ANALYSIS:A weekend of snow-filled fun for some, cold misery for others. Is it all related to global warming? asks RAY BATES
COMING AFTER the mild Irish winters of recent decades, the frequent frosts, widespread snow and unusual sea ice of our current winter have led many to wonder if something contrary to expectation is happening to our climate.
Figures recently released by Met Éireann confirm that the number of ground frosts in January was considerably greater than normal and February has so far shown no sign of milder conditions. In addition, 2008, though considerably warmer than the climate norm, was the coolest year of the present century so far in Ireland.
Data released by the Nasa Goddard Institute for Space Studies similarly show that 2008, though ranking as the ninth warmest year in the period of instrumental measurements, was the coolest year since 2000 globally.
These facts have been taken by climate sceptics as grounds to question the predictions of continued global warming due to greenhouse gas emissions, a frequently heard claim being that "there has been no global warming since 1998". William Reville has argued more than once in his weekly column in Science Today in The Irish Timesthat ignoring the climate sceptics is bad science and has called for a more open debate on the global warming question.
I will here present some considerations that are relevant to this issue.
As far as Irish conditions are concerned, the change from our recent mild winters to our current icy one is no particular reason for surprise and has little bearing on the question of man-made global warming.
Seasonal extremes at our Atlantic location are attributable much more to variations in regional circulation than to the signal of man-made warming, whose effects here are for the present small by comparison. The circulation changes are linked with natural modes of atmospheric and oceanic variability in the North Atlantic region with timescales ranging from inter-annual to multi-decadal.
Just as our anomalous Irish summer rainfall of 2008 falls well within the range of natural seasonal variability and is not to be seen as a sign that global warming has arrived, the extremes of our current winter also fall well within the range of natural variability and are not to be seen as a sign that global warming is ending.
The signal of climate change as measured by the global and annual mean temperature, on the other hand, has clearly emerged from the noise of natural variability over the past century. The Intergovernmental Panel on Climate Change (IPCC) has attributed the observed global warming of 0.7 degrees over this period with more than 90 per cent certainty to the increase in greenhouse gases due to human activities. While the underlying trend is clearly upwards, the global mean temperature in individual years, or even for some years in a row, can go counter to the trend.
The irregular ups and downs superimposed on the underlying trend are due mainly to the oscillation between El Niño (when a large area of anomalously warm surface water covers the tropical Pacific) and its opposite, La Niña. This oscillation is an aspect of natural variability and occurs irregularly with a period of three to seven years.
The warmest El Niño of the 20th century occurred in 1998 and caused a record peak in the global-mean temperature in that year. Since then, La Niña has predominated and no comparable El Niño has occurred. It was not until 2005 that the underlying upward trend of the global warming signal was sufficient to bring the global mean temperature above its 1998 peak, establishing a new record by a slight margin.
The dip in the global mean temperature in 2008 that makes it the coolest year since 2000 was mainly due to a significant La Niña event during the past year. In addition, the sun has just been passing through a minimum in its 11-year cycle, with slightly reduced energy output, causing an additional slight cooling influence.
Assuming past patterns are repeated, a significant El Niño event will occur during the next few years and solar activity will again increase. These influences will add to the warming effects of the increasing man-made greenhouse gases and there is little doubt that the current masking of the global warming trend by natural variability will disappear and global mean temperatures will reach new records.
In the meantime, more than adequate reason for maintaining the impetus to reduce greenhouse gas emissions is provided by the dramatic retreat of the Arctic sea ice, the accelerated melting of glaciers worldwide and the continued rise in global mean sea level.
- 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. He was a contributor to the 2001 IPCC Report as a nominee of Denmark