The climate in Switzerland has shown a marked change in mean annual temperature in the past 30 years, with a steep rise in the late 1980s. Winters are becoming shorter, vegetation develops earlier in the year, and glaciers are in rapid retreat.
A network of weather stations has been established in Switzerland ever since record keeping began in 1864. Thanks to this information, we have an excellent level of knowledge about the Swiss climate in the past decades. The mean annual temperatures for the reference period 1981–2010 are 8–10 °C on the Central Plateau. Somewhat higher temperatures are recorded in the areas around Geneva and Basel, known for their mild climate. Temperatures are warmest in the lowlands of Ticino.
Above-average temperatures since the 1990s
The period since the 1993–1996 atlas surveys has been characterised by exceptionally warm years compared to the 1961–1990 period, although the greatest rise in temperature occurred in the 1980s. The 1990s entered the history books as a series of exceptionally warm years. Since then, mean annual temperatures have risen yet again, albeit not as steeply as in the 1980s. 2011 and 2015 were the warmest years so far, with temperatures just over 2 °C above the long-term average of 1961–1990.
The increase in average temperatures applies to all seasons, but is more pronounced in spring and summer than in autumn and winter. The warmest summer by far since records began was in 2003, when temperatures rose 5 °C above the long-term average. The second warmest summer with a plus of 3.5 °C followed in 2015. Throughout the 1990s, winters were generally mild, while a few colder winters with below-average temperatures occurred after the turn of the millennium. The last severe cold spell took place in Switzerland in February 2012 and counts among the ten coldest two-week periods since 1864.
The number of extremely hot days, i.e. days when temperatures rise above 30 °C, has increased considerably since 1990 – from five to 12 per year in Lucerne, for example. At the same time, the number of frost days with below-zero temperatures appears to have declined somewhat. In Davos GR, the decline amounts to about 10 % since 1990 (from about 200 to 180 days per year). Many weather stations on the Central Plateau have recorded a fairly constant number of 80–100 frost days per year since 1990 – albeit with annual fluctuations; there were 10–15 frost days more between 1960 and 1990.
The temperature increase is apparent in waterbodies as well, as the average water temperature is correlated to the average air temperature. In rivers and streams, a temperature increase of 0.1–1.2 °C was recorded between 1970 and 2010. The ice cover on mountain lakes also thaws earlier in the year, as the example of Lake St. Moritz GR illustrates. The ice currently melts ten days earlier on average than in 1980.
A further consequence of the higher temperatures is the retreat of glaciers. Between 1973 and 2010, the area covered by glaciers in Switzerland shrank by 27.7 %, or 0.75 % per year.
The melting of the ice masses causes ground that is normally permanently frozen (permafrost) to gradually thaw, leading to unstable ground and an increase in debris flows and landslides.
Earlier onset of spring
The higher temperatures have an effect on the vegetation. Today, hazel bushes below 600 m generally bloom in mid-February, 13 days earlier than in 1951. This trend, together with the flowering and new leaf formation of eight other plant species, is revealed by the spring index. An abrupt rise in the late 1980s is particularly striking: since then, spring has begun much earlier than the 1981–2010 average. At the same time, weather stations below 800 m have observed a marked drop (minus two weeks) in the number of days with snow cover. A similar trend is visible with regard to snow cover between 1100 and 2500 m of altitude. Today, snow melts an average of 25 days earlier in spring than it did in 1970. In autumn, snow cover begins 12 days later on average; the change in autumn is thus only half as large as in spring. Since 1990, most weather stations have recorded no further decrease in the number of days with snow cover. The average number of days with snow cover per year between 1981 and 2010 was 79 in St. Gallen, 37 in Bern, and 17 in Geneva. A further difference can be observed at various altitude levels. In the mountains, the onset of spring has advanced more rapidly than in the valleys. In 1960, the difference in the timing of new leaf formation was 34 days per 1000 m of altitude; today, the difference has dropped to 22 days per 1000 m.
Constant precipitation levels in the long term
The Central Plateau experiences 110–150 days of precipitation per year on average. Precipitation levels range between 900 and 1200 mm. Higher levels of precipitation are reached in the Jura mountains and in the southern Alps. The Säntis area AR/AI/SG tops the list with an average of 2837 mm per year. The driest locations are in Valais and Engadine GR. With a yearly average of 545 mm, Stalden VS records the lowest amounts of precipitation. The southern Alps occupy a special position, with high levels of precipitation falling on relatively few days. Accordingly, strong rainfall is most frequent in Ticino. At the same time, the areas south of the Alps are among the sunniest in Switzerland. In Switzerland as a whole, the greatest amount of rain falls in summer.
In contrast to the rising temperatures, no trend is visible in terms of rainfall in Switzerland overall. South of the Alps, spring has become drier on average, while slightly more rain has fallen in spring in the north of the country in recent years. However, more relevant than average values in terms of breeding success are extreme amounts of precipitation as well as the number of days with rainfall. Heavy rain occurs mainly in summer and autumn in the form of thunderstorms. While the highest levels of daytime rainfall per year have increased since 1980, they have remained fairly constant since 1995. The largest changes occurred on the Central Plateau and in the Pre-Alps, while many inner-Alpine weather stations show no trend towards more heavy rain. However, there are indications that the timing of flood events is changing, with floods occurring at unusual times.
What lies ahead?
In Switzerland, average temperatures have risen by 1.8 °C since the mid-19th century. That means that climate warming is about twice the global average. Climate models project that temperatures in Switzerland will increase by another 0.5–3.6 °C up until 2060. Warm weather spells are expected to become more frequent in Switzerland in the next few years. The temperature increase will probably be more pronounced in summer than in the other seasons, and more rainfall is expected in winter and less in summer. Southern Switzerland will presumably become drier and warmer, and a greater increase in temperatures is expected at higher altitudes.
The vegetation period, which currently lasts about 250 days on the Central Plateau and 180 days in the Pre-Alps, is expected to increase by about 40 days, in the Alps and Pre-Alps by as much as two months. In the distribution of Alpine plants, an upward shift of 20–35 m between 1911–1970 and 2014–2015 has been observed. The tree line has moved upwards by 5 m per decade on average, mainly due to the abandonment of cultivated land, but also because of climate change. These trends are expected to continue. The composition of tree species will also gradually change. Of the three most common tree species in Switzerland, firs are expected to cope best with warmer and drier conditions, while the climatic changes will be more problematic for beech and especially spruce. Warmth-loving species such as oaks could stand to benefit. In the case of prolonged dry periods, we may see further areas of Scots pine forest dying off, as harmful organisms can spread more easily. This is a well-known phenomenon in the Valais. Following the dry and hot summer of 2003, the mortality rate of Scots pines near Visp VS was almost 30 %. Similar instances occurred in 2010 and 2016 after prolonged periods of dry weather. As summers become drier, the risk of forest fires will increase throughout the country. According to model calculations, glaciers will shrink by another 50 % between 2000 and 2050, and small glaciers will disappear completely. As permafrost continues to thaw, landslides, slope instability and rockfall will likely occur more often.
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