Sweden

Storms Sweden

There is a lot of cross-border information on storms in Northern, Western and Central Europe. This information is summarized on the page for Europe in the window 'Storms: European scale'. Additional information that specifically refers to individual countries is presented on the Storm pages of these countries.

Trends and variability: no increase in storm frequency

From a study on the history of storminess in Northern Europe it was concluded that there are no robust signs of any long-term trend in the storminess indices. The results from this study were derived from local pressure observations in Lund since 1780 and Stockholm since 1820 (Sweden). It was concluded that the 1980’s–mid 1990’s were a period of enhanced storminess, mainly seen in the Stockholm record, but this period is within the natural variability of the records (1).

Storminess is during the entire historical 200 year period remarkably stable, with no systematic change and little transient variability. There is no indication of a long-term robust change towards a more vigorous storm climate (1).

The results for Lund and Stockholm do, however, show a general increase in the storminess indices for Stockholm and only weak indications of this increase in the Lund indices, which instead show a return to low levels again in the 1990’s. This points at a south – north gradient in the recent storminess variation, consistent with the conclusion of earlier research (2): the wave conditions of the northern North Sea has become more rough compared to the southern North Sea (1).

Previous research has shown that the storm and wave climate in most of the Northeast Atlantic and in the North Sea has undergone significant variations on time scales of tens of years. Part of the variability was found to be related to the North Atlantic Oscillation. Thus, though it has indeed roughened in the past decades, the present intensity of the storm- and wave climate seems to compare with the intensity at the beginning of this century. Therefore, changes in storminess could not be found for the area Northeast Atlantic, North Sea and Baltic Sea (3).

Winter storminess using mean sea level pressure data over the Northeast Atlantic showed decadal scale variations and large geographical variability in storms (4). It was concluded that a modest increase in storminess over the past two to three decades occurred in the region of Scandinavia (4), but this was less detectable in and near the United Kingdom, supporting earlier results (5). Overall, a modest increase in winter storminess over the Northeast Atlantic was found from 1875 to 1995 (4).

In the Azores high frequencies of storms occurred from about 1875 to 1905, very low frequencies in the 1920s to the early 1940s, then returning to a regime of higher frequencies, peaking around 1980 (6).

From analyzed wind data a substantial increase in average annual wind over the North Sea has been reported: about a 10% increase from 1958 to 1997 (7). From examined wind data in western France, increasing occurrences of strong winds in western Brittany, but decreasing trends in Normandy and Pays de Loire have been found (8).

Whereas tropical cyclones develop over warm tropical oceans, extra tropical cyclones, also called mid-latitude cyclones, evolve along the polar front, which is defined as a semi continuous boundary in the mid-latitudes that separates cold polar air masses from warm subtropical air masses (9).

There is strong suggestion that extra tropical systems in the North Atlantic Basin have declined overall over the past 50–100 years, but that there is an increase in frequency of really powerful storms (10). Some geographical variability is apparent in these trends, whereas higher latitudes in the northern hemisphere are experiencing upward trends in cyclone frequencies, including the regions off of the eastern Canadian coast, Iceland, and Scandinavia, while decreasing trends are more evident at middle latitudes.

Both ENSO (El Niño Southern Oscillation) and the NAO have impacts on frequencies and tracking of these systems. These empirical results are in general agreement with GCM forecasts under global warming scenarios, as extra tropical storms are expected to displace northward along with the jet stream and regions of baroclinicity. Analyses of wave climatology in the North Atlantic Basin show that the last two to three decades have been rougher at high latitudes than several decades prior, but this more recent sea state is similar to conditions about 100 years ago. The recent roughness at sea seems to be related to high NAO index values, which are also expected to increase with global warming.

The last two to three decades have been rougher at high latitudes than several decades prior, but this more recent sea state is similar to conditions from about 100 years ago. The recent roughness at sea seems to be related to high NAO index values, which are also expected to increase with global warming. Thus, when coupled to an anticipated continued rise in global sea level, this trend will likely result in increasing loss of sediment from the beach-nearshore system resulting in widespread coastal erosion.

