Take cold comfort . . . humans not solely to blame for bad winters


ANALYSIS:After exceptionally cold winters in 2009/2010 and 2010/2011, are such Arctic conditions going to be more frequent?

THE YEAR 2010 will be remembered in Ireland as one that began and ended with freezing conditions, burst pipes and widespread water shortages.

Confirming this grim picture, Met Éireann’s climate summaries tell us that the cold spell of January 2010 (beginning in the previous month) was the most extreme in Ireland since 1963, while the November-December cold spells set new records, with air temperatures below -15 C in many places.

A glance at global maps of temperature anomalies shows that the cold winter conditions at the beginning and end of 2010 were not just confined to Ireland, but were experienced over large areas of the Northern Hemisphere.

On the other hand, data just released by Nasa’s Goddard Institute for Space Studies tell us that, globally, 2010 was one of the two warmest years in the instrumental record, joining 2005 for this distinction.

Thus the global warming trend of recent decades, which the scientific consensus as represented by the Intergovernmental Panel on Climate Change attributes to man-made greenhouse gas emissions, is being sustained on a decadal-average basis.

Is this paradoxical concurrence of man-made global warming and cold northern winters one that involves some cause and effect relationship, is there some other external cause independent of man-made greenhouse gases in operation, or is what we’re seeing just a curious aspect of the natural variability of our climate?

These are questions of enormous interest to climate professionals as well as being of considerable public concern.

Among recent theories that have been put forward suggesting a causal link between man-made global warming and northern winter cooling are that reduced sea ice over the Barents Sea, or enhanced snow cover over Siberia, both seen as resulting from the warming, are giving rise to cold northerly and easterly airflows over Eurasia in winter.

It seems to the present author that these theories are rather far-fetched.

If there is an external cause of our cold winters, I believe most meteorologists would agree that it should be sought through the North Atlantic Oscillation (NAO).

The NAO is a climatic phenomenon influencing the entire north Atlantic and surrounding areas. Its simplest measure is the deviation from normal of the surface pressure difference between the Azores High and the Icelandic Low.

It is well known that the variability of the NAO, which occurs on both interannual and interdecadal timescales, exerts a major influence on the variability of our winters. When the NAO is positive (Azores-Iceland pressure difference larger than normal), our winters tend to be dominated by mild westerly winds; when it is negative they tend to be marked by spells of cold winds coming from the north and east.

The NAO has exhibited large variability during the entire instrumental record but the winter of 2009/10 showed the most extreme negative value ever recorded. The value for the current winter is not yet available, but it seems set to show an even more extreme negative excursion.

Recent research using global climate models at the UK Met Office shows that the variability of the NAO is significantly influenced by conditions in the stratosphere (the region of the atmosphere lying above about 10km). This confirms previous research using simple theoretical models (in which the present author was involved) suggesting the possibility of such a “top-down” influence.

The stratosphere is much more susceptible to direct external influences than the lower levels of the atmosphere.

The major signal of increasing greenhouse gases, for example, is found not in the surface warming but in the stratospheric cooling to which they give rise.

The stratosphere is also significantly influenced by variations in the ultraviolet (UV) component of the solar energy output. Such UV variations are now known from satellite observations to be as large as 6 per cent over the 11-year solar cycle. Thus there is a mechanism whereby greenhouse gas increases and solar variability, separately or in combination, may affect our winters through a “top-down” influence exerted on the NAO from the stratosphere.

Meteorologists and solar physicists are at present actively collaborating with a view to establishing how important the effect of this mechanism may be.

Recent statistical studies at the University of Reading using temperature and sunspot data going back to the Little Ice Age (about 1650–1700) have shown that cold winters in Europe tend to be associated with low solar activity.

During the sunspot minimum of 2009 solar activity fell to levels unknown since the start of the 20th century, and still remains at relatively low levels. The results of these statistical studies are suggestive of an external mechanism but far from conclusive.

The question of whether our recent cold winters have an external cause remains a complex one, awaiting the results of future research for clarification.

I would venture the answer that the anomalous conditions we’ve been seeing have been due mainly to the natural variations of the North Atlantic Oscillation, but with some “top-down” external influence involving both greenhouse gases and low solar activity as described above.

Our burst pipes and water shortages will inconvenience us from time to time, but the recent conditions that gave rise to them are unlikely to become a habitual feature of our winters.

Ray Bates is adjunct professor of meteorology at UCD. He was formerly professor of meteorology at the University of Copenhagen and a senior scientist at Nasa’s Goddard Space Flight Centre