Severe Weather

Impacts of Extreme Events in The United States

Event	Annual Mean Loss of Life (period)	Annual Mean Current $ Loss (period)	Recent extreme event, $ loss, deaths (date)
floods	96 ('86-'95)	$2.4B ('84-'93)	$20B ('93) 156 ('76)
hurricanes	20 ('86-'95)	$6.2B ('89-'95)	$30B ('92) 256 ('69)
winter storms	47 ('88-'95)	>$1B (est.)	$6B ('93) 200+ ('93)
tornadoes	44 ('85-'95)	$2.9B ('91-'94)	$3.8B ('93) 94 ('85)
extreme heat	384 ('79-'92)	?	>$15B ('80) 522 ('95)
extreme cold	770 ('68-'85)	?	>$30B ('76-'77) ?
lightning	175 ('40-'81)	>$1B (est.)	?
hail	-	$2.3B (est.)	$650M ('90)
Annual Averages	>1500	>$15.8B	-

Billion dollar weather disasters: by state and by year.

Temporal Fluctuations in Weather-Related Damages

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.

In recent years, extreme weather events have led to even larger economic losses in the United States.

Multi-billion dollar losses now occur with increasing frequency.

The trend in losses has led many to conclude that the United States has witnessed changes in the frequency and/or intensity of extreme events.

The table above estimates national losses experienced in the early 1990's on the order of $300 million per week (current dollars).

Trend data can mislead. Underlying the data on extreme event impacts are sub-trends in climate patterns and changes in society.

In recognition of our relatively poor understanding of trends in impacts and the underlying vulnerability to those impacts. A qualitative summary of trends in impacts, event frequency and intensity, and causes for the trends is shown in Table 2 below.

	OBSERVED FREQUENCY	OBSERVED INTENSITY	DOCUMENTED SOCIETAL IMPACTS	POSSIBLE CAUSES OF TRENDS IN IMPACTS
FLOODS	Weak		Deaths Damages	Land Use Population Shifts Changes in Climate(?)
TORNADOES	Strong Weak		Deaths Damages	Warning Time Mobile Homes
HURRICANES	Intense Weak		Deaths Damages	Coastal Development Population Growth
EXTREME TEMP			Deaths Damages	Aging Population Heat Island
WINTER STORMS			Deaths Damages
LIGHTNING			Deaths Damages	Infrustructure Recreation
HAIL			Damages	Agriculture Urbanization
= variable with no discernable trend
= unknown

Lets review recent work on trends in economic losses associated with extreme events in the United States. These phenomena are floods, hurricanes, winter storms, extreme heat and cold, and thunderstorm related events (tornadoes and lightning).

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 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 flood policies.

Another significant hypothesis is that increasing population and urbanization in the United States has led to a commensurate increase in population at risk.

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.

Hurricanes are 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, to best capture the year-to-year variability in tropical cyclone damage, consideration must also be given toward 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.

Over the long term, the average annual impact of damages in the continental United States is about $4.8 billion (1995 $), substantially more than previous estimates. 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

Figure 4: Tornado statistics: (a) annual number of deaths due to tornadoes in the United States; (b) annual values of number of tornadoes (solid line) and tornado days (dashed line) for the United States (1953 96); (c) annual number of tornadoes causing fatalities for the United States (1953 96).

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.

Annual lives lost due to lightning, as reported by the National Oceanographic and Atmospheric Administration show no trends, up or down, since 1975.

Tornadoes are a product of thunderstorms, and most tornadoes are small in scale (covering less than a square mile) and are extremely infrequent events at any given location.

Figure 4 illustrates that the number of deaths due to tornadoes has decreased from the 1950s to the mid-1990s (Grazulis 1993b, 1997), perhaps reflecting the effects of improved storm warnings and detection on public safety.

Extreme heat and cold

Extremes of temperature have well-known impacts on human health. There are indications that the number of heat-related deaths in the United States have been increasing in recent years, but heat-related death tallies are suspect.

The hot summers of 1952-55 each resulted in more than 500 deaths, with >5000 heat deaths estimated in 1963 and 6700 in 1966. More than 15,000 heat-related deaths are estimated to have occurred in the heat waves of 1980.

The number of deaths due to extreme cold during the 1979 94 period varied from 267 deaths in 1992 to a high of 524 deaths in 1989, based on statistics from the CDC. The time distribution over this brief period does not suggest any obvious trend.

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