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Storms in the Future: Changes in Intensity, Cloudiness, Rainfall and Economic Costs


Results, Part 2

Team 2: How have the damages due to storms changed over time, and how are these changes related to the storms?

Actual damage costs increased dramatically during the past 30 years from approximately $200 million to more than $10 billion in 1998, dropping to $4 billion in 2000, as indicated by the dark blue line in the graph in Figure 7. Part of this increase is due to inflation and part of it is due to the growth in prosperity in the United States. The damage costs for each year were adjusted for inflation (using the value of the 1970 dollar as the baseline) and for prosperity (using the Gross Domestic Product (GDP) of 1970 as the baseline). These adjustments are also plotted in Figure 8, and show the same pattern of increase over the time period, though the magnitudes of the changes are much smaller. Clearly damages due to storms have been increasing from 1970 to 2000 for reasons other than just inflation and increased prosperity.

Time series plot of storm damages in the US, 1970 - 2000.
Figure 7: Annual Storm Damages Over the Unitred States, 1970 - 2000
Time series plots of average temperature and storm damages in the US, 1970 - 2000.
Figure 8:Average Temperature and Storm Damages in the US Midlatitudes, 1970 - 2000

Seasonal variations were also examined, and after removing damages caused by hurricanes in the summer and fall seasons, the same increasing trend was observed in each season, though that of the winter showed the highest rate of increase. The rest of the investigation centered on the changes occurring in winter. This also allowed for comparisons with the results of team 1.

An obvious connection to the increasing damages over the past 30 years is the increase in temperatures due to global warming over the same time. Is there a direct relationship between the two? In Figure 8 the adjusted damages are compared to the average midlatitudes temperatures for January of each year. While the trends show a similar increase over time, the correlation (0.058) between the average temperatures and storm damages for this time period was not significant. This lack of correlation indicates that changes in the damages are due to reasons other than just the increase in temperature.

Further research was directed toward investigating other factors that may have contributed to the rise in damages. An increase in the number and /or strength of the storms occurring over this time period would cause the damages to increase. Analysis showed that the number and strengths of storms (higher sea level pressures indicate weaker storms) actually decreased over this time period (Figures 9, 10), contradicting the idea that the increasing storm damages were due to changes in storm properties.

Time series plot of average frequency of storms over the US, 1970 - 1998.
Figure 9: Average Frequency of Storms Over the United States, 1970 - 1998
Time series plot of average central sea level pressure of storms over the US, 1970 - 1998.
Figure 10: Average Central Sea Level Pressure of Storms Over the US, 1970 - 1998

However, when examining the plots in Figures 9 and 10, the last part of each graph shows a trend that is opposite to the trend for the entire 30-year period, storm frequencies and storm intensities appear to be increasing. Based on this observation, the data was analyzed separately for each decade.

Figure 11 shows the trends of the average frequency of the storms throughout the winters of the 1990s. The trend is increasing. Figure 12 illustrates the trends in average intensity and average maximum intensity of the storms during the same period. In both cases the sea level pressure is also decreasing, implying that the average intensity of the storms is increasing during the winters of the 90s. During the 90s there were more and stronger storms than before, and this increase may be partially responsible for the rising costs of the damages.

Time series plot of average frequency of storms over the US, Winter 1970 - 1998.
Figure 11: Average Frequency of Storms Over the United States, Winter 1970 - 1998
Time series plot of average intensity and average maximum intensity of US Winter storms, 1970 - 1998.
Figure 12: Average Intensity and Average Maximum Intensity of US Winter Storms, 1970 - 1998.

The sea level pressures of the winter storms were then examined in more depth. Histograms were produced for the distributions of the sea level pressures of the storms fro each year. No trends were found and no correlations with damage or temperature could be found for either strong or weak storms over this period.

Difference plot of storm frequencies over the US, Winter 1993 - 1985.
Figure 13: Difference in Storm Frequency Distribution Over the US, Winter 1993 - 1985.

The last factor that was examined was the geographical location of the storms for winters in years with high damages and for those in years with low damages. The winter of 1985 (low damages) and the winter of 1993 (high damages) were selected not only for the magnitudes of the damages those years, but also because these years are included in the ISCCP data set and would enable a direct comparison of the clouds of those storms. The frequency of storms occurring in 1985 was subtracted from the frequency of storms occurring in 1993. By studying the resulting difference in frequencies shown in Figure 13, the areas impacted by storms over the US can be analyzed. The red regions in this figure show the areas where the storms occurred more often in the high damage year; the blue regions are areas where the storms occurred less frequently in the high damage year. Clearly there were more storms over the eastern coast of the United States in the high damage year. This region is highly urbanized, and the increase in damages for the high damage year could be due partially to the fact that the storms occurred over areas where more damages could be inflicted.