The Quaternary versus the Instrumental Record

The historical record of tropical cyclones in the Australian region shows some degree of decadal variability over the last 130 years especially in connection with variations in ENSO (e.g. Solow and Nicholls, 1990). The longest complete data set of land falling cyclones occur from northeast Queensland (Callaghan, 2005) and here the usual pattern of many more lower magnitude events occurring compared to higher magnitude ones exists. The record is interesting as it shows that during this time period only two category 5 and two category 4 cyclones struck the northeast Queensland coast. The rarity of these extreme events has led to a relatively blase attitude towards the severity and consequences of such a hazard in some regional centres (Anderson-Berry, 2003).

Using only instrumental (last 30-40 years) and/or historical data sets (last approximately 100 yrs), several studies have determined the recurrence intervals of cyclones in this region. This has usually involved extrapolating centuries to millennia beyond the short record to determine the frequency of the most extreme events i.e. the 0.1% Annual Exceedence Probability event (AEP) (Harper, 1999; Mclnnes et al., 2000). The most recent magnitude frequency analysis of cyclones for the Queensland region used only the last 33 years (AD 2004-1970) of record, being the instrumental period (Queensland Government, 2001). Based on these analyses Cairns can expect a severe Category 5 cyclone (900 hPa) approximately every 1,000 yrs. Likewise, the Queensland Govt. (2004) showed that the probability of a category 5 cyclone crossing at Cairns and generating a storm tide equal to that needed to generate the sedimentary ridges discussed in this study was 0.001 (annual exceedence probability) or once every 1,000 years.

The Quaternary sedimentary record for the Queensland region suggests that over the past 5,000 years a category 5 cyclone occurs at any one location on average every 200-300 years (Nott and Hayne, 2001) which is approximately four times more frequent than that suggested by the instrumental period. In the Cairns region the sedimentary record suggests a category 5 cyclone made landfall here sometime between AD 1800-1870, the latter being the date of first European settlement. The isotope stalagmite record suggests that the last intense cyclone to occur near Cairns was AD 1801. This record also suggests that Cairns has experienced between 5 and 7 intense cyclones which were likely to be category 4 or 5 events over the past 800 years.

The clear message is that the Quaternary record demonstrates that the historical/ instrumental record substantially underestimates the frequency of the most extreme tropical cyclone events. It also suggests that the size of the 1% annual exceedence probability (AEP) event is much more intense than had been previously estimated. Present policies and guidelines for hazard risk mitigation and urban planning, particularly for storm tides, are based upon the estimates from the historical/ instrumental records despite the fact that the Quaternary records cast strong doubt upon the veracity of this approach. There is a reluctance to accept the Quaternary record as realistic because it is less precise than the instrumental record. Nott (2006)

has suggested that this may be because of the lack of familiarity with reconstructing long-term time series from natural records by those undertaking the risk assessments.

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