Severe Weather

Impacts of Extreme Events in The United States

The table below is a summary of severe weather fatalities in the United States based on the 1991-2000 period. The source of this information is the National Weather Service. Because there are many different ways to estimate the impacts of severe weather, if you check other sources, you will likely see different numbers.

Severe Event	Ave # fatalities/year
Heat Related	206
Flooding	89
Tornadoes	57
Lightning	55
Winter Weather	54
Cold Related	28
Hurricanes	14

The following are links show the National Weather Service's severe weather statistics reports for the last several years showing loss of life and economic losses for each category of severe weather. Note that there can be some rather large fluctuations among the various categories from year to year.
NWS Severe Weather Statistics for 2001

NWS Severe Weather Statistics for 2002

NWS Severe Weather Statistics for 2003

Billion dollar weather disasters shows the most costly weather related events since 1980 by state and by year.

Temporal Fluctuations in Weather-Related Damages

We will now attempt to answer the following questions:

• Are there any significant trends in either loss of life or property damage related to severe weather events?
• If trends exist, what are the possible causes?
• Have there been changes in the frequency or intensity of severe weather events?

Figure 1: Weather-related insurance statistics: (a) annual weather-related losses to the insurance industry for 1950-96; (b) number of insurance-defined catastrophes and the dollar losses for 1950-94 for catastrophes causing losses between $10 and $100 million, compared to the U.S. population.

Another problem with determining trends in the impacts of severe weather events is that there are large fluctuations from year to year in the number and severity of extreme weather events. For example consider hurricanes. Some years there are no major hurricanes that strike the United States, while in other years, multiple major hurricanes may strike. Exceptionally extreme events occur infrequently, perhaps once a decade or once a century. Data collected by the insurance industry is shown in the figure 1.

We will now comment on some of the trends noted in recent studies for each of the categories of severe weather events listed in the table above.

Floods

Figure 2: Flood statistics: (a) annual values and linear trend for flood damages in the United States for 1903 97; (b) annual values (dashed line) and 5-yr average values (solid line) of flood-related fatalities in the United States for 1913-94;

Annual flood damages for the period 1903-97 (Figure 2a), as tabulated by the National Weather Service (NWS), have been increasing steadily (using constant or inflation-adjusted dollars).

Flood-related fatalities in the United States (Figure 3b) from 1913-94 have been high since the early 1970s (compared to the period prior to 1970) due to an increased frequency of years with high deaths.

Of the annual deaths related to floods, 80%-90% are caused by flash floods and 40% of these are related to stream crossing or highway fatalities.

Why have these trends occurred? Some have speculated that more heavy precipitation is the cause, while others point to increased societal vulnerability due to growth and participition in recreational activities.

There is some evidence that the frequency of heavy rain events over the United States has been increasing recently, although this is difficult to prove scientifically. This has been suggested to be a consequence of global warming. But most likely, the increasing property loses due to flooding result from the following factors: (1)Increase in overall population and the desire to live and build structures in flood prone areas and (2)Much of the property built in flood prone areas is more extravagent and valued higher (even after adjusting for inflation). The increase in loss of life associated with flooding is partially due to increased population, but also because more people seek recreational opportunities in flash flood prone areas (hiking, river trips, etc.). The loss of life is probably offset somewhat because weather forecasting and public warnings have improved in recent years.

Hurricanes

Figure 3: Hurricane statistics: (a) decadal totals of damages (left graph) and fatalities (right graph) due to hurricanes for 1900 95; (b) annual hurricane damages normalized for inflation, wealth, and coastal population for 1925 96 (from Pielke and Landsea 1998); (c) annual number intense U.S. landfalling hurricanes.

The information about trends related to hurricanes is summarized in figure 3. I realize the figure is difficult to read. The significant trend information for hurricanes is shown in panel a, which shows that property damage associated with hurricanes is increasing sharply, while deaths due to hurricanes is decreasing sharply with time. The explanation for these inverse trends is simple. The increase in property damage results from the fact that much building is occuring along America's coastlines. People want to live on the coast and vacation on the coast. As competition for this property increases, the value of the property increases. High priced homes and resorts are built in hurricane prone areas. Even considering inflation, the value of coastal properties continues to increase sharply. The sharp decreases in deaths is directly related to improvements in hurricane tracking, public warnings, and evacuation planning. With the use of satellites, hurricanes can be tracked for days before making landfall. Forecasting the future movement of tropical storms has also improved dramatically. Thus, people evacuate, but leave behind their high priced property.

