Examensarbete vid institutionen för geovetenskaper ISSN 1650-6553 Nr 90 Can effects from global warming be seen in Swedish snow statistics? Mattias Larsson - 1 - Sammanfattning Denna studie är ett resultat av en omfattande undersökning av snöförhållandena i Sverige sedan början av 1900-talet. Jag har för detta ändamål analyserat data av maximala snödjup och antalet dagar med snötäcke per kalenderår från ett 40-tal utvalda stationer. Dessa stationer har sedan delats upp på olika regioner varefter medelvärden har räknats fram i resp. fall. Datamaterialet illustreras här i form av olika stapeldiagram uppdelat på fyra följande kategorier; hela tidsserien, perioden 1961-2003, konsekutiva 10-årsmedelvärden samt en tidsserie med de mest högfrekventa svängningarna bortdämpade. För att kunna bedöma eventuella trender i de uppritade tidsserierna så har jag använt mig av de båda statistiska metoderna enkel linjär regression resp. Mann-Kendall's test. Tillhörande beräkningar redovisas på tabellform. För att svara på frågan om den globala uppvärmningen kan sättas i samband med de senaste 30- 40 årens övervägande snöfattiga vintrar i södra Sverige så har jag studerat korrelationen av snödata gentemot det norra halvklotets vintermedeltemperatur. Motsvarande beräkningar av korrelationskoefficienter har också genomförts för den svenska vintermedeltemperaturen Utslaget på testerna visar att det inte har skett så dramatiska förändringar i snöförhållandena på lång sikt. Magnituden på lutningskoefficienten för de anpassade regressionslinjerna tyder på att det maximala snödjupet och antalet dagar med snötäcke i medeltal har legat på en ganska konstant nivå under de senaste hundra åren. När det gäller maximala snödjup så kan man paradoxalt nog se en tendens till en svag uppgång för Götaland och norra Norrland. Det är också de enda fallen som är statistiskt säkerställda för tidsserien som helhet. För den kortare perioden 1961-2003 så kan man däremot se att antalet dagar med snötäcke har minskat relativt kraftigt i södra Sverige motsvarande en nedgång på cirka 40 % i Götaland och 20 % i Svealand. Test med enkel linjär regression ger signifikanta resultat i båda fallen medan Mann-Kendall endast fastställer trenden för Götaland. En närmare undersökning av det maximala snödjupet för den kortare tidsserien ger dock inte lika tydligt utslag i statistiken men man kan trots allt urskilja en signifikant minskning för Svealand i testet med enkel linjär regression. Det rör sig här om en nedgång på cirka 30% efter 1960. Det går inte att omedelbart relatera förändringarna i det svenska snöklimatet till den globala uppvärmningen. Beräknade värden på korrelationskoefficienten ger inte ens signifikant utslag för perioden 1961-2003 trots att den globala medeltemperaturen har ökat ganska markant sedan 1970. Motsvarande beräkningar för den svenska vintermedeltemperaturen visar att den har väldigt stor betydelse för om nederbörden i Götaland och Svealand faller som regn eller snö medan det för norra Norrland inte har någon nämnvärd påverkan. - 2 - Abstract This study is a result from a major investigation about the snow conditions in Sweden since the beginning of the twentieth century. For this purpose, data were analysed with respect to the maximum snow depth and the number of days with snow cover per year from some more than forty selected stations. These stations were then divided into different regions and means were calculated for each series. The data are presented in the shape of different histograms in the four following categories; the whole period in request (1900-2003), the latest 43 years (1961-2003), and consecutive mean values for every decade and time series with the highest frequented fluctuations equalized. To be able to detect any trends in the plotted time series two statistical methods, simple linear regression and Mann-Kendall’s test, were applied. The calculations belonging to these tests are showed in tables. To be able to answer the question if the global warming can be related to the latest 3-4 decades predominantly warm winters in the southern part of Sweden I have been studying correlations in snow data with respect to the northern hemispheres mean temperature for the winter season. Corresponding estimates of the correlation coefficients have also been made with respect to the Swedish winter mean temperature. The response of the tests shows that it has not been such dramatic change in the snow conditions in the long run. The magnitude of the slope for the adjusted regression lines implies that the maximum snow depth and the number of days with snow cover in average have been on a fairly constant level during the latest hundred years. When it comes to the maximum snow depth one can distinguish a tendency for a small rise in Götaland and northern