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26 Mar 15 12:18

When you think of severe thunderstorms, you might think of that hail dent in the hood of your car you keep meaning to get repaired, the devastating tornadoes of Moore OK or Joplin MO, or maybe even the box office hit Sharknado! But what about baseball sized hail in Argentina or deadly tornadoes in Bangladesh? While it's true that the US experiences the most severe thunderstorm activity worldwide, what is less known is that many other countries also feel the devastating impacts.

What exactly is a "severe" thunderstorm, sometimes called severe convective storm? This designation is given based on whether the storm reaches a threshold where it causes physical damage to structures, and varies from country to country(generally hail that is 1 inch or 2cm in size or larger, and wind gusts 56 mph or 90 km/hr or higher).

Insured losses from severe thunderstorms in recent years have been steadily increasing. Between 1990 and 2014, the global insured losses from severe thunderstorms grew 9% on average each year, compared to 6.6% on average for all natural catastrophes. Also, for weather-related catastrophes, the share of losses from severe thunderstorms has grown steadily since 1990: 29% in the1990's, 38% in the 2000's, and over 45% so far in the 2010's, as shown in the figure from Swiss Re Sigma. Excluding 2005, every year since 2000 there has been at least one severe thunderstorm event resulting in USD 1 billion or more in insured losses.And since 2008, there have been 4-7 such events annually. The losses are largely driven by events in the US and Europe. However, the deadliest tornadoes have largely been in Bangladesh, where it is not uncommon for there to be several hundred deaths resulting from a single event. The deadliest tornado on record occurred 26 April 1989 in the Manikganj district of Bangladesh killing 1,115people and injuring 12,000. 

There has also been a lot of press recently regarding the link between El Niño/La Niña phase (often referred to as ENSO) and the frequency of hail and tornadoes in the US based on a recently published study. However,there have been half a dozen or more similar studies published since 2000, and there appears to be little consensus between them. For example, the most recent study, published in Nature Geoscience by Allen, Tippett, and Sobel, claims that the most activity in the Central US is correlated with La Niña conditions (colder than normal sea surface temperatures in the east Pacific), while another study published in 2013 in the Journal of Climate by Lee, Atlas, Enfield, Wang, and Liu, found that activity was more correlated with warmer than normal sea surface temperatures in east Pacific along with colder than normal sea surface temperatures in the central Pacific (what is known as the positive phase of the Trans-Niño Index). Yet another study published in Monthly Weather Review in2008 by Cook and Schaefer found that neutral ENSO conditions in winter favored stronger and more frequent tornado outbreaks. All this to say, there is significant year to year variability in severe thunderstorm activity, and in my opinion the ENSO is only one piece in a larger complex climate puzzle.

Regardless of the year-to-year variability, the increasing losses from severe thunderstorms are a growing challenge for society and insurers alike. The events of recent years demonstrate that it doesn't need to be an active year to have major losses, and reaffirms the need for stronger mitigation measures worldwide, such as improved building codes and construction practices. Additionally, more comprehensive risk modeling is needed to better understand the risks posed to communities and to minimize the impacts.


Category: Climate/natural disasters: Disaster risk, Floods/storms, Resilience


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