Vulnerabilities – Trends of storm frequency and intensity in the past

In the Netherlands, the total damages due to the storm Daria in 1990 amounted to over 1 billion Euros (7).

Trends of moderate wind events (that occur on average 10 times per year) and strong wind events (that occur on average twice a year) have been analyzed from a high quality dataset of near-surface wind observations at Dutch meteorological stations for the period 1962–2002. From this analysis a decrease in storminess over the Netherlands between 5 and 10%/decade over the period 1962–2002 was concluded (1). … The trends identified only represent the linear change over the past 40-year period. The authors did not investigate whether these trends are part of longer term changes or whether decadal fluctuations are present as well.

This result is inconsistent with the results of other studies that are based on a different method (reanalysis data) and that conclude that storminess has increased during the same 41 year period. … The authors of the study on Dutch station records of near-surface wind in the past four decades concluded that their results, pointing at a  decrease in storminess, is closer to reality than the increase suggested by the reanalysis data (1).

Other studies on systematic changes in storminess on the basis of station observations report: a significant negative trend in the number of winter storms in Switzerland north of the Alps between 1864 and 1994 (2); a decrease in wind activity for the central Mediterranean and Adriatic region between 1951 and 1970 and an increase from 1970 onwards (3); no sustained trends over the UK over the 20th century up to 1990 (4) and in Dublin (Ireland) up to 2000 (5).

On the basis of several studies, it was concluded that the wind climate along the European coast has not become more severe in the past 100 years or so (1881–1995). Increasing strong geostrophic wind speeds were found from around 1965 onward, but these were not labeled alarming when compared with conditions earlier in the 20th century and at the end of the 19th century (6).

Meso-scale surface roughness over the Netherlands has increased between 1962 and 2002 due to the increase in the percentage of built-up area. However, the effect is too small to explain the observed decrease in storminess of between 5 and 10%/decade, as it can be shown that this decrease would imply an increase in surface roughness that at present would correspond with a covering of the whole territory of the Netherlands with skyscrapers, which is clearly not the case (1).

The winter atmospheric circulation itself is governed by the North Atlantic oscillation (NAO), but correlations between the NAO index and the annual number of wind events over the Netherlands are <0.4 and mostly not significant (α= 0.05). Apparently, even in situations of strong westerlies (high NAO index), the exact position of the jet stream and accompanying storm systems determines the number of wind events over the Netherlands in a more subtle way (1).

Vulnerabilities – future storm frequency and intensity

In 1999 it was concluded that the number of severe storms in the Netherlands could increase by up to 20-30% by the end of the century - if CO2 emissions continue to rise unabated. This translates into up to nearly 10 more storms – from 30 to 40 – over the period 2071-2100. Top wind speeds would likely rise by 2-16%. A mere 2% increase in storm wind speeds could already increase average annual damage by 80%, and a 6% increase even up to 500% (7). Recent studies confirm these projected trends (9,10,11,12).

As a consequence of higher storm intensity, the return period of damaging storms is projected to reduce significantly (9). For example in the British Isles/North Sea/Western Europe region, high intensity storms with an average return period of 20 years under the 20th century climate would become a 10 year event by 2040 and 2030 (A1B and A2 scenarios), respectively. The return period for such strong storms would further decrease to 5.3 and 5.8 years by 2100 under the two climate scenarios (9).

From statistical analyses (ranking and extreme value statistics) for a large part of Europe a general and consistent tendency towards an increased frequency of windstorm-related losses over most of Western, Central and Eastern Europe was concluded for IPCC B1 and A2 scenarios, and slightly inconsistent findings for A1B scenario. From these analyses it was concluded that losses may reach unseen magnitudes at the end of the 21st century, which for some countries (e.g. Germany) may exceed 200% of the strongest event in present day climate simulations. In these analyses, it was assumed that storm damages occur only at 2% of all days; the minimum wind speed that is expected to produce any loss, therefore, is defined as the regional 98th percentile of the daily maximum wind speed (10).

