12.03.2012
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.
Original article
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.
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