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A New History of Major Atlantic Hurricane Activity Over the Past 270 Years
Reference
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: 698-701.

Background
In a commentary on the study of Nyberg et al., Elsner (2007) states that "increases in oceanic heat from global warming will raise a hurricane's potential intensity, all else being equal." However, as he continues, "increases in wind shear - in which winds at different altitudes blowing in different directions may tear apart the developing storm - could counter this tendency by dispersing the storm's heat." Consequently, he asks the important question addressed by Nyberg and his collaborators: "In the long run, which effect will win out?"

What was done
In a study that contributes much to answering this question, the team of six scientists from Sweden, Puerto Rico and the United States developed a history of the frequency of major (category 3-5) Atlantic hurricanes over the past 270 years based on proxy records of vertical wind shear and sea surface temperature that they derived from corals and a marine sediment core. These parameters are the primary controlling forces that set the stage for the formation of major hurricanes in the main development region westward of Africa across the tropical Atlantic and Caribbean Sea between latitudes 10 and 20°N, where 85% of all Atlantic major hurricanes and 60% of all non-major hurricanes and tropical storms are formed.

What was learned
The researchers report that the average frequency (number per year) of major Atlantic hurricanes "decreased gradually from the 1760s until the early 1990s, reaching anomalously low values during the 1970s and 1980s." More specifically, they note that "a gradual downward trend is evident from an average of ~4.1 (1775-1785) to ~1.5 major hurricanes [per year] during the late 1960s to early 1990s," and that "the current active phase (1995-2005) is unexceptional compared to the [many] other high-activity periods of ~1756-1774, 1780-1785, 1801-1812, 1840-1850, 1873-1890 and 1928-1933." Hence, they conclude that the recent ratcheting up of major Atlantic hurricane activity appears to be simply "a recovery to normal hurricane activity [our italics]."

What it means
Al Gore and other climate alarmists claim that the increase in major Atlantic hurricanes over the past decade or so, which they portray as truly unique, has been driven by what they call unprecedented CO2-induced global warming. This view, however, can now be seen to be not only incredibly myopic but downright wrong, for there have been many such increases in major Atlantic hurricanes over the past quarter of a millennium that have had nothing whatsoever to do with either rising atmospheric CO2 concentrations or increasing air temperatures. Hence, it is fitting that Elsner concludes his assessment of the issue by stating that "the assumption that hurricanes are simply passive responders to climate change should be challenged," and that the findings of Nyberg et al. do so in such a convincing manner.

Reference
Elsner, J.B. 2007. Tempests in time. Nature 447: 647-649.

Reviewed 4 July 2007