These statistical analyses show 3 different tendencies for the period 2060-2100 compared with 1960-2000 (10):

1. Countries with shorter return periods of storms and higher losses for all 3 climate scenarios: Germany, Belgium, the Netherlands, Poland, Estonia, Austria, Croatia, Bosnia and Hungary;
2. Norway with longer return periods and lower losses for all 3 climate scenarios;
3. All other countries in the studied part of Europe (Czech Republic, Finland, Great Britain, Ireland, Italy, Latvia, Lithuania, Portugal, Slovakia, Slovenia, Spain, Switzerland) have typically higher losses under future climate conditions and in some cases shorter return periods. Some countries, e.g. Italy and Sweden,  actually show a tendency to longer RPs (A1B scenario).

In addition to these statistical analyses, simulations by a global climate model for the period 2060-2100 show that maximum storm losses for countries of Western Europe could increase by ~65% by the end of the 21st century, according to the IPCC A1B and A2 scenarios (10). Similar results were found in earlier studies for Central Europe (11), and some European countries (12). The significance of changes in storm magnitude strongly depends on country and scenario. For many countries, findings point towards higher loss events, significant for at least one of the tree studied IPCC climate change scenarios (B1, A1B, A2). An exception is Norway, for which weaker losses are found (10).

More hurricanes

Model simulations (based on a climate change scenario showing 1°C less global warming than the SRES A1B scenario) suggest that tropical hurricanes might become a serious threat for Western Europe in the future (13). An increase in severe storms of predominantly tropical origin reaching Western Europe is anticipated as part of 21st global warming. An eastward extension of the development region of tropical storms is projected. In the current climate, the main genesis region for hurricanes is confined to the western tropical Atlantic, where sea surface temperatures are above the threshold (27°C) required for tropical cyclones to develop. Future tropical storms that reach western European coasts (and cause hurricane-force storms) predominantly originate from the eastern part of the tropical Atlantic. This is because climate warming in the eastern tropical Atlantic causes sea surface temperatures to rise well above the 27°C threshold. In addition to an increase in the frequency of severe winds (Beaufort 11–12), a shift is projected of the season of highest occurrence from winter to autumn (13). Scientists stress that both natural variability and human influences (including climate change) play a role in determining the frequency, strength and trajectory of hurricanes on the Atlantic Ocean (15).    

After their formation, tropical cyclones move in a north-westerly direction. When they reach the mid-latitudes they are caught by the predominant westerly winds, thereby veering their track in a north-easterly direction, with the possibility of reaching Western Europe. Geometrically, this likelihood increases if their genesis region in the tropical Atlantic is further to the east. In addition, the shorter travel distance in the mid-latitudes will enable the “tropical” characteristics of hurricanes to be better preserved along their journey to Western Europe. Hence, the likelihood of these storms maintaining their strength when reaching Western Europe will increase, because there is simply less time for them to dissipate (14).

Adaptation strategies

Insurance

In the Netherlands insurance coverance (in % of forest area) is 12% (8).

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 the Netherlands.

1. Smits et al. (2005)
2. Schiesser et al. (1997), in: Smits et al. (2005)
3. Pirazzoli and Tomasin (2003), in: Smits et al. (2005)
4. Hammond (1990), in: Smits et al. (2005)
5. Sweeney (2000), in: Smits et al. (2005)
6. WASA Group (1998), in: Smits et al. (2005)
7. Dorland et al. (1999)
8. Gardiner et al. (2010)
9. Della-Marta and Pinto (2009), in: Gardiner et al. (2010)
10. Pinto et al. (2012)
11. Schwierz et al. (2010), in: Pinto et al. (2012)
12. Leckebusch et al. (2007); Pinto et al. (2007a); Donat et al. (2011), all in: Pinto et al. (2012)
13. Haarsma et al. (2013)
14. Hart and Evans (2001), in: Haarsma et al. (2013)
15. Rosen (2017)