flooding we are now well into our third week of an exceptional cold spell, with no end in sight.

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This cold spell will last for the next 10 days at least and possibly even much longer.

One commentator suggested that it may last until the end of February and even into March. This might sound like nonsense, but the reality is that when conditions are like this they tend to stabilise and be stationary for long periods of time.

There are a number of reasons which contribute to why this cold spell is of such an exceptional nature in terms of duration and the sustained low temperatures. Sustained daytime temperatures below 0 degrees Celsius are unusual, but sustained night temperatures below minus 5C and now minus 10C are very rare indeed for Ireland.

The reasons why such a sustained spell has occurred after a long series of very mild winters include, firstly, that we have a settled period of dominant high pressure, with polar air moving southwards.

This has the obvious effect of reducing temperatures and blocking our more normal winter pattern of winds from the southwest and west.

Also, given that the days are very short and the strength of the sun's rays — because of their low angle — is relatively weak at this time of year, it is easier for a high pressure cold system at mid to high latitude to be sustained over a relatively long period of time, running to weeks or even months.

It has been suggested that one of the contributing factors to this mode becoming stable and sustained is the occurrence of a moderate El Nino, which has the effect of distorting world climate. The influence of El Nino on European and high latitude climates is, as yet, poorly understood, yet it would be a strange coincidence indeed if it was not playing a role.

The fact that many northern countries across Europe and Asia, including Japan, China and northern India, are also affected shows the size of the climatic disturbance and also the size of this polar air mass that is causing all the problems. However it must be noted that temperatures in the northern USA and Canada are well above average for this time of year, by as much as 10C in some places.

But what of global warming? Surely this shows global warming has weaknesses.

Of course, you cannot relate one particular weather event, even one of this nature, directly to global warming which is concerned with longterm changes over years, decades and centuries.

There is always going to be year-to-year variability, and, since we have had a very long run of very mild years, then the likelihood of a colder winter was always possible once in a while. Even in North Africa snow might fall once every 50 years or so.

However, the frequency of these cold spells in an Irish context will decline as the century progresses and as the overall warming process progresses.

This is already occurring, given the general decline in frost days and days with snow over the last 40 years or so, and the increase in the length of the grass-growing season by several weeks.

The question that remains of most importance is how long will this spell last. I have already suggested that these spells can become stable and stationary over several weeks and even months. The history of cold spells in Ireland with a similar meteorology bears this out to some extent.

The Irish Annals record great snow and freeze events going back to the 1st millennium BC. Major events of this nature occurred in AD 436, 582, 798 and 960. The winter of 821-822 was thusly described in the Annals of Ulster: “A wonderful frost froze the seas, lakes and rivers so that the herds, horses and flocks were led across them.”

More recently Gerald Boate wrote of the great snow of 1635, which lasted throughout part of January and into February. More severe was the great frost of 1739-1740, which lasted from the end of December into May. Here the cause was a volcanic eruption, but was also part of the Little Ice Age, with much colder temperatures at times. The eruption caused a further downturn in global climate which lasted for at least two years.

Probably the lowest temperatures ever to occur in Ireland occurred during this event. Manley's Central England temperature record, which dates from 1659 up to the present, shows 1740 as the coldest year on record. The effects on Ireland were devastating, resulting in a famine in 1740-1741 which caused the deaths of around 25 per cent of the population of Ireland — a higher percentage figure than the Great Famine of the 19th century. David Dickson's book on this period is well worth consulting.

In living memory, the cold spell of 1947 which lasted from late January to mid-March is similar to what is happening at present, during this event the temperatures at Dublin Airport did not rise above 5C between January 22 and March 7.

The cold spell of 1962-63, which also coincidentally started in late December, lasted until mid-March. During this, conditions were so severe that the Shannon was crossed on foot near the city of Limerick for the first time that anyone could remember.

These two events show that these spells, once established, can survive for weeks or even months, and it is no coincidence that January features in all of these events.

All this suggests that, although we can only forecast with some certainty for the next 10 days, and there is unlikely to be any significant change, there is a clear need for serious and rapid planning to be undertaken for a much more prolonged cold spell with a considerable matter of urgency.

It might not happen but the precautionary principle should apply given our past history of these types of prolonged cold spells.

Dr Kieran Hickey lectures in physical geography and is author of ‘Five Minutes to Midnight? Ireland And Climate Change’ published by White Row Press, Belfast

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