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Fifteen Hundred Years of Atlantic Tropical Cyclones
Volume 13, Number 10: 10 March 2009

In an intriguing paper recently published in Nature, Mann et al. (2009) developed two 1500-year histories of North Atlantic tropical cyclone (TC) activity. The first of these proxy records, as they describe it, was derived from "a composite of regional sedimentary evidence of landfalling hurricanes," which included "a site from the Caribbean (Vieques, Puerto Rico), one from the US Gulf Coast, one from the southeastern US coast, three from the mid-Atlantic coast (one from New York and two from New Jersey) and two from southeastern New England (one from Rhode Island and another from Massachusetts)."

The second of the two estimates employed "a previously published statistical model of Atlantic tropical cyclone activity driven by proxy reconstructions of past climate changes," the three climate factors being "(1) the sea surface temperature over the main development region for tropical Atlantic tropical cyclones, which reflects the favorability of the local thermodynamic environment, (2) the El Niņo/Southern Oscillation, which influences the amount of (unfavorable) vertical wind shear, and (3) the North Atlantic Oscillation, which affects the tracking of storms, determining how favorable an environment they encounter."

The results of this enterprise revealed, in their words, "periods of high [TC] activity (that is, comparable to current levels) during a medieval era of roughly AD 900-1100." And because they found that the level of medieval activity "matches or even exceeds [italics added] current levels of activity within uncertainties for the statistical model," it is highly likely that the temperatures of the North Atlantic's main TC development region, as well as the Niņo3 region, were equivalent to, or even greater than, those of the recent past, implying much the same thing about the rest of the globe, as may be inferred from the Interactive Map and Time Domain Plot of our Medieval Warm Period Project.

Even more support for this conclusion is provided by the study of Landsea et al. (2009), who explored the influence of TC duration on observed changes in TC frequency, using the HURDAT Atlantic TC database. Their work revealed that "the occurrence of short-lived storms (duration two days or less) in the database has increased dramatically, from less than one per year in the late-19th/early-20th Century to about five per year since about 2000, while moderate to long-lived storms have increased little, if at all." Hence, they concluded that "the previously documented increase in total TC frequency since the late 19th Century in the database is primarily due to an increase in very short-lived TCs," which they attribute to "improvements in the quantity and quality of observations, along with enhanced interpretation techniques."

Interestingly, just as in the case of the discredited "hockey stick" temperature history of Mann et al. (1998, 1999) and Mann and Jones (2003), the Atlantic TC history of Mann et al. (2009) consists of reconstructed results ("apples") up until the mid-19th Century, after which observational results ("oranges") are employed. And those oranges pile up ever faster over the most recent 150 years of observational data, leaving the prior apples in their dust, just like the results of Landsea et al.'s analysis suggest they should. But when the latter researchers adjust for this artifactual phenomenon, they find that "no significant [TC] trend remains using either an 1878 or a 1900 starting point." And this development suggests that earth's Medieval Warm Period may well have been much warmer than what the Current Warm Period has been to date.

Sherwood, Keith and Craig Idso

Landsea, C.W., Vecchi, G.A., Bengtsson, L. and Knutson, T.R. 2009. Impact of duration thresholds on Atlantic tropical cyclone counts. Journal of Climate, in press.

Mann, M.E., Bradley, R.S. and Hughes, M.K. 1998. Global-scale temperature patterns and climate forcing over the past six centuries. Nature 392: 779-787.

Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations. Geophysical Research Letters 26: 759-762.

Mann, M.E. and Jones, P.D. 2003. Global surface temperatures over the past two millennia. Geophysical Research Letters 30: 10.1029/2003GL017814.

Mann, M.E., Woodruff, J.D., Donnelly, J.P. and Zhang, Z. 2009. Atlantic hurricanes and climate over the past 1,500 years. Nature 460: 880-883.