The past few years have seen considerable debate over whether anthropogenically induced global climate change has already influenced the behaviour of tropical cyclones. Much of the debate has focused on the veracity and length of the time series used. Emanuel (2005) and Webster et al., (2005) used time series a few decades in length and in the case of the Nth Atlantic since approximately AD 1930. Landsea (2005) challenged the interpretation of the earlier part of the Atlantic record and suggested that calculations by Emanuel to estimate the PDI were incorrect and re-assessment showed that the PDI over the past 70 or so years appears to have changed little for this basin. Suggestions were then made that fluctuations in the Atlantic PDI were in tune with the Atlantic Multidecadal
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Oscillation (AMO) although recent suggestions have been made that recent fluctuations in the AMO may be an artifact of aerosols emitted from the USA (Mann & Emanuel 2006).
Each of these debates, and those that continue (Landsea, 2007; Mann et al., 2007, Kossin et al., 2007), have been concerned with a relatively short historical record (100-150 yrs). As such it has been difficult to discern whether the apparent upswing in Atlantic tropical cyclone activity since the 1970s is abnormal in terms of the natural variability of events in this basin. Nyberg et al. (2007) have shown however, based upon the character of a 270 year luminescence line record in Caribbean corals, that the 1970s and 1980s experienced the quietest phase in tropical cyclone activity for nearly the past 300 years and the recent upswing is nothing unusual. This evidence, despite its significance has not figured prominently in the debate regarding anthropogenic change versus natural variability. The same is true for other century to millennial scale records of tropical cyclones such as the overwash deposits in the Gulf of Mexico (Liu and Fearn, 2000; Liu, 2004) and the US Atlantic coast (Donnelly and Webb, 2001; Donnelly et al. 2004), tree ring records for the US (Miller et al., 2006), sedimentary deposits in Australia (Nott and Hayne, 2001; Nott, 2003) and also an oxygen isotope record from a calcium carbonate stalagmite in NE Queensland (Nott et al. 2007). The reasons behind this are unclear but may be because these data are in a proxy form and hence do not deal with instrumented or historical data. However this is not true of Nyberg et al. (2007) record for there is a close correlation between the latter part of their time series and the instrumented record.
A similar debate concerning global climate change and tropical cyclone behaviour has not as yet occurred in the Australian region. Here the instrumented record extends back a little over 30 years and the historical record just over 100 years, and on the basis of these records there doesn't appear to have been a recent upswing in tropical cyclone activity as has occurred in the NW Pacific and Atlantic Basins. A considerable archive of paleo-proxy data on the long-term natural variability of tropical cyclones is being amassed from northeast Queensland to Western Australia (Nott, 2006). The results of this work will be useful to better ascertain when and if global warming begins to have an effect on tropical cyclone behaviour in this region.
This chapter outlines these various types of long-term natural records of tropical cyclones from across the Australian continent and discusses their trends in relation to the trends seen in the shorter historical and instrumental records.
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