Extreme events over time - Extreme Events: A Physical Reconstruction and Risk Assessment - page 253

however, highlights that here too there have been periods when the frequency
of high-intensity events has increased. Liu et al .(2001) showed that clusters of
increased cyclone frequency occurred between AD 1660--1680 and 1850--1880. At
other times there were quiescent periods in the record. With a record of this
length it is possible to obtain a reasonable reflection of the nature of this haz-
ard for it displays non-stationarity over the longer term. These periods of regime
shifts can then be incorporated into any risk assessment of the hazard and a
more realistic view of the level of exposure of people and infrastructure along
with community vulnerability can be made.
Hayne and Chappell (2001)havesuggested that the occurrence of tropical
cyclones along parts of the Great Barrier Reef have not shown any appreciable
change in frequency over the past 5000 years. Nott and Hayne (2001)subsequently
showed that these coral shingle ridge deposits were most likely emplaced by
themost extreme cyclones to strike the region and hence preferentially record
only this spectrum of the cyclone climatology. Therefore, on average, it would
appear that the most extreme tropical cyclones in this region have not shown
any appreciable change in frequency of occurrence and, therefore, could be
regarded as belonging to the one hazard regime over this extended period of
time. However, while these events have occurred on average every 200--300 years,
there is considerable spread in the data showing that the time interval between
actual events has varied from 50 to 500 years. More importantly though, and

despite the fact that these events at present appear to display randomness, their
frequency of occurrence is an order of magnitude higher than that suggested
by thehistorical record. Hence, the historical record is a poor reflection of the
true behaviour of the most extreme tropical cyclones in this region. Again, the
prehistoric record shows that the historical record does not display a complete
pattern of the true variability of this hazard. Any risk assessments, therefore,
that rely solely upon the short historical record here are almost certainly likely
to underestimate the risk of the hazard.
Of course any interpretations of the nature of the hazard are dependent
upon the scale and resolution of the record. The fact that no extreme cyclone
events have occurred at many of the sites analysed by Nott and Hayne (2001)
over historical times (i.e. the last 130 years) suggests that if the historical record
could be extended to about 300 years then it would be likely to show at least one
and maybe two extreme cyclone events during this period. It may also be the
case that these two events occurred over a relatively short time interval. Nott
(2003) showed that natural cyclone records spanning the last 200 years near
Cairns, North Queensland reveal that two extreme tropical cyclones occurred
between about AD 1800 and 1870, and none have occurred since (being the period
covered by the historical record). The radiocarbon chronology is not sufficiently

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