# planetwater

ground- water, geo- statistics, environmental- engineering, earth- science

## Magnitudes of Extreme Weather Events

The invention what we now call insurance was a big step in human development: People who potentially suffer from losses due to the same reason pay money into a pot. When somebody experiences such a loss gets, he gets money from the pot, and more money than he puts in on a regular basis.

Things get problematic, when losses occur more frequently than expected or are more severe and lead to more severe losses than expected. Then there is not enough money in the pot. This could happen either because the expected frequency or severity were estimated wrong, or because the underlying process that causes the losses has changed.

In one of the stories Kaiser Fung describes how there is not enough money in the pot for people who suffer from storms in Florida. I think when dealing with the magnitude of extreme events, especial care has to be spent on the way of “statistical thinking”, hence I’d like to expand on this topic a little bit to Kaiser Fung’s writing.

Generally, the severity of storms is measured by the “return period”. This is statistically speaking, the number of years that pass on average until a storm of such a severity occurs again.

There are a few problematic things:

• One problem is the “on average” part, because it means the average if we had an infinitely long time series of observations of annual storms. Unfortunately, a really long time-series of measurements of naturally occurring phenomena is 100 years old. The magic of statistics comes into play when we want to estimate the magnitude of a storm with a 100 year return period or even a 1000 year return period.
• Another problem is, that really severe storms can occur in consecutive years. For example, two storms, each with a return-period of about 100 years could occur this year and next year. Generally, the property of storms occurring in consecutive years is covered by the statistics-side, but is generally perceived wrongly in the public.
• Expanding on the last problem: such two really severe storms could even occur both in the same year. Or even more than two storms in one year. This is a property that is covered only in more complex statistical models.
• In doing all of this, in order for statistical models to work, we have to assume that all the storms are “from the same homogeneous population”. This has a couple of implications! For one, every storm is independent of every other storm. This might be ok, if there is only one big storm every year. But what if similar or one set of conditions leads to multiple really big storms? Or what if the underlying process that causes storms, such as the weather patterns off-shore of Florida, change? We base our estimates of a storm with a return period of 100 years on data gathered in the past, and that’s as good as we can do for data collection. But if the underlying process started changing during the last say 20 years and such that the severity of storm generally increases, then our estimates based on our data consistently underestimate the severity of storm.
• Finally, one problem I want to only mention and not go into depth, because it is too deep for this post is the problem of making statements about extreme events in an areal context. Is the severity of a storm with a return period of 100 years the same everywhere in Florida? Everywhere in the USA? Everywhere in the world?

A novel concept for me about which Kaiser Fung wrote is that storms are can be classified differently: Not according to the return period in terms of severity of the natural phenomenon, measured for example by wind speed, but according to the economic loss they cause. This doesn’t solve the problems outlined above, but is at least an interesting different yardstick.

Written by Claus

May 4th, 2010 at 8:54 pm

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