Background
The authors write that "the assessment of changes in tropical cyclone [TC] activity within the context of anthropogenically influenced climate change has been limited by the short temporal resolution of the instrumental TC record (less than 50 years)," citing Nott and Hayne (2001) and Knutson et al. (2010), while they also state that "controversy exists regarding the robustness of the observational record, especially before 1990," citing Kamahori et al. (2006), Landsea et al. (2006) and Harper et al. (2008).

What was done
In an effort to remedy this situation, the three researchers "developed a new technique, which calibrates high-resolution, long-term palaeo-records of TC activity against the instrumental TC record." This was done using a scale that "allows for a direct comparison between the past and present, and enables an examination of TC climatology at higher temporal resolution and on annual, decadal or millennial scales simultaneously. And in doing so, they created a new index based on earlier TC activity indices that "describe the severity of a season in terms of the number of storms, their intensity, their size and their longevity," which indices include "the accumulated cyclone energy index (Bell et al., 2000), the revised accumulated cyclone energy index (Yu et al., 2009), the power dissipation index (Kantha, 2006) and the hurricane intensity index (Kantha, 2006)," which creation was based on "a direct comparison between the modern instrumental record and long-term paleotempest (prehistoric TC) records derived from the ¹⁸O/¹⁶O ratio of seasonally accreting carbonate layers of actively growing stalagmites."

What was learned
Haig et al.'s results, derived from stalagmites collected from regions in Queensland and Western Australia that are prone to TC activity, revealed a repeated multi-centennial cycle of TC activity, the most recent of which cycles commenced around AD 1700 and included a sharp decrease in activity after 1960 in Western Australia, which they described as being "lower than at any time over the past 550-1500 years."

What it means
In light of what they learned, Haig et al. concluded that the Australian region seems to be experiencing the most pronounced phase of tropical cyclone inactivity since the industrial revolution, adding that the dramatic reductions in TC activity "suggest that climate change cannot be ruled out as a causative factor" of the decline in TC activity in that part of the world that occurred as the earth was in process of rebounding from the global chill of the Little Ice Age.

References
Bell, G.D., Halpert, M.S., Schnell, R.C., Higgins, R.W., Lawrimore, J., Kousky, V.E., Tinker, R., Thiaw, W., Chelliah, M. and Artusa, A. 2000. Climate Assessment for 1999. Bulletin of the American Meteorological Society 81: 1328.

Harper, B.A., Stroud, S.A., McCormack, M. and West, S.A. 2008. A review of historical tropical cyclone intensity in north-western Australia and implications for climate change trend analysis. Australian Meteorological Magazine 57: 121-141.

Kamahori, H., Yamazaki, N., Mannoji, N. and Takahashi, K. 2006. Variability in intense tropical cyclone days in the western North Pacific. SOLA 2: 104-107.

Kantha, L. 2006. Time to replace the Saffir-Simpson hurricane scale? EOS Transactions, American Geophysical Union 87 (1): 3-6.

Knutson, T.R., McBride, J.L., Chan, J., Emanuel, K., Holland, G., Landsea, C., Held, I., Kossin, J.P., Srivastava, A.K. and Sugi, M. 2010. Tropical cyclones and climate change. Nature Geoscience 3: 157-163.

Landsea, C.W., Harper, B.A., Hoarau, K. and Knaff, J.A. 2006. Can we detect trends in extreme tropical cyclones? Science 313: 252-254.

Nott, J. and Hayne, M. 2001. High frequency of 'super-cyclones' along the Great Barrier Reef over the past 5,000 years. Nature 413: 508-512.

Yu, J.Y., Chou, C. and Chiu, P.G. 2009. A revised accumulated cyclone energy index. Geophysical Research Letters 36: 10.1029/2009GL039254.