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Global Warming and Atlantic Hurricane Intensity
Reference
Vecchi, G.A., Swanson, K.L. and Soden, B.J. 2008. Whither hurricane activity? Science 322: 687-689.

Background
"A key question in the study of near-term climate change is whether there is a causal connection between warming tropical sea surface temperatures (SSTs) and Atlantic hurricane activity." Thus begins the Climate Change Perspective of Vecchi et al. (2008) in the 31 October 2008 issue of Science. As they go on to explain in more detail, there are two schools of thought relative to this topic. One posits that the intensity of Atlantic Basin hurricanes is directly related to the absolute SST of the basin's main development region, which would be expected to rise in response to global warming. The other posits that Atlantic hurricane intensity is directly related to the SST of the Atlantic basin's main development region relative to the SSTs of the other tropical ocean basins, which factor could either rise or fall to a modest degree in response to global warming -- and possibly even cycle between the two modes.

What was done
Based on pertinent data obtained between 1946 and 2007, Vecchi et al. plotted Atlantic hurricane power dissipation index (PDI) anomalies calculated from both the absolute SST values of the Atlantic Basin and the relative SST values derived from all tropical ocean basins as a function time, extending them throughout most of the current century based on projections of the two parameters obtained from 24 different climate models; and comparing the results they obtained between1946 and 2007 with the measured PDI anomalies.

What was learned
"Between 1946 and 2007," in the words of the three researchers, the relative SST "is as well correlated with Atlantic hurricane activity as the absolute SST." However, they report that the "relative SST does not experience a substantial trend in 21st-century projections," and, therefore, they say that "a future where relative SST controls Atlantic hurricane activity is a future similar to the recent past, with periods of higher and lower hurricane activity relative to present-day conditions due to natural climate variability, but with little long-term trend."

What it means
This result, as Vecchi et al. describe it, "suggests that we are presently at an impasse," and that "many years of data will be required to reject one hypothesis in favor of the other," as the projections derived from the absolute and relative SST parameters "do not diverge completely until the mid-2020s." Consequently, if the absolute SST ultimately proves to be the proper forcing factor, the scare stories of Al Gore and James Hansen relative to this topic would have some validity. But if the relative SST proves to be the controlling factor, the researchers say that "an attribution of the recent increase in hurricane activity to human activities is not appropriate, because the recent changes in relative SST in the Atlantic are not yet distinct from natural climate variability." As for which of these two alternate futures awaits us, we suggest that a perusal of the materials we have archived under the heading of Hurricanes (Atlantic Ocean - Global Warming Effects: Intensity) may help you reach at least a tentative conclusion.

Reviewed 28 January 2009