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A Service of The Greening Earth Society
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Does global warming threaten US coastal regions with stronger hurricanes? The facts will blow you away! by Each year, Dr. William Gray and the hurricane forecast team in the Atmospheric Science Department at Colorado State University and the Tropical Prediction Center issue predictions regarding the expected hurricane frequency within the Atlantic Ocean basin for the upcoming hurricane season. These predictions are based on many different global or local climatic variables and parameters. The forecasts usually are issued near the beginning of the season (early June) and are revised early in August. This year (1999), the Atlantic Hurricane season is expected to be busy because there is a strong connection between the absence of an El Niño event and increased Atlantic hurricane activity. In an attempt to remain on the cutting edge of climate science, Vice President Al Gore has made even longer-range predictions regarding global hurricane activity. In many speeches on the subject of the environment, the Vice President predicts more and stronger hurricanes as part of our globally-warmed future. This was especially true in the wake of Hurricane Mitch. That record-breaking storm devastated Central America in October 1998 and the Vice President linked it to global warming in his speeches. His predictions were featured in Newsweek and resulted in his front-cover appearance. Intense media coverage of that devastating hurricane brought home horrifying images of disaster. On many occasions, the package came complete with apocalyptic pronouncements from the Vice President and others about the likelihood of more "Mitches." Even before Mitch was a cluster of thunderstorms in the Caribbean, concern had been raised about the future occurrence and intensity of Atlantic Ocean basin (and to some extent, global) hurricane activity. This heightened interest is a result of the growth in population centers along the East and Gulf Coasts of the United States and the damage brought by recent storms such as Hugo (1989), Andrew (1992), and Fran (1996). Each brought with it damage totaling billions of dollars even though none were in the same category as Hurricane Mitch or Hurricane Camille, which raked the Gulf Coast in 1969. Hurricane Camille is regarded as the strongest hurricane to have made landfall along the US coast. Recent articles, such as that by Van Der Vink et al. (1998), show that damage totals and property losses from hurricanes are increasing steadily. Kunkel et al. (1999) found, "There has been a steady and substantial increase in hurricane losses. However, there has been no corresponding upward trend in hurricane frequency and intensity. The observed increases are due entirely to increased societal exposure along the vulnerable coastlines." In plain language, the increase in population along the coasts, combined with increased ownership of property – and more expensive property – means that there is a greater likelihood of property losses as hurricanes move inland. Kunkel, et al., go on to cite other work showing that if damage estimates from previous years are scaled to mid-1990’s conditions, all else being equal, there is little trend in hurricane damage estimates along the US coastline, although the 1940’s and 1950’s stand out as periods of significant loss, relatively speaking. Despite these facts, one cannot help but notice that media coverage of weather events and weather-related disasters has increased dramatically. Studies of this phenomenon, e.g. Ungar (1999), demonstrate that news coverage of these events has increased at least three-fold in the last 25 years or so. The proliferation of ‘round-the-clock news sources is a large contributor to this phenomenon. Also, sources such as The Weather Channel increase their coverage of major storms, including hurricanes, as they threaten US population centers. It is likely that this greater news coverage combined with substantial improvements in forecasting technique and skill by the National Weather Service (including The National Hurricane Center) has increased public awareness of the dangers that hurricanes pose and the lead time available for evacuation of population centers. The result is a substantial decrease in loss of life from recent storms. Unfortunately, the increase in media coverage has also brought with it the spread of misinformation regarding the possibility of future hurricane disasters – as evidenced by coverage of the Vice President’s linkage of global warming to more and stronger hurricanes along the US coast in the future. While bona fide hurricane forecasts by scientific groups such as Colorado State University’s Department of Atmospheric Science and the Tropical Prediction Center have a measurable record of success, it is difficult to project with any certainty far into the future as to whether or not there will be more or stronger hurricanes. This is especially true for hurricanes striking the US coastline. Only by studying past trends of hurricane activity and their linkages to global and local environments are we provided with vague clues about future hurricane activity. A number of studies – e.g. Landsea (1993), O’Brien et al. (1996), Bove et al. (1998), and Landsea et al. (1999) – have examined the issue of past Atlantic Ocean Basin hurricane activity, especially those that strike the coast. Landsea et al. (1999) found that there to have been a decrease in the number of US landfalling hurricanes between 1944 and 1996, with no significant trend in hurricane intensity. Landsea (1993) found that there has been a tendency toward fewer intense hurricanes over a similar time period (1944 – 1991). This was reflected by a tendency toward fewer landfalling, intense hurricanes during the 1970’s and 1980’s when compared to the 1940s and 1950s. It is now well known that there is a connection between the El Niño/La Niña oscillation and hurricane activity in the Atlantic Ocean Basin. For example, there is a tendency toward fewer hurricanes during El Niño years and more hurricanes during La Niña years. O’Brien et al. (1996) and Bove et al. (1998) studied this variation with respect to landfalling US hurricanes. They found that fewer hurricanes make landfall along the US coast during an El Niño year, and that the chance for a major hurricane is reduced as well. Landsea (1993) demonstrates that hurricane frequency and intensity may follow even longer-term cycles than the El Niño-related variability. This study and others have linked long-term pressure and sea surface temperature trends and oscillations in the Atlantic Ocean Basin to hurricane frequency. As mentioned above, these factors are the basis for the statistically-based annual hurricane forecasts. Johnston and Lupo (1999) examined variations in Atlantic Ocean Basin hurricane activity with respect to longer-term sea surface temperature variations in the Pacific Ocean Basin. A 50 – 70 year sea surface temperature variation, the North Pacific Oscillation (NPO), recently has been shown to modify and modulate the well-known shorter-term oscillation known as El Niño (Gershanov and Barnett, 1998). They found that when the NPO generally weakens El Niño events (as was the case during the period 1947 – 76), there tended to be more Atlantic hurricanes, annually, and little variation in year-to-year hurricane activity and intensity between El Niño and La Niña years. Their study also found that there was decreased Atlantic Ocean Basin hurricane activity during the periods from 1938 – 46 and 1977 – 98. These periods also exhibited strong El Niño-related variability with fewer and weaker hurricanes occurring during El Niño years than during La Niña years. The Johnston and Lupo (1999) study also found little long-term trend in the overall frequency of hurricane occurrence over the Atlantic Ocean Basin (Figure 1), and any trend was found to be statistically insignificant. The data in Figure 1 can be downloaded directly from the Colorado State University Archive compiled by Dr. Christopher Landsea (Landsea, 1993) http://typhoon.atmos.colostate.edu or through the UNISYS weather processor (Vietor, 1999). 
Fig. 1: (Taken from Johnston and Lupo, 1999) The 61-year Atlantic Ocean Basin hurricane frequency distribution and trend. Data cover the period from 1938–98. All of these studies demonstrate the complex relationship between long-term variations in hurricane frequency and intensity, and the global environment. Only by understanding these and many other factors can we properly assess the future risk faced by US coastal areas. Only by being prepared for the possibility of a landfalling hurricane every summer and fall will the loss of life and property be minimized. In assessing the Vice President and his followers’ hurricane forecasts, it is best to read the words of the scientists who study hurricanes at Colorado State University and the Tropical Prediction Center http://typhoon.atmos.colostate.edu/forecasts :
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