Impacts

The simplest characterization of hurricane intensity is embodied in the Saffir-Simpson scale: from Category 1 ---barely a hurricane--- to Category 5 ---the worst imaginable. "Major Hurricanes" are those in Categories 3, 4, and 5 with winds stronger than 110 miles per hour equivalent to 100 kt or 50 m s^-1. Category 5 hurricanes are the most extreme and also the most rare. Only two, the 1935 Labor Day Storm and Camille in 1969 are recorded to have struck the United States. Andrew, at the very top of Category 4 was the third strongest U.S. landfall, and the second strongest on the mainland, given that the 1935 storm hit the Florida Keys.

In the 20^th century, U.S. hurricanes destroyed > $73 billion in property, not corrected for inflation. During the 70-year period from 1925 through 1995, the toll was $61 billion. If the damage from historical hurricanes is normalized for inflation, increased population, and greater individual wealth (Pielke and C. Landsea 1998), the estimate of total damage for the shorter period is $340 billion, equivalent to an average annual loss of $5 billion. During these 70 years, 244 landfalls occurred. The average landfall would have resulted in $1.5 billion in damage with today's prices and costal development. But the average doesn't tell the story. Major hurricanes accounted for 80% of the normalized damage, although they represented only 20% of occurrence.

The 1995 through 1999 seasons inclusive have been the five most active in the > 100-year quantitative climatology. Historically, hurricane landfalls on the U.S. east coast were common during the 1940s through the mid 1960s. In the 1970s and 1980s, landfalls were few. Now activity appears to have returned to the high level that characterized the immediate post-World War II period. These fluctuations in activity are most pronounced for major hurricanes. They also correlate with the observed "North Atlantic Mode", a coherent, multidecadal fluctuation of global sea-surface temperatures. During the active portion of the long-term record, Atlantic Sea-Surface Temperature (SST) anomalies in tropics and high latitudes were warm, and conversely. If the hurricane climatology and the Multi-Decadal Mode prove to be reliable guides, we may expect the first decade or two of the 21^st Century to produce as many of the most damaging major hurricanes annually as the last 5 years have.

In terms of hurricane-related mortality, the 20^th Century started badly. In Galveston Texas, on a single windy Saturday night, 9 September 1900, the "Great Hurricane" washed > 6,000 souls to their deaths. The total mortality for the century was just a bit more than twice this figure, 13,306 U.S. residents. During the first three decades of the century, the average annual loss of life was 329, or discounting the Galveston tragedy, 129. In the forty years from 1930 through 1969, it was 70. Since 1969, the average annual loss of life has been < 20, notwithstanding a 10-fold increase in coastal population from 1930.

The reason for the dramatic reduction has been effective warnings and timely evacuation from coastal areas inundated by storm surge. Invariably, large loss of life in hurricanes before 1970 stemmed from wind-driven flooding. Since 1970 drowning from inland flooding caused by torrential hurricane rains has come to predominate. Experience shows that when storm surge (or wind for that matter) completely flattens buildings, about 10% of the people present die. Evacuation insures that virtually nobody is present. On the other hand, extreme ( > 30 cm in 6 hr) rainfall places a much larger population at individually smaller risk. For example, Hurricane Floyd, the deadliest U.S. hurricane since Agnes of 1973, killed 49 of the more than 5 million people that it affected in the North Carolina Coastal Plain and Piedmont, for an average risk of dying of less than one in a hundred thousand. People in the developing world, who generally do not have the benefit from farsighted land-use policies or effective building standards, are at greater risk. The tragedy of Hurricane Mitch of 1998, which took 10,000 lives in Honduras, demonstrates that among the many bad consequences of poverty is vulnerability to natural disaster.