What was done
The authors examined newspaper accounts, ships' logbooks, meteorological journals and other documents in order to reconstruct a history of tropical cyclones passing through the 61.5°W meridian between the coast of South America (~9.7°N) and 25.0°N over the period 1690-2007, which they describe as "the longest and most complete record for any area of the world."

What was learned
Chenoweth and Divine say they could find "no evidence of statistically significant trend in the number of tropical cyclones passing through the region on any time scale," but they note that "hurricane frequency is down about 20% in the 20th century compared to earlier centuries," and that "this decline is consistent with the 20th century observed record of decreasing hurricane landfall rates in the U.S. (Landsea et al., 1999; Elsner et al., 2004) and proxy reconstruction of higher tropical cyclone frequency in Puerto Rico before the 20th century (Nyberg et al., 2007), as well as model-simulated small changes in Atlantic basin tropical cyclone numbers in a doubled CO2 environment (Emanuel et al., 2008; Knutson et al., 2008)." In addition, they report that "the period 1968-1977 was probably the most inactive period since the islands were settled in the 1620s and 1630s," which finding, in their words, "supports the results of Nyberg et al. (2007) of unprecedented low frequency of major hurricanes in the 1970s and 1980s."

What it means
In discussing the latter observation, the two researchers state that it "may be of more than passing interest given the interest in increased tropical cyclone activity reported in the satellite era (Emanuel, 2005; Mann and Emanuel, 2006; Kossin et al., 2007) and the significance of detected trends in tropical cyclone numbers since this time (Trenberth et al., 2007)." In fact, it strongly suggests that the latter short-term increases in cyclone numbers have absolutely nothing to do with the supposedly unprecedented concurrent warming of the globe, as they appear to be nothing more than a simple recovery from a short-term dip (within a century-scale lull) in yearly cyclone numbers that reduced them to their lowest levels of the past three centuries.

References
Elsner, J.B., Xufeng, N. and Jagger, T.H. 2004. Detecting shifts in hurricane rates using a Markov Chain Monte Carlo approach. Journal of Climate 17: 2652-2666.

Emanuel, K.A. 2005. Increasing destructiveness of tropical cyclones over the past 30 years. Nature 436: 686-688.

Emanuel, K., Sundarrajan, R. and Williams, J. 2008. Hurricanes and global warming: Results from downscaling IPCC AR4 simulations. Bulletin of the American Meteorological Society 89: 347-367.

Knutson, T.R., Siutis, J.J., Garner, S.T., Vecchi, G.A. and Held, I.M. 2008. Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions. Nature Geoscience 10.1038/ngeo202.

Koissin, J.P., Knapp, K.R., Vimont, D.J., Murnane, R.J. and Harper, B.A. 2007. A globally consistent reanalysis of hurricane variability and trends. Geophysical Research Letters 34: 10.1029/2006GL028836.

Landsea, C.W., Pielke Jr., R.A., Mestas-Nunez, A.M. and Knaff, J.A. 1999. Atlantic basin hurricanes: Indices of climatic changes. Climatic Change 42: 89-129.

Mann, M. and Emanuel, K. 2006. Atlantic hurricane trends linked to climate change. EOS, Transactions, American Geophysical Union 87: 233,241.

Nyberg, J., Malmgren, B.A., Winter, A., Jury, M.R., Kilbourne, K.H. and Quinn, T.M. 2007. Low Atlantic hurricane activity in the 1970s and 1980s compared to the past 270 years. Nature 447: 698-702.

Trenberth, K.E. et al. 2007. Climate Change 2007. The Physical Science Basis. Contribution of WG 1 to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Solomon, S. et al. (Eds.). Cambridge University Press, Cambridge, UK.