An increasing trend in significant wave heights (SWH) at several Northeast Atlantic locations since 1960 has been reported (11). This trend was believed to be related to the systematic deepening of the Icelandic low and intensification of the Azores high over the last three decades, leading to high positive NAO values (11). The northeast North Atlantic Ocean has experienced significant multidecadal variations in wave height activity over the last century, and it has indeed roughened in winters of the last four decades (12). It was concluded that the northeastern Atlantic has indeed roughened in recent decades, but that recent wave climates are comparable to the climatology around the turn of the 20th century, and that at least part of this variability is associated with the NAO (3).

Projected changes: reduction in storm frequency and increase in intensity

The climate scenarios do not point unambiguously to an increase in the frequency of strong winds (13).

The North Atlantic Oscillation (NAO) is the main source of interannual climate variability in the North Atlantic region. The NAO is essentially a measure of the atmospheric pressure difference between the Icelandic Low and the Azores High. A large pressure gradient between a well-developed Icelandic Low and a strong Azores High (termed a positive NAO) results in a strong westerly air flow on a more northerly track over the eastern North Atlantic and Europe; this brings warm, wet winters to all of Europe except the southern part. When both pressure systems are weak, this is termed a negative NAO, and the westerly air flows are also weak; this results in colder, drier winters in Northern Europe.

The NAO exhibits considerable seasonal and interannual variability, with prolonged periods of domination of positive or negative phases, influencing different components of the ocean-atmosphere-sea-ice system, including, for example, the amount of ice on the Baltic Sea. It is not known whether there is a link between the NAO and the increasing concentrations of greenhouse gases in the atmosphere (14).

Future changes in wind conditions are highly uncertain as global models show great differences in change in large-scale circulation over the North Atlantic/Europe. A feature common to most scenarios is a decrease in wind speed in the Mediterranean and some increase in the North Sea area and increased wind speeds over those parts of the Baltic Sea that remain ice-free in a future warmer climate (Bay of Finland, Bothnian Sea and Bay of Bothnia) (15).

Projected changes in wind differ widely between various climate models. The spatial resolution of both GCMs and RCMs is far too coarse to accurately represent the fine scales of extreme wind. As the downscaled projections differ widely, there is no robust signal seen in the RCM results. Looking at projected changes in large-scale atmospheric circulation from numerous GCMs, they indicate that an increase in windiness for the Baltic Sea basin would be somewhat more likely than a decrease. However, the magnitude of such a change is still highly uncertain and it may take a long time before greenhouse gas-induced changes in windiness emerge, if ever, from background natural variability (14).

GCM simulations of increasing CO₂ indicated heightened storminess in the Northeast Atlantic, as well as in northwestern Europe because of a potential intensification and eastward extension of the North Atlantic storm track (16). It was also demonstrated that a high North Atlantic Oscillation (NAO) Index results in an intensification of cyclonic circulation in both the Baltic and North Seas and that under a global warming scenario, the NAO index values are expected to rise (17).

Furthermore, it was reported that in a doubled CO₂ GCM simulation, there is a reduction in the total number of winter cyclones in both hemispheres, but the frequency of intense cyclones increases, especially in the Northern Hemisphere, which is clearly in general agreement with the empirical record (18). The Ireland–Scotland region may experience fewer, but more powerful storms in a future dominated by global warming (19).

A doubling of CO₂ on earth would not likely change the geographical patterns and seasonality of storms, but the overall number of events may significantly reduce. Besides, more powerful storms may result (20).

It was concluded that cyclones in the middle latitudes (30–60jN) in the northern hemisphere have significantly declined in frequency, but have increased significantly at higher latitudes (60–90jN). Time series of winter cyclone frequency and intensity were correlated with the time series of mean Northern Hemisphere winter temperature. Results indicate statistically significant correlations for both the high and mid latitudes: negative correlation between mid-latitude cyclone frequency and winter temperature, and positive correlation between high-latitude cyclone frequency and winter temperature (21). One interpretation of these results is that increasing levels of atmospheric CO₂ have forced increases in Northern Hemisphere temperature, which in turn force changes in circulation and cyclone activity. This interpretation is consistent with model sensitivity studies. Thus, a northward shift of storm tracks (extra tropical cyclonic activity) in the Northern Hemisphere would take place due to global warming. The researchers, however, stressed that cyclone activity and temperature could simply covary on interannual timescales, the observed trends not being related to greenhouse forcing in any way (5).

