Extreme events over time - Extreme Events: A Physical Reconstruction and Risk Assessment - page 251

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Extreme events over time
The physical processes operating during extreme events (or natural
hazards when they affect people) are now relatively well understood. There are
also a broad array of approaches and methodologies available for unravelling
thelong-term history of extreme events from natural records. As a corollary,
sufficient data has now been collected on the long-term behaviour of these phe-
nomena to allow more realistic assessments of their frequency and magnitude
compared to that achievable from short historical records alone. To date, how-
ever, few risk assessments, especially those dealing with atmospheric hazards
utilise this long-term data.
One of the most interesting aspects of the long-term records is that they
often show that short historical records do not accurately reflect the behaviour
of a hazard over longer periods of time. Hazard behaviour appears to differ
between the two temporal records in terms of variability and stationarity or
non-stationarity. Short historical records often do not display the full variability
of a hazard and they also often give the impression that the hazard displays
stationarity. Of course a short historical record may reflect the true behaviour
of a hazard, but a risk assessor can never be sure of this unless comparisons
are made with actual records of the longer term behaviour. Many of the long-
term records discussed in earlier chapters show, however, that short historical
records poorly reflect the true nature of the hazard. As a consequence, problems
can arise when risk assessors extrapolate from historical records and assume
that the return interval of extreme events is much longer than is realistically
the case. In such situations people and property are unnecessarily exposed to
hazards resulting in increased community vulnerability and risk.
Short historical records can also give the impression that natural hazards
occur in a stochastic fashion over time. However, long-term records regularly
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