The frequency and intensity of hurricanes (as well as the number hitting U.S. coastlines and inflicting major damage) have been rising during recent years, in an uneven trend. Any study that takes the record back to the 1970s indicates a very tight relationship between ocean warming, hurricane intensity, and air temperatures. However, during the 1950s and 1960s, air temperatures were generally cooler than during the 1980s, butwater temperatures and hurricane intensity were higher— again, on an average.
By 2005, the complexity of this issue had provoked a vibrant (some might even say, "testy") debate between some hurricane experts regarding whether and to what degree, hurricane intensity and frequency was related to the overall warming trend. This debate often spilled over into the public realm as Florida and surrounding areas were smacked by four major hurricanes in 2004 and the 2005 hurricane season set records for the number of severe hurricanes. The same year also included some of the severest hurricanes on record in the Atlantic Basin, including Katrina, which killed more than 1,000 people in and near New Orleans. The city also lost more than half of its population (falling from about 470,000 to about 220,000 according to the U.S. Census) between 2005 and 2006.
The relationship between hurricane intensity and increasing temperatures is complicated by the fact that other factors have an important role in hurricane formation and strength. For example, El Nino (warming of the Pacific Ocean near the equator) has a strong influence on hurricanes in the Atlantic Ocean because it intensifies wind shear in the atmosphere which tears the storms apart as they form. While the summers of 2004 and 2005 were notable for several devastating hurricanes, the 2006 hurricane season was relatively quiet, with very little loss of life or property to tropical storms. Water temperatures were similar during all of these summers. The major difference was El Nino conditions during the 2006 hurricane season. The strength of the West African monsoon, which spins off low-pressure systems that may become hurricanes, also plays a role (Donnelly and Woodruff, 2007, 465-468).
All other things being equal, however, warmth does intensify hurricanes. They thrive on heat and fall apart ifwater temperatures fall below 80°F. Water temperatures (like air temperatures) sometimes vary, over periods of several decades, as the long-term trend "signal" provoked by warming raises them on average. For example, water temperatures in the Atlantic Ocean, which produces nearly all the hurricanes that have an impact on the United States of America, have been rising steadily, but gradually, since the 1970s, along with a general global rise in air temperatures.
A study published in Nature on August 4, 2005 (Emanuel, 2005, 686688) indicated that the "dissipation of power" of Atlantic hurricanes had more than doubled in the previous 30 years, with a dramatic spike since 1995, due to global warming and other variations in ocean temperatures working together. The study, by Massachusetts Institute of Technology climate scientist Kerry Emanuel, was the first to indicate a statistical relationship between warming and storm intensity (Merzer, 2005).
Hurricane Frances nears Florida, 2004 (NASA image courtesy Jacques Descloitres, MODIS Land Rapid Response Team at Goddard Space Flight Center)
This trend reflects longer storm lifetimes and greater intensities, both of which Emanuel associates with increasing sea-surface temperatures. The large upswing in the last decade is unprecedented and probably reflects the effect of global warming. "My results suggest that future warming may lead to an upward trend in tropical cyclone destructive potential and—taking into account an increasing coastal population— a substantial increase in hurricane-related losses in the 21st century," Emanuel wrote (2005, 686).
Thomas R. Knutson and Robert E. Tuleya's models indicate that given sea-surface temperature increases of 0.8oC to 2.4°C, hurricanes would become 14 percent more intense (based on central pressure), with a 6 percent increase in maximum wind speeds and an 18 percent rise in average precipitation rates within 100 kilometers of storm centers. Tuleya is a hurricane expert who recently retired after 31 years at the Fluid Dynamics Laboratory and teaches at Old Dominion University in Norfolk, Virginia; Knutson is at Princeton University. "One implication of the results," they wrote, "is that if the frequency of tropical cyclones remains the same over the coming century, a greenhouse -gas-induced warming may lead to a gradually increasing risk in the occurrence of highly destructive category-5 storms" (Knutson and Tukeya, 2004, 3477).