Vulnerabilities: more damage

The climate scenarios for Sweden show that weather-related events such as floods, storms and landslides will increase over the next hundred years (13). Extreme weather events are already causing extreme weather events today, examples being the floods around Lake Vänern in the winter of 2000/01 and the winter storm Gudrun in 2005. The increased forest growth, combined with wetter ground and fewer frosts, leads to increased storm-felling of trees, affecting systems with overhead power lines, regardless of the intensity and frequency of the storms.

Very severe storms with extensive uprooting of trees are rare, and it is difficult to identify trends for these. The storm Gudrun, however, caused by far the greatest number of uprooted trees in southern Sweden in the past hundred years. Just two years later, southern Sweden was struck by another powerful storm (15).

An increase in damage of these kinds in the future can be anticipated. In many cases the harmful effects can be reduced by adapting society to the expected new circumstances, while in others it may be a better strategy to wait and deal with the damage when it occurs (13).

Hurricane Gudrun in 2005 caused twice as much damage as all the other weather events in the extreme weather period 1997/2007. Yet this does not include a large proportion of the losses incurred by the forest owners, which totaled SEK 16 billion. Sweden’s costs arising from Hurricane Gudrun were calculated in total to SEK 20.8 billion (21). The European Commission estimated the total damage in Southern Sweden to be 2.3 billion EUR, making it the county’s worst natural disaster in modern time (22). This adds up to nearly 0.8% of Sweden’s GDP.

The largest individual costs from Gudrun is damage to forests, although several other sectors also incurred considerable costs. In Sweden, Gudrun was probably the most serious storm in 35 years and probably the costliest in history, exceeding the costs of 1999’s Anatol by at least two times. Gudrun was among the 10 largest storms ever experienced in this region (23).

In particular in Sweden the storm caused huge forest losses (24): the storm fell about 75 million m³ of trees, totaling the normal annual harvest in the whole country. In Sweden, during the past century the area of planted forest has arisen considerably. Old fields and meadows have been forested and ditches dug in the forest floors themselves. This has increased the area of forests in Sweden, but also brought to forestry soils that are less steady for the trees to grow in. Because of the heavy machinery used for harvesting, the amount of dozy timber vulnerable to storm winds is on the rise (25). It can thus be said that the actions of humans have greatly decreased the natural adaptive capacity of the forests towards extreme weather events. Natural processes remain the greatest reason for forest damages however. As climate change progresses, the rate of harmful storm events is seen to be on the rise.

The estimates of potential costs for disasters in the British Stern Review (26) are based in part on the insurance sector’s costs as a consequence of extreme weather events, which have increased by 2% annually since the 1970s. The report also maintains that if this trend continues, the annual costs caused by extreme weather events could increase to 0.5–1% of global GNP by 2050.

Adaptation strategies Sweden

Insurance

In France insurance coverance (in % of forest area) is 70% (27).

References

The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Sweden.

1. Bärring and Von Storch (2004)
2. Vikebø et al. (2003), in: Bärring and Von Storch (2004)
3. WASA Group(1998), in: Keim et al. (2004)
4. Schmith et al. (1998), in: Keim et al. (2004)
5. McCabe et al. (2001)
6. Andrade et al. (2004), in: Keim et al. (2004)
7. Siegismund and Schrum (2001), in: Keim et al. (2004)
8. Pirazzoli et al. (2004), in: Keim et al. (2004)
9. Ahrens (2000), in: Keim et al. (2004)
10. Keim et al. (2004)
11. Kushnir et al. (1997), in: Keim et al. (2004)
12. Wang and Swail (2002), in: Keim et al. (2004)
13. Swedish Commission on Climate and Vulnerability (2007)
14. HELCOM (2007)
15. Ministry of the Environment of Sweden (2009)
16. Carnell et al. (1996), in: Keim et al. (2004)
17. Schrum (2001), in: Keim et al. (2004)
18. Lambert (1995), in: Keim et al. (2004)
19. Lozano et al. (2004), in: Keim et al. (2004)
20. Bengtsson et al. (1996); Emanual (1987), both in: Keim et al. (2004)
21. Ministry of Enterprise, Energy and Communications (2005), in: Swedish Commission on Climate and Vulnerability(2007)
22. European Commission (2006a), in: Haanpää et al. (2007)
23. Suursaar et al. (2006b)
24. Haanpää et al. (2007)
25. SMHI (2005), in: Haanpää et al. (2007)
26. Stern (2007)
27. Gardiner et al. (2010)