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Climate Change and Natural Disasters in Switzerland Past and Present Climate has changed, worldwide … Since the late 19th century, the global mean surface temperature has risen by 0.8°C. There is a very high probability that the warming of the past 50 years is a consequence of emissions of greenhouse gases and aerosols caused by human activity. These emissions are very likely also the cause of the melting of polar sea ice and of the widespread retreat of glaciers1. … and in Switzerland • Since the beginning of systematic recordings in 1864, the annual mean temperature has increased in Northern Switzerland by between 1.2°C and 1.5°C and in Southern Switzerland by approx. 0.9°C 2; • The past 15 years were among the warmest in the past 500 years. The four warmest years ever were all recorded after 1990 3; • During the 20th century, winter precipitation in the region North of the Alps and in Western Switzerland has increased by 10 – 30% 4; • Below the altitude of 1300 m, the periods with snow covering have become significantly shorter 5. Debris flow in Brienz (BE) following the heavy precipitation event in August 2005. © Schweizer Luftwaffe At present it is not possible to prove that these regionally observed changes are also man-made, since the natural climate variability (mainly the influence of the North Atlantic Oscillation) plays an important role6. In the light of what is known today, however, such a connection is plausible. In the long series of measurements available for Switzerland today, there is no evidence for a systematic change in the frequency of rare weather extremes. Since a statistical detection can only be expected in the case of massive changes7, however, it cannot be excluded that climate change has already influenced the occurrence of rare weather extremes. Annual Temperature in Switzerland 1864-2006 Deviation from the mean 1961-1990 2.5 Deviation, degree C Extreme weather events have probably changed Independently of climate change, there have always been periods of frequent and less frequent extreme weather events. Especially where rare extremes are concerned – for instance those that cause substantial damage – these random variations are very pronounced. Eventual changes due to climate change remain difficult to identify. 2.0 years below the norm 1961-1990 smoothed evolution in Switzerland 1.5 years above the norm 1961-1990 smoothed evolution over the Northern Hemisphere 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 1860 1880 1900 1920 1940 1960 1980 2000 year Intense weather events have changed in the past decades In contrast to rare weather extremes, systematic changes in intense weather events can already be observed in the long series of measurements available for Switzerland. These are events that do not, as a rule, cause significant damage. • The number of exceptionally cold days has decreased in the course of the 20th century 8; • The duration and intensity of heat waves has increased 9; • In autumn and winter (but not in summer), intense precipitation has become more frequent 10; • In rivers north of the Alps, whose catchment areas are still in a natural state, peak volume has increased in winter 11. Whether this is a signal of the influence of human activity on the climate remains unclear. However, the changes correspond qualitatively to the expectations formulated by studies on climate change. In Northern Switzerland the frequency of intense precipitation events in winter has clearly increased in the course of the 20th century 9. 2 What does the future have in store? The climate will continue changing … n the coming decades, the increase of greenhouse gases will continue to influence the global climate. Worldwide, this will have an impact on the atmosphere, on the water cycle, including snow and ice, on the oceans and on the biosphere. In some areas, the changes are well understood and a quantitative estimate is possible. In other areas there are major uncertainties. At present, the following developments are expected for Switzerland, for the mid 21st century 12,13: …and with it the future weather extremes In the future, there will be shifts both in the frequency and in the occurrence of extreme weather events. The extent and character of these modifications will vary depending on the place and nature of the events. There are major uncertainties when one tries to quantify these changes. Based on present knowledge, however, Switzerland can expect 14,15,16: • Fewer cold spells and frost days in Switzerland. • More frequent heat droughts in the summer. waves and These developments have an impact on the natural disasters caused by the weather Whether Switzerland will be affected more frequently by natural disasters in the future will depend on the interplay between the different climatic factors and on local sensitivities. In order to make quantitative estimates, detailed model analyses will be generally necessary. On the whole, it can be expected that the following natural events will increasingly threaten the Alpine region 17,18: • An increase in precipitation and the shift from snow to rain in the low lands and the pre-Alps will probably lead to more flooding along medium-sized to large rivers. • a warming by 1.0°C to 3.5°C; • an increase of winter precipitation by up to 20 %; • a decrease of summer precipitation by 5 to 30 %. • In the winter half-year, more frequent and more intense heavy precipitation. • More intense precipitation is likely to produce more mudslides and landslides. Where other types of extreme weather are concerned, such as storms or hailstorms, the influence of climate change is not yet sufficiently understood. • The melting of permafrost will reduce the stability of mountain slopes. • Hotter summers will lead to more heatinduced stress for humans, animals and plants and accordingly have an impact on their health. • Summer drought will influence forestry and agriculture, navigation and water resources. In part, the expected changes significantly exceed the natural climate variations we have known so far. It can therefore be assumed that society will perceive these changes very clearly. Change of heavy precipitation in winter, 2071-2100 with respect to 1961-1990. Scenario of a selected regional climate model. 