Background
The authors state that "it has been proposed that an increase in sea surface temperatures caused by anthropogenic climate change has led to an increase in the frequency of intense tropical cyclones," citing the studies of Emanuel (2005) and Webster et al. (2005), which is also the view Al Gore expressed in his 21 March 2007 testimony to the U.S. Senate's Environment & Public Works Committee.

What was done
Cognizant of the need to have a much longer record of the frequency of occurrence of intense hurricanes than that used by Emanuel and Webster et al. to draw the politically-charged conclusions the latter investigators and Gore have since been championing, Donnelly and Woodruff developed "a record of intense [category 4 and greater] hurricane activity in the western North Atlantic Ocean over the past 5,000 years based on sediment cores from a Caribbean lagoon [Laguna Playa Grande on the island of Vieques, Puerto Rico] that contains coarse-grained deposits associated with intense hurricane landfalls."

What was learned
The two researchers from the Woods Hole Oceanographic Institution detected three major intervals of intense hurricane strikes: one between 5,400 and 3,600 calendar years before present (yr BP, where present is AD 1950), one between 2,500 and 1,000 yr BP, and one after 250 yr BP. They also report that coral-based sea surface temperature (SST) data from Puerto Rico "indicate that mean annual Little Ice Age (250-135 yr BP or AD 1700-1815) SSTs were 2-3°C cooler than they are now," and they say that "an analysis of Caribbean hurricanes documented in Spanish archives indicates that 1766-1780 was one of the most active intervals in the period between 1500 and 1800 (Garcia-Herrera et al., 2005), when tree-ring-based reconstructions indicate a negative (cooler) phase of the Atlantic Multidecadal Oscillation (Gray et al., 2004)."

What it means
Donnelly and Woodruff conclude that "the information available suggests that tropical Atlantic SSTs were probably not the principal driver of intense hurricane activity over the past several millennia." Indeed, there is no compelling reason to believe that the current level of intense hurricane activity is either unprecedented or that it has been caused by global warming, in contrast to what Al Gore continues to claim. Quite to the contrary, the two researchers write that "studies relying on recent climatology indicate that North Atlantic hurricane activity is greater during La Niņa years and suppressed during El Niņo years (Gray, 1984; Bove et al., 1998), due primarily to increased vertical wind shear in strong El Niņo years hindering hurricane development."

References
Bove, M.C., Elsner, J.B., Landsea, C.W., Niu, X.F. and O'Brien, J.J. 1998. Effect of El Niņo on US landfalling hurricanes, revisited. Bulletin of the American Meteorological Society 79: 2477-2482.

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

Garcia-Herrera, R., Gimeno, L., Ribera, P. and Hernandez, E. 2005. New records of Atlantic hurricanes from Spanish documentary sources. Journal of Geophysical Research 110: 1-7.

Gray, S.T., Graumlich, L.J., Betancourt, J.L. and Pederson, G.T. 2004. A tree-ring-based reconstruction of the Atlantic Multidecadal Oscillation since 1567 A.D. Geophysical Research Letters 31: 1-4.

Gray, W.M. 1984. Atlantic seasonal hurricane frequency. Part I: El Niņo and 30 mb quasi-biennial oscillation influences. Monthly Weather Review 112: 1649-1668.

Webster, P.J., Holland, G.J., Curry, J.A. and Chang, H.-R. 2005. Changes in tropical cyclone number, duration, and intensity in a warming environment. Science 309: 1844-1846.