Martin Beniston of the University of Fribourg analysed data from 11 climate recording stations in Switzerland and found that the incidence of warm winter spells has increased since 1970. In 2003, for instance, the average temperature recorded at Saentis, a town in the northwest of Switzerland, was 15°C above the seasonal average during the winter, compared with 11.5°C for the summer. A similar pattern was observed in data from other stations. Moreover, climate simulations for the period 2071--2100 suggest warm winter spells are going to become more common.

Beniston says that these anomalies could be caused by the North Atlantic Oscillation or Atlantic El Niño - a cyclic pattern of atmospheric pressure variability that affects the weather of North America and Europe. According to pressure records from Bern and Zurich, persistent high pressure over Switzerland has more than doubled since the 1960s, leading to higher mean temperatures, lower than average precipitation and reduced snow cover in the Alps.

"Warm winter spells can lead to early snow melt, winter-time floods, problems for the ski industry, an early start to the vegetation cycle of alpine plants and so on," says Beniston. "Understanding the causes and consequences of these events could lead to environmental and economic strategies for the future as the climate continues to warm."