3 Implications for society The risk of natural disasters does not only depend on the climate The climate is not the only factor which determines to what extent our society is at the risk due to natural disasters. Changes in society play an important part as well. As a consequence of increased human settlement pressure, construction in areas that are more exposed to potential danger has increased. In addition, improvements in the standard of living imply that property of ever increasing value is exposed to danger. The potential for damage has grown, and society has become more sensitive to natural disasters. Depending on future social developments, the effects of climate change could either intensify or be mitigated17. Climate change demands flexible solutions The time scale of the expected climate change is comparable to the planning horizon for new buildings, infrastructure, and land use decisions. Where planning is concerned, it will become ever more important to take into account the changing conditions of weather, climate and natural disasters. This is especially true for constructions that provide protection from natural disasters. Since there are still many uncertainties concerning the future development of the climate, it is at present difficult to define measures, which are concrete and universally valid. At the moment, the focus should therefore be on flexible solutions that are geared towards present needs and that can be adapted to more severe future requirements . There are solutions already available that provide better protection for little extra expenditure, and are useful to society regardless of climate change. References 1 IPCC, 2007:Climate change 2007. Working Group 1, Fourth Assessment Report. WMO, UNEP, (in press). 2 Begert, M., et al., 2005: Homogeneous temperature and precipitation series of Switzerland from 1864 to 2000. Int. J. Climatol., 25, 65-80. 3 Casty, C., et al., 2005: Temperature and precipitation variability in the European Alps since 1500. Int. J. Climatol., 25, 1855-1880. 4 Schmidli, J., et al., 2002: Mesoscale precipitation in the Alps during the 20th century. Int. J. Climatol., 22, 1049-1074. 5 Scherrer, S., et al., 2004: Trends in Swiss Alpine snow days: The role of local- and large-scale climate variability. Geophys. Res. Lett., 31, L13215, doi:10.1029/2004GL020255. 6 Wanner, H., et al., 2001: North Atlantic Oscillation – concepts and studies. Surv.Geophys. 22, 321-382. 7 Frei, C., et al., 2001: Detection probability of trends in rare events: Theory and application to heavy precipitation in the Alpine region. J. Climate, 14, 1564-1584. 8 Jungo, P., et al. , 2001: Changes in the anomalies of extreme temperature anomalies in the 20th century at Swiss climatological stations located at different lati-tutdes and altitudes. Theor. Appl. Climatol., 69, 1-12. 9 Della-Marta, P.M., et al., 2006: The length of western European summer heatwaves has doubled since 1880. J. Geophys. Res. (submitted). 10 Schmidli, J., et al. , 2005: Trends of heavy precipitation and wet and dry spells in Switzerland during the 20th century. Int.J. Climatol., 25, 753-771. 11 Birsan, M.-V., et al., 2005: Streamflow trends in Switzerland. J. Hydrol., 314, 312-329. 12 Frei C., 2005: Die Klimazukunft der Schweiz – Eine probabilistische Projektion. Bericht erhältlich unter www.occc.ch und www.meteoschweiz.ch . 13 OcCC, 2006: Die Schweiz im Jahr 2050. Beratendes Organ für Fragen der Klimaänderung. (in Vorbereitung). 14 Schär, C., et al., 2004: The role of increasing temperature variability in European summer heatwaves. Nature, 427, 332-336. 15 Frei, C., et al., 2006: Future change of precipitation extremes in Europe: An intercomparison of scenarios from regional climate models. J. Geophys. Res., 111, D06105, doi:10.1029/2005JD005965. 16 Beniston, M., et al., 2006: Future extreme events in European Climate: An exploration of regional climate model projections. Clim. Change, (in press). 17 OcCC, 2003: Extremereignisse und Klimaänderung. Beratendes Organ für Fragen der Klimaänderung. 88pp. Erhältlich unter www.occc.ch. 18 Defila C., 2004: Der Sommer und Herbst aus phänologischer Sicht. Schweiz. Z. Forstwesen, 155 (5), 142-145. 19 PLANAT 2004: Strategie Naturgefahren Schweiz. Synthesebericht. 81 pp. Erhältlich unter www.planat.ch PD Dr. Christoph Frei MeteoSwiss Krähbühlstrasse. 58 8044 Zürich T 044 256 91 11 www.meteoswiss.ch Climate Information MeteoSchweiz [email protected] Mai 31st 2006 around 06.45 a rockfall occurred at the autobahn A2 in Gurtnellen. Picture: Walter Arnold, Tiefbauamt des Kanton Uri Florian Widmer National Platform for Natural Hazards PLANAT Federal Office for the Environment FOEN CH-3003 Bern T 031 324 17 81 [email protected] www.planat.ch 01/2007 © MeteoSchweiz, PLANAT 4