Hurricanes and Extreme Weather Events

As discussed briefly in Chapter 1, one of the most serious worries about rising temperatures is that they will not only increase rainfall and flooding, but that they may contribute to extreme weather events like hurricanes. The primary fear is that rising temperatures may warm the ocean's surface. University of California professor of geography Catherine Gautier, in her book Oil, Water, and Climate, explains that "a hurricane is essentially a heat engine with the warm tropical ocean as its primary energy supply. Hurricanes require a sea surface temperature above 28°C [82.4°F] to develop and so are bred in warm tropical regions. . . . Both theory and computer modeling suggest that a warmer ocean (as is expected in a world with a higher concentration of greenhouse gases) could increase the strength of hurricanes because more water vapor is added to the atmosphere by additional evaporation____"12 More evaporation means more air is rising and mixing, spreading heat and energy throughout the atmosphere—a process known as atmospheric convection. The increase in energy and air movement spurs the development of hurricanes.

The possible link between global warming and hurricanes has led some commentators to attribute Hurricane Katrina, the 2005 hurricane that destroyed New Orleans, to the effects of global warming. Nobel Peace Prize winner and former vice-president Al Gore, for example, suggests in the documentary film An Inconvenient Truth that Katrina may have been worsened by climate change. Reporter Jeffrey Kluger also suggests that warmer ocean waters may have contributed to the storm and concludes, "While the people of New Orleans may not see another hurricane for years, the next one they do see could make even Katrina look mild."13

In contrast, Christopher Essex, a professor of mathematics at the University of Western Ontario, and Ross McKitrick, a professor of economics at the University of Guelph, argue that Katrina

Following page: Hurricane Katrina, which devastated New Orleans in 2005, may have gained greater force from warmer ocean waters. Michael Lewis/National Geographic/Getty Images.

had nothing to do with global warming. Essex and McKitrick point out that hurricanes are not simply a function of ocean temperature; they also require specific atmospheric humidity, specific wind conditions, and so forth.14 These myriad conditions suggest why hurricanes are so difficult to predict. Furthermore, Essex and McKitrick argue there is no evidence that the number of hurricanes is actually increasing. Although 2005, when Ka-trina hit land, was a bad year for hurricanes, 2006 was actually a very mild year. In fact, for the United States, "the data show that the 1940s were the worst in terms of both total and major hurricanes, followed by the 1950s, 1930s, and 1890s."15 In comparison, recent decades have seen relatively mild hurricane activity.

Gautier agrees with Essex and McKitrick that "No sound theoretical grounds yet exist to predict how climate change will affect the number of hurricanes, if at all."16 She notes, however, that recent satellite data does seem to indicate an increase in the strength, though not the number, of hurricanes. Whether this increase is due to global warming or to natural hurricane cycles is difficult to determine. Gautier concludes that "combined natural and anthropogenic [man-made] effects are likely to induce increased Atlantic hurricane activity in the forthcoming decades" except in years affected by an atmospheric phenomenon called El Niño, which involves the periodic heating of portions of the Pacific Ocean, and which tends to moderate storms.

Other data also raises concerns about the possible effect of temperature rise on storms. For example, researchers at the Carnegie Institution for Science found in a 2008 study that jet streams have risen and shifted toward the poles. Jet streams are high bands of wind that have a major effect on the paths of weather systems and storms. Jet streams tend to weaken storms, so as those air currents move toward the poles and weaken, hurricanes, which usually develop closer to the equator, may become stronger. Researcher Ken Caldeira has stated, "At this point we can't say for sure that this is the result of global warming, but I think it is."17

NASA researcher Hartmut Aumann also collected evidence that suggested that warming may increase hurricane activity. Aumann and his team tracked the formation of high clouds associated with severe storms over the course of the year. They found that when sea surface temperature increased, so did the formation of clouds. They concluded that if temperatures increased because of global warming by .23°F (.13°C) per decade, there was likely to be an increase in storm activity of 6 percent per decade. Aumann has been careful to note the difficulty of making predictions in this area, however. "Clouds and rain have been the weakest link in climate prediction," he said. "The interaction between the daytime warming of the sea surface under clear-sky conditions and increases in the formation of low clouds, high clouds, and ultimately rain is very complicated."18 Researchers, then, do generally believe that increases in ocean temperatures affect hurricanes. There is still much work to be done, however, before scientists can determine whether, or to what degree, hurricanes are likely to worsen.

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