Will global warming increase hurricane activity? Two studies published in the last week arrived at opposite conclusions. A link between warmer sea surface temperatures and increased North Atlantic hurricane activity “has been quantified for the first time,” according to a study by University College London researchers that was published in Nature (Jan. 30). They claim to have associated a 0.5 degree Celsius increase in sea surface warming with a 40 percent increase in Atlantic hurricane activity during 1996-2005 as compared to the average activity during 1950-2000. “The scientists who have linked global warming to stronger storms said the study makes sense, and is, if anything, just repeating and refining what they have already said,” the Associated Press reported (Jan. 30). But the study result isn’t surprising considering it was derived from a computer model that included only two variables -- sea surface temperature and atmospheric wind field -- which the researchers claim explain about 75 percent of the variance in Atlantic hurricane activity between 1965-2005. They claim to have teased out the association between sea surface temperature and hurricane activity by statistically removing the influence of wind from the model. Sea surface temperatures and wind, however, aren’t the only factors affecting hurricane activity. The model omitted at least two other known factors -- atmospheric humidity and sea level pressure -- and other more mysterious factors such as the tendency of hurricane activity to occur in cycles that are decades long.
Even though sea surface temperatures seem to have warmed, it’s not at all clear that Atlantic hurricane activity has truly increased. As recently described in World Climate Report, the average hurricane activity during 1995-2005 was greater than that during 1971-1994, but the 1970s and 1980s witnessed unusually low hurricane activity. So the increased hurricane activity of 1995-2005 “thus appears to represent a recovery to normal hurricane activity, rather than a direct response to increasing sea surface temperature,” according to World Climate Report. Finally, regardless of whether warmer sea surface temperatures are associated with increased hurricane activity, the University College London researchers admitted that, “Our analysis does not identify whether greenhouse gas-induced warming contributed to the increase in water temperature and thus to the increase in hurricane activity.” Since the entire global warming debate depends on whether manmade greenhouse gas emissions drive climate change, without a link between such emissions and sea surface temperature changes, the claimed sea surface temperature-hurricane activity link is, at best, an academic point. The other hurricane study, published in Geophysical Research Letters (Jan. 23) and not widely reported by the media, comes from climate scientists at the National Oceanic and Atmospheric Administration (NOAA). The NOAA researchers compared sea surface temperatures with hurricanes that made U.S. landfall -- the most reliable hurricane measurement over the long-term, according to the researchers. They found a slight decrease in the trend of landfalling hurricanes with warmer sea surface temperatures. “This paper uses observational data to demonstrate that the attribution of the recent increase in Atlantic hurricane activity to global warming is premature and that global warming may decrease the likelihood of hurricanes making landfall in the United States,” the researchers concluded. As leading hurricane forecaster William Gray of Colorado State University put it, “Meteorologists who study tropical cyclones have no valid physical theory as to why hurricane frequency or intensity would necessarily be altered by small amounts (plus/minus 0.5 degrees Centigrade) of global mean temperature change.” Dr. Gray continued, “In a global warming or global cooling world, the atmosphere’s upper air temperatures will warm or cool in unison with the sea surface temperatures. Vertical-lapse rates [differences between the atmospheric and sea temperatures that, when increased, tend to favor storm formation] will not be significantly altered.” In observing that there were 80 major hurricanes during 1945-1969 when the global temperature was cooling, but only 38 major hurricanes during 1970-1994 when global temperature was warming, Dr. Gray note that “Atlantic sea-surface temperatures do not necessarily follow global mean temperature trends.” Even the United Nations Intergovernmental Panel on Climate Change (IPCC) acknowledged in its most recent assessment that although average Northern Hemisphere temperatures in the last half of the 20 century were very likely higher than at any other time during the last 500 years, “There is no clear trend in the annual number of tropical cyclones [hurricanes]. And, of course, Al Gore learned this lesson the hard way. His attempt in “An Inconvenient Truth” to link manmade greenhouse gas emissions with the Hurricane Katrina tragedy was sound rejected by a British High Court judge who succinctly ruled that, “In scene 12 Hurricane Katrina and the consequent devastation in New Orleans is ascribed to global warming. It is common ground that there is insufficient evidence to show that.” As Sen. John McCain emerged from the Florida Republican primary as the Republican front runner, Politico.com observed that “After hitting it in most every appearance he made in New Hampshire and Michigan, John McCain now rarely brings up the topic of global warming.” In talking to reporters after a campaign event in West Palm Beach, McCain said, “I try to bring it up in areas that I think that it is of great import to people.” Given the scientific evidence, it’s quite easy to understand why Floridians might not think that alleged global warming-hurricane link is of great import.