Hurricane Death and Destruction
Hugh E. Willoughby
Florida International University, Miami, FL

The number of deaths or amount of destruction caused by hurricanes in the US can vary by 4 orders of magnitude from season to season. Since 1900, hurricane-inflicted mortality has decreased exponentially while inflation-adjusted damage has increased exponentially. These characteristics suggest that the common logarithm is a useful transformation. The logarithmic representation can treat only years where impacts are nonzero. Thus, the resulting probability density functions must be multiplied by the probability of nonzero damage to calculate true probabilities. From 1900-2005, the probability of at least one death increased from 53% to 97% and of nonzero damage increased from 33% to 99%.

For both mortality and damage, seasons in which at least one major hurricane (maximum winds > 48 m s-1) made landfall (MJ) lie above an exponential trend line fitted to the complete data set; most seasons with damage > 0, but without a major hurricane landfall (HH), lie below the trend line. MJ mortality decreases exponentially with a halving time of 26.4 years. HH mortality is essentially constant. The reason for the decrease in MJ mortality is increasingly effective prevention of disasters that kill large numbers (> 100) of people.

If historical damage is normalized for population growth and increasing individual wealth as well as for inflation, neither MJ nor HH damage exhibits a significant trend based upon data from 1900-2005. Based upon a shorter record, 1900-2000, HH damage decreases with time as a result of inclusion of years that would have produced no reported damage in former times. This effect saturates by the early 21st century.
The common logarithms of death and damage appear to be normally distributed. Thus, the peak of the log-normal distribution represents the geometric mean impact, and the antilog of the standard deviation is a factor that multiplies or divides the mean to produce the range. Extrapolated along the exponential trend to 2005, MJ mortality has a geometric mean of 11.8 souls with a range from 2 to 67. HH mortality has geometric mean 8.9, with range of 3 to 25 deaths. MJ normalized damage has a geometric mean of $4 billion ($4x109), with range $0.9 to $20 billion. HH normalized damage has a geometric mean of $ 0.12 billion, with range $ 0.026 to $0.56 billion. Base upon these distributions, the probabilities of > 1,000 deaths or > $100 billion damage in a single year are ~ 1% in the early 21st Century.

The 2004 and 2005 hurricane seasons killed 1500 US residents and destroyed $160 billion in property. By contrast, the years 1970 -2003 were relatively benign. Compared with calculations based upon 1900-2000, calculations of exceedance probabilities based upon 1900-2005 data produce impact estimates between 40% and 100% higher on the high-impact, low probability tails of the distributions. The relative roles in this result of chance, undersampled extremes, and nonstationarity on the 30-100 year timescale are not clear.