Hurricanes: Their Nature and Impacts on Society - Climate Science ...

140 HURRICANES: THEIR NATURE AND IMPACT ON SOCIETY
as an average, but it will also have a variance (i.e. spread around an average).
Often, variability is just as or more important to decision-makers than the
average (Katz <strong>and</strong> Brown 1992)!
What, then, is a "normal" (i.e. typical) weather event? There are different
ways to define normal weather. In some cases, "normal" is implicitly defined
as the climate over a recent 30-year period, e.g. as has been done with respect
to floods (FIFMTF 1992). Of course, it is possible to argue that on planet
Earth all weather events are in some sense normal; however, such a definition
has little practical utility for decision-makers. One way to refine the concept is
to define normal weather events as those events that occur within a certain
range within a distribution, such as, for instance, all events that fall within one
st<strong>and</strong>ard deviation of the mean (Pielke <strong>and</strong> Waage 1987). In practice, historical
records of various lengths <strong>and</strong> reliabilities have been kept around the
world for temperature, precipitation, storm events, etc. When data are available,
such a statistical definition lends itself to equating normal weather with
"expected" weather, where expectations are set according to the amount of
the distribution defined as "normal". For example, about 68% of all events
fall within one st<strong>and</strong>ard deviation of the mean of a bell-shaped distribution.
A change in the statistical distribution of a weather variable -such as that
associated with a change in climate -is troubling because decision-makers
may no longer expect that the future will resemble the past. For the insurance
industry, as well as other decision-makers who rely on actuarial information,
such a possibility of a changing climate is particularly troubling. A climate
change is thus a variation or change in the shape or location (e.g. mean) of a
distribution of discrete events (Katz 1993).
"Extreme" weather events can simply be defined as those "not normal",
depending on how normal is chosen to be defined. For instance, if normal
weather events are those which occur within two st<strong>and</strong>ard deviations of the
mean, then about 5% of all events will be classified as extreme. While it is
possible to define hurricanes as "normal" <strong>and</strong> "extreme", the simple fact is
that for most communities any l<strong>and</strong>falling hurricane would qualify as an
extreme event, because of their rarity at particular locations along the coast.
From the st<strong>and</strong>point of those human activities sensitive to atmospheric
processes, it is often the case that decisions are made <strong>and</strong> decision processes
established based on some set of expectations about what future weather or
climate will be like. Building codes, l<strong>and</strong> use regulations, insurance rates,
disaster contingency funds are each examples of decisions that are dependent
upon an expectation of the frequency <strong>and</strong> magnitude of future normal <strong>and</strong>
extreme events. Decision-making will take a different form in a situation
where expectations are reliable <strong>and</strong> thought to be well understood than in a
situation of relative ignorance (Camerer <strong>and</strong> Kunreuther 1989).
In short, many of society's decision processes are established based upon an
expectation of "normal" weather. Yet for most coastal communities "normal
weather" has historically (or at least over the time of a human memory) meant