Winter began in November with freezing temperatures and snow in the mountains. That is unusual for a year that will likely go down in history as one of the warmest on record.This content was published on January 24, 2011 - 09:33
Climatologist Heinz Wanner says the past few years are among the warmest since the first measurements were taken in 1860, and he tells swissinfo.ch that “manmade greenhouse gases are the cause”.
The emeritus professor at Bern University said the emissions have led to a rise of the global mean temperature.
That partly justifies how November temperatures in Europe north of the Alps could be four degrees celsius below the long-term average for the month.
Wanner, former president of the university’s Oeschger Centre for Climate Change Research, says the unusually cold November can be explained when one looks at the larger picture – how events are interconnected within the northern hemisphere.
The so-called North Atlantic Oscillation (NAO) – a climatic phenomenon of fluctuations in the difference of atmospheric pressure at sea level – decides whether winter in Europe will be cold or mild.
A low-pressure system over Iceland and a high-pressure system over the Azores adds up to a “positive phase” within the NAO. This produces strong westerly winds and creates ideal conditions for cool summers and mild, wet winters.
In contrast, a negative phase of the NAO can suppress the westerlies allowing winds from the north and east to put Europe north of the Alps into a deep freeze.
The NAO does not only seesaw back and forth over a short period of time, but also has what can be considered a long-term memory.
Wanner says the variability can be seen over an entire generation. “Having been born in 1945, I know that we had a phase of very cold winters between 1950 and 1975, with 1956 and 1963 being the coldest.”
A phase of warm winters followed between 1975 and 1998. “In the second half of the 90s I indicated that it was probable that there would again be several cold winters if the climate system did not change,” the expert said.
The NAO is not only a seasonal phenomenon. According to the authorities in the United States, this past December was the 15th month in a row in which the NAO remained in a negative phase.
The difference in pressure between Iceland and the Azores is influenced by a complex interaction of numerous air, land and sea factors.
Key are the surface temperature and currents of the North Atlantic, tropical Atlantic and Pacific including the El Nino phenomenon, according to the climate researcher.
Also playing a role is the distribution of sea ice as well as influences from the stratosphere and snow cover on the Eurasian continent.
Wanner says US meteorologist Elmar Reiter showed many years ago that large-scale snow cover and strong reflection of the sun would cool Eurasia over a prolonged period.
“That leads to the creation of a high pressure area over the continent which directs masses of cold air to western Europe,” the Swiss scientist explained. This tendency contributes to the continuation of the negative NAO.
Wanner says Switzerland could see its smaller lakes freeze once again thanks to the current longer phase of cold winters. However, he warns that the greenhouse effect will probably not allow for sustained freezing periods.
Solar activity will also have a say in whether winters are cold or mild. The sun is in a cycle in which scientists expect will be below average in intensity.
“On top of this interplay, we have the NAO. This is what makes the future so fascinating,” Wanner said.
Leading scientists who have contributed to findings by the Intergovernmental Panel on Climate Change (IPCC) agree with Wanner’s assessment that Europe could have several very cold winters ahead.
One of the predictions contained in the 2007 IPCC report was that we will not experience a positive phase of the NAO until the second half of the 21st century, when we will again see mild and wet winters.
Wanner has discussed the decadal variability and forecasts with one of Switzerland’s leading professors of tourism, Hans-Rudolf Müller at Bern University.
“In the short term, the tourist industry[ski resorts] has prepared itself for a variable climate and are putting their hopes on snow cannon based on the motto, ‘if it doesn’t snow, then we’ll snow ourselves’.”
From 2001 to 2008 Heinz Wanner was the acting director of the Swiss National Climate Research Programme NCCR. He was also president of the Oeschger Centre of Climate Change Research at Bern University.
Wanner is a honorary member of the Swiss Academy of Science. He got the honorary medal of the Maszaryk University Brno in 2005 and won the Vautrin Lud prize, the inofficial nobel prize in geography, in 2006. In 2009 he was awarded with the honorary doctor of the Humboldt University of Berlin.End of insertion
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