As an indication of how complex the origin of hurricanes can be, Johan Nyberg and colleagues reconstructed hurricane activity in the North Atlantic Ocean for 270 years into the past, using proxy records for vertical wind shear and sea-surface temperature from corals and a marine sediment core. In an exercise of what scientists call "paleotem-pestology," samples are taken from lagoons into which storm tides wash, an event associated with strong winds and storm surges that occur only during very strong tropical storms. Like all proxies, these are far from perfect. They do not, for example, account for changes in severe hurricanes' paths, since they sample only a very small fraction of the area over which the storms move (Elsner, 2007, 648). Records would be required over a much larger area to give them value.
Nyberg and colleagues found that the average frequency of major hurricanes decreased gradually from the 1760s until the early 1990s, reaching a long-term low cycle during the 1970s and 1980s. After 1995, frequency increased to levels similar to other periods of high intensity in their record, "and thus appears to represent a recovery to normal hurricane activity, rather than a direct response to increasing sea-surface temperatures" (Nyberg et al., 2007, 698). The upshot of this and other research is that while hurricanes are sustained by warm water, vertical wind shear (winds blowing from different directions at various heights that disturb hurricanes' circulation) can tear them apart, dispersing storm-sustaining heat. This research raises other questions: El Nino conditions may be fostered by warming oceans, but El Nino conditions in the Pacific tend to cause above average wind shear in the Atlantic, which seems to tear up hurricanes' circulation. The picture is not as simple, therefore, as equating warmer water with more frequent and intense hurricanes.
William M. Gray, professor emeritus of Atmospheric Sciences at Colorado State University, is a long-standing opponent of the idea that warming temperatures have anything to do with hurricanes. According to his tally, between 1957 and 2006, 83 hurricanes hit the United States, 34 of them major. Between 1900 and 1949,101 hurricanes hit the same area, 39 of which were major, with wind speeds above 110 miles an hour. From 1966 to 2006, says Gray, only 22 major hurricanes hit the United States, whereas between 1925 and 1965, 39 such storms hit the same area. "Even though global mean temperatures have risen by an estimated 0.4°C and CO2 by 20 percent, the number of major hurricanes hitting the United States declined," Gray (2007, A-12) wrote. Since 1995, however, the number of major storms hitting the U.S. Atlantic and Gulf of Mexico coasts has risen sharply. Gray associates the increase with strengthening circulation in the Atlantic Ocean.
Tropical cyclones have also been forming during recent years in places where they occur very rarely, if at all. During June 2007, for example, Tropical Cyclone Gonu, with sustained winds ofmore than 120 miles per hour, churned 35-foot-high waves and then struck Oman, on the Arabian peninsula, causing at least 13 deaths. The storm was the strongest on record in the northwestern Arabian Sea. The cyclone hit the Omani coastal towns of Sur and Ras al Hadd with sustained winds over 100 miles an hour. Judith Curry, a hurricane expert at Georgia Tech, said the cyclone's strength was "really rather amazing" for the region, and appeared to be amplified by sea temperatures hovering around 87°F. Even when weakened, she said, the storm could prove disastrous in Oman or Iran. "Cyclones are very rare in this region and hence governments and people are unprepared," she said (Revkin, 2007). --
Cumate Change in New EngLand; Goodbye, MapLE Syrup
New England's maple trees require cold weather to yield the sap that becomes syrup; they yield less sap in warmer winters. An analysis of syrup production between 1920 and 2000 indicated a decline in every New England state except Maine. At the same time, titmice, red-bellied woodpeckers, northern cardinals, and mockingbirds are being observed more often at bird feeders in Vermont. All of these birds have migrated from more southerly latitudes as temperatures have increased.
University of New Hampshire forester Rock Barrett, who supervised the survey, said that pervasive warming already might have doomed New England's maple syrup industry. "I think the sugar maple industry is on its way out, and there isn't much you can do about that," he said. Even in 2002, however, roughly one in four Vermont trees still was a sugar maple. Vermonters made almost 60 percent of New England's 850,000 gallons of syrup that year, according to federal farm data (Donn, 2002).
Much of New England could lose its maple forests during the twenty-first century in favor of the oak and hickory that are dominant further south. Already, during recent decades, the greatest expansion of syrup production has occurred to the north, in colder Quebec. During a decade ending in 2002, yearly production there has doubled to satisfy a booming market, which by the year 2000 surpassed the United States fivefold, according to the North American Maple Syrup Council (Donn, 2002). Over the last 80 years, New England's typical syrup output has dropped by more than half, from more than 1.6 million gallons a year to less than 800,000 gallons. --
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