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Global warming: A sharp drop in snowfalls at low and intermediate altitudes
Since the end of the 1970s, the proportion of snow compared to precipitation in the form of rainfall has dropped sharply in Switzerland, especially at low altitudes. A team from the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) and the WSL Institute for Snow and Avalanche Research (SLF) has used a new method to analyze the relationship between snow and rising temperatures.
An abundance of snow at altitude, but hardly any in the lowlands: that, in a nutshell, sums up the current situation to the north of the Alps. Between December and February there was higher-than-average precipitation, but below 700 m higher temperatures meant that it mostly took the form of rain, not snow. Consequently, lowland regions have once again experienced a relatively snowless winter, whereas copious amounts of snow have fallen at altitude, where winter is still very clearly present.
The impact of global warming on snow is a highly complex area of research because temperature is not the only parameter to be taken into consideration, with the frequency and intensity of precipitation being just two of the other factors that play a key role here. So the reason why some winters are relatively snowless may be just as attributable to dry periods as to temperatures that are too high for snow. This makes the impact of global warming often hard to quantify.
Scientists from the two Swiss research institutes WSL and SLF have succeeded in analyzing the relationship between snow and global warming independently of the quantities of precipitation that have fallen.
Thanks to long-term observations gathered by the 76 weather stations run by MeteoSwiss and SLF, Gaëlle Serquet, Christoph Marty, Jean-Pierre Dulex and Martine Rebetez have demonstrated that, over the past 60 years, snow has increasingly given way to rain. This change has been accelerating since the 1970s in particular, in direct correlation with rising temperatures.
So far, this shift has primarily become apparent at low and intermediate altitudes, i.e. particularly below 1000 m and up to around 1500 m, because the temperatures there were frequently close to melting point (0°C). As a result, the 0.57°C temperature increase observed every decade for the past 30 years has frequently resulted in the snows of the 1970s turning into rain in the 2000s.
The researchers’ findings show that, in places where temperatures back in the 1970s exceeded -2.7°C in winter, snowfall has frequently been replaced by rainfall in the course of the last 30 years. Indeed, some observation stations at low altitudes have seen a reduction of more than 60% in their snow, whereas at high altitudes, where winter temperatures average -8°C or -10°C, the drop has remained insignificant.
In future, researchers will thus be able to predict the average reduction in snow anticipated over the coming decades, depending on the expected temperature increase.
Serquet, G., C. Marty, J.‐P. Dulex, and M. Rebetez (2011), Seasonal trends and temperature dependence of the snowfall/precipitation‐day ratio in Switzerland, Geophys. Res. Lett., 38, L07703, doi:10.1029/2011GL046976.
The full article will be provided by M. Rebetez upon request.