Vulnerabilities – Trends of storm frequency and intensity in the past
According to measurements of significant wave height between 1978 and 2007, a slight reduction in significant wave height seems apparent at Westhinder, but the temporal series are too short to be able to provide a definitive response. Similarly, wind speed on the Belgian coast has shown a slight reduction, in particular since 1990-1995. This supports recent research suggesting that there has been a reduction in the frequency of storms in the Southern Bay of the North Sea (1).
Vulnerabilities – Future storm frequency and intensity
Projections of changes in storm frequency and intensity are still very uncertain and model-dependent. From different studies different, and sometimes contradictory, results have been reported. In an official Belgian report it was stated that more recent climate models no longer project an increase in extreme winds and North Sea storms, but a decrease instead (1). The latest model projections, however, do project an increase of storm intensity and frequency, however (2,3,4,5,).
As a consequence of higher storm intensity, the return period of damaging storms is projected to reduce significantly (2). 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 (2).
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 (3).
These statistical analyses show 3 different tendencies for the period 2060-2100 compared with 1960-2000 (3):
- 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;
- Norway with longer return periods and lower losses for all 3 climate scenarios;
- 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 (3). Similar results were found in earlier studies for Central Europe (4), and some European countries (5). 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 (3).
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 (6). 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 (6). 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 (8).
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 (7).
The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Belgium.
- Ministry for Social Affairs, Health and Environment (2009)
- Della-Marta and Pinto (2009), in: Gardiner et al. (2010)
- Pinto et al. (2012)
- Schwierz et al. (2010), in: Pinto et al. (2012)
- Leckebusch et al. (2007); Pinto et al. (2007a); Donat et al. (2011), all in: Pinto et al. (2012)
- Haarsma et al. (2013)
- Hart and Evans (2001), in: Haarsma et al. (2013)
- Rosen (2017)