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Tropical Cyclones (Atlantic Ocean - Global Warming Effects: Frequency, The Past Few Millennia) -- Summary
Has the warming of the past century increased the yearly number of intense Atlantic Basin hurricanes? We investigate this question via a brief review of some studies that have explored this question via thousand-year reconstructions of the region's intense hurricane activity.

Liu and Fearn (1993) analyzed sediment cores retrieved from the center of Lake Shelby in Alabama (USA) to determine the history of intense (category 4 and 5) hurricane activity there over the past 3,500 years. This work revealed that over the period of their study, "major hurricanes of category 4 or 5 intensity directly struck the Alabama coast ... with an average recurrence interval of ~600 years." They also report that the last of these hurricane strikes occurred about 700 years ago. Hence, it would appear that 20th-century global warming has not accelerated the occurrence of such severe storm activity.

Seven years later, Liu and Fern (2000) conducted a similar study based on sixteen sediment cores retrieved from Western Lake, Florida (USA), which they used to produce a proxy record of intense hurricane strikes for this region of the Gulf of Mexico that covered the past 7000 years. In this study, twelve major hurricanes of category 4 or 5 intensity were found to have struck the Western Lake region. Nearly all of these events were centered around a 2400-year period between 1000 and 3400 years ago, when 11 of the 12 events were recorded. In contrast, between 0 to 1000 and 3400 to 7000 years ago, only one and zero major hurricane strikes were recorded, respectively. According to the two researchers, a probable explanation for the "remarkable increase in hurricane frequency and intensity" that affected the Florida Panhandle and the Gulf Coast after 1400 BC would have been a continental-scale shift in circulation patterns that caused the jet stream to shift south and the Bermuda High southwest of their earlier Holocene positions, such as would be expected with global cooling, giving strength to their contention that "paleohurricane records from the past century or even the past millennium are not long enough to capture the full range of variability of catastrophic hurricane activities inherent in the Holocene climatic regime."

Consequently, the next time a major Atlantic hurricane makes landfall somewhere on the Gulf Coast of the United States, don't give in to climate-alarmist hype and immediately point the finger of blame at global warming. Such occurrences may well fall within the range of millennial-scale climate variability. And remember, also, that from 3400 to 7000 years ago, the earth was experiencing what is known as the Holocene Maximum, which was the warmest period of the current interglacial. And that 3600-year period saw zero category 4 or 5 intensity hurricanes in the Western Lake region of Florida.

Last of all, we have the study of Donnelly and Woodruff (2007), who 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. Thus cognizant of the great 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 Mr. Gore and others 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."

Based on this work, 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)."

In light of these findings, Donnelly and Woodruff concluded 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 in any way unprecedented or that it has been caused by global warming, in contrast to what climate alarmists continue 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 [cooler] La Niņa years and suppressed during [warmer] El Niņo years (Gray, 1984; Bove et al., 1998), due primarily to increased vertical wind shear in strong El Niņo years hindering hurricane development."

In summary, millennial-scale reconstructions of intense hurricane activity within the Atlantic Basin provide no support for the climate-alarmist claim that global warming will lead to the creation of more intense Atlantic hurricanes that will batter the east, southeast, and southern coasts of the United States. In fact, they suggest just the opposite.

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.

Donnelly, J.P. and Woodruff, J.D. 2007. Intense hurricane activity over the past 5,000 years controlled by El Niņo and the West African Monsoon. Nature 447: 465-468.

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.

Liu, K.-b. and Fearn, M.L. 1993. Lake-sediment record of late Holocene hurricane activities from coastal Alabama. Geology 21: 793-796.

Liu, K.-B. and Fearn, M.L. 2000. Reconstruction of prehistoric landfall frequencies of catastrophic hurricanes in northwestern Florida from lake sediment records. Quaternary Research 54: 238-245.

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.

Last updated 24 December 2008