Norrland. This is also the only cases which are statistical significant for the period in request (1905-2003). For the shorter period 1961-2003 however, the number of days with snow cover has decreased quite substantially in the southern part of Sweden corresponding to a decrease about 40% in Götaland and 20% in Svealand. The test based on simple linear regression gives significant results in both cases while Mann-Kendall only establishes the trend for Götaland. A closer view of the maximum snow depth for the shorter period (1961-2003) does not give the same response but there is at least evidence for a significant decrease in Svealand in the test with simple linear regression. It corresponds to a decrease of about 30% since 1960. One cannot immediately relate the changes in the Swedish snow climate to the global warming. Estimated values of the correlation coefficient do not even give significant results for the period 1961-2003 despite of the fact that the global mean temperature has raised quite considerably since 1970. The corresponding calculations for the Swedish winter mean temperature show that it plays a very important roll if the precipitation in Götaland and Svealand is coming as rain or snow while it does not matter at all in northern Norrland. - 3 - A winter vignette from Gyllbergen, southwestern Dalarna. The illustration on the front page, taken by Lars Andersson in Borlänge, shows the snow conditions in Gyllbergen about 20 km southwest of the town 27 February 1988. The place is known as a forest area with lots of snow in the winter and there is a well-prepared track system for cross-country skiing. In the area there is now also the automatic weather station of SMHI (= Swedish Meteorological and Hydrological Institute) Stora Spånsberget. Earlier there was a manual station measuring of precipitation and temperature that was established in 1964. Quotation of the diary notes from Lars Andersson from 27 February: "Check of snow depth in Gyllbergen: 90-110 cm on the highest areas situated 450-500 meter above the sea. All trees/bushes completely encapsulated in dry snow + some ice coverings. Because of strong winds there are snowdrifts here and there on exposed places. As the sun was shining constantly that day one could go skiing around and look at all beautiful snow formations and take photographs in colour". A supplement noted on the 29:th: "A pole was broken about one kilometre from the TV-mast on Stora Spånsberget at 02:30 a.m. at which time the radio sound disappeared in Dalarna. Reports in the local newspapers that the Telecommunications Administration had to turn out and that they first thought that the 300 metres high mast had collapsed, but it was not that bad. According to the newspaper the snow depth there was 1-1,5 meter." The winter of 1987-88 was in southern Sweden relatively warm with plenty of precipitation and lots of snow lying in the higher terrain, where especially northern Svealand recorded large snow depths. Borlänge received entirely 79,5 millimetres (mm) during January followed by 69,1 mm in February and 33,7 mm in March. The corresponding figures for the weather station in Idkerberget were 90,5 - 106,2 - 53,5 mm. (Idkerberget is situated 5 km east of the TV-mast on Stora Spånsberget.). The day when the picture was taken in Gyllbergen Borlänge had only 32 cm snow that is considerably less than at the location on the photo. - 4 - Table of contents Sammanfattning .....................................................................................................2 Abstract...................................................................................................................3 Table of contents ....................................................................................................5 1. Introduction........................................................................................................6 2. A short description of the Swedish snow climate............................................7 3. Collecting of data..............................................................................................13 4. Theory. ..............................................................................................................17 4.1 Simple linear regression. ..............................................................................17 4.2 Definition of the correlation coefficient. ......................................................19 4.3 Mann-Kendall’s test......................................................................................20 4.4 Calculation of filtered mean values..............................................................21 5. Results. ..............................................................................................................22