Hurricanes are by far the costliest natural disasters in the United States. Previous research into long-term trends in hurricane-caused damage along the U.S. coast has suggested that damage has been quickly increasing within the last two decades, even after considering inflation. However, the researchers who put together panel b attempt to consider two additional factors: coastal population changes and changes in wealth. Both population and wealth have increased dramatically over the last several decades and act to enhance the recent hurricane damages preferentially over those occurring previously. With this normalization, the trend of increasing damage amounts in recent decades disappears. Instead, substantial multidecadal variations in normalized damages are observed: the 1970s and 1980s actually incurred less damages than in the preceding few decades. Only during the early 1990s does damage approach the high level of impact seen back in the 1940s through the 1960s, showing that what has been observed recently is not unprecedented.

I am personally skeptical about the method used to normalize the coastal population changes and changes in wealth. What I think they are tying to show is that the 1970s and 1980s were rather tranquil times with respect to major hurricanes striking the United States. There were more prior to the 1970s and again since 1990. This is probably nothing more than a natural decades-long cycle in hurricanes striking the United States (mostly Atlantic Ocean hurricanes), again indicating how sporatic these events can be. No one has credibly shown that this has any relation to global climate change.

Over the long term, the average annual impact of damages in the continental United States is about $4.8 billion (using 1995 dollar equivalents), Of these damages, over 83% are accounted for by the intense hurricanes (Saffir Simpson categories 3, 4, and 5), yet these make up only 21% of the U.S.-landfalling tropical cyclones.

Thunderstorm Related

Thunderstorms are a fundamental part of the nation's climate, producing between 15% (West Coast) and 70% (high plains) of the average precipitation across the nation. Thunderstorms also produce five weather phenomena that damage property and crops and kill and injure humans and livestock: lightning, tornadoes, high winds, heavy rainfall (and flash floods), and hailstorms. Thunderstorm-related damages occur across all parts of the nation and are common each year, causing 45% of all weather-related insured property losses in the nation.

Although the number of severe thunderstorms and tornadoes show much fluctuation from year to year, there is no indication of trends in either the number or severity of these events. As with other types of severe weather property damage is increasing primarily due to the fact that there are more structures around today (higher density of buildings). The number of deaths due to tornadoes has been slowly decreasing due to better warnings from the weather service and better preparedness training within the public sector. The annual number of deaths due to lightning has been nearly constant.

Winter Weather

Over the entire United States, there has been little change in the total number of winter storms or their severity, however, there has been an interesting regional shift in winter weather patterns over recent years. It appears that number of winter storms that affect the eastern and northeastern United States has increased recently, while a decreasing number winter storms have impacted the great lakes region over the same time period. This is probably nothing more than a natural cycle in weather patterns and not related to global warming. However, because the eastern part of the United States is more densely populated than the great lakes region, the monetary losses related to winter weather has increased over this time period. There has been no observed change in the total number of deaths related to winter weather.

Extreme heat and cold

Perhaps because extreme heat and cold do make spectactular news videos, many people are surprised when they hear that temperature extremes, particularly heat waves, are the largest weather related cause of death. This was pointed out in a recent New York Times article Most Deadly of the Natural Disasters: The Heat Wave. (Make sure you read this article, I may write exam questions from it). In recent decades, heat related deaths have been increasing, while cold related deaths have remained fairly steady.

Heat related deaths fluctuate quite a bit from year to year. Most deaths are associated with prolonged periods of hot temperatures that occur sporatically. In the hot summers of 1952-55, 1966, 1980, 1988, and 1995, the death tolls due to heat were estimated to be as high as several or even tens of thousands per year, while in cooler summers very few are killed by heat. Thus, most deaths occur in a small percentage of years with extreme heat.

Several factors are thought to contribute to the recent increase in heat related deaths: (1)An aging population (older folks are more susceptible); (2)The heat island effect in urban areas (buildings absorb much more of the sun's energy than trees or grasslands); and (3) A shift toward higher concentrations of people, especially poor people, living in large cities. The problem seems to be most severe in large northern cities such as Chicago and New York because the people that live there and the buildings in which they live are not well adapted to the infrequent heat waves that strike. In southern cities high temperatures are more common, and people are more prepared to deal with the heat.

Although many proponents of global warming would like you to believe that the recent increases in heat related deaths are due to human induced climate change, to my knowledge there have been no studies to indicate that the frequency or severity of heat waves has increased significantly.

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