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![[nws fatalities]](nws_30year_2006.gif)
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| Severe weather fatalities in the United States. (Source: National Weather Service) |
The figure to the left gives the number of severe weather fatalities in the United States per year based on the 1997-2006 period (10 year average) and the 1977-2006 period (30 year average). 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 slightly different numbers.
To put these numbers into perspective, the average number of United States deaths related to vehicle accidents has averaged about 42,000 per year over the last ten years, with an average of about 16,000 murdered per year over the last decade. See the following links: 2002 year summary for vehicular deaths, 2004 year summary for vehicular deaths FBI Crime stastics for 2005.
There are rather large changes in loss of life for each category of severe weather from year to year. This is shown in the National Weather Service's compilation of severe weather deaths from 1940 - 2006.
Beside deaths, severe weather is the cause of extreme economic loss as well. Available statistics on deaths, injuries, and property damage for recent years broken down by type of severe weather are given below:
The most costly weather disasters in the United States in terms of economic damage
are shown in the following links:
United States billion dollar weather disasters from 1980 - 2005 by location
United States billion dollar weather disasters from 1980 - 2005 by year
We will now attempt to answer the following questions:
One 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. Thus, the choice of time period over which to analyze trends will affect the results.
A second problem is that the choice of geographical area selected for study will affect the results, since extreme weather events are often localized. For example, we already pointed out that hurricane activity in the Atlantic has been high since 1995 (probably due to a natural 20-30 year cycle), but there has not been a world-wide increase in hurricane activity. This is also a problem when focussing on small regions of the United States, as extreme events may have increased in one state over the last decade, but decreased in another state.
We can also argue about how to answer the following questions: What constitutes a "cost"? Do we want to look at insured losses or all losses? How can we be sure that loss estimates are accurate for individual disasters and/or that they are comparable across disasters? How can we possibly compare the relatively high property losses in developed countries with the relatively high social costs (such as deaths, injuries, and homelessness) in developing nations? Which indirect costs should be included? How can we be sure that a loss of life is directly caused by the severe event?
Keeping these difficulties in mind, a summary of trends in property damage, loss of life, and frequency/severity of severe weather events for the United States only is provided below. The information is surprisingly scarce and different sources are often contradictory. A qualitative summary of trends in impacts, event frequency and intensity, and causes for the trends for the United States only is shown in the table below. The scientific group that put this table together has not updated their material since 1997, but many of the major features have not changed.
| OBSERVED FREQUENCY | OBSERVED INTENSITY | DOCUMENTED SOCIETAL IMPACTS | POSSIBLE CAUSES OF TRENDS IN IMPACTS | |
|---|---|---|---|---|
| FLOODS |  Weak |
 |
DeathsDamages |
Land Use Population Shifts Changes in Climate(?) |
| TORNADOES |  Strong Weak |
|  Deaths Damages |
Warning Time Mobile Homes |
| HURRICANES | Intense Weak |
|
DeathsDamages |
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 |
||||
I will now provide a summary based on my reading and interpretation of the available information on trends in severe weather and severe weather impacts. In recent years, most types of extreme weather events have led to larger economic losses in the United States (even after adjusting for inflation). Multi-billion dollar losses now occur with increasing frequency (see information provided in previous section). 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. However, the majority of evidence suggests that it is not changes in the frequency/severity of extreme weather events that are responsible for the increasing cost, but rather much of the increase in property damage probably reflects from the fact that property values and population are increasing.
Trends in loss of life due to extreme weather events show mixed results. Little change is found in loss of life due to lightning and extreme cold; loss of life related to tornadoes has decreased with time; loss of life due to hurricanes decreased sharply since the early part of last century (due to better warnings and preparedness), but has started to show signs of increasing recently. On the other hand, loss of life due to flash flooding and extreme heat may be increasing with time. Both the downward and flat trends in loss of life (in the context of an increasing population) are most likely the result of improved warnings and disaster education and preparedness, whereas the increase in heat deaths is probably a function of societal conditions in large cities and the increase in flood deaths may be due to changes in recreational activities.
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.
Annual flood damages for the period 1903-97, as tabulated by the National Weather Service (NWS), have been increasing steadily (using inflation-adjusted dollars). Flood-related fatalities in the United States have in general increased 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. An increasing trend in heavy precipitation events worldwide has been labeled as "likely" in the IPCC report: Climate Change 2007 (summary for policymakers). This has been suggested to be a consequence of global warming. Even if the frequency of very heavy rain events has been increasing, 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 (and possibly the increase in heavy rain events), but more importantly because more people seek recreational opportunities in flash flood prone areas (hiking, river trips, etc.). The loss of life (although increasing) is probably offset somewhat because weather forecasting and public warnings have improved in recent years.
Hurricanes are by far the costliest natural disasters in the United States. Over the long term, the average annual impact of damages in the continental United States is about $5 billion (using 2002 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.
Prior to hurricane Katrina, which is too recent to be factored into trend estimations, the significant trend information for hurricanes shows that property damage associated with hurricanes is increasing sharply, while deaths due to hurricanes have decreased 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.
Katrina will undoubtedly be the costliest natural disaster in United States history. But different from recent hurricanes, Katrina also caused a significant loss of life (current estimates are approaching 1800). Many of these people would not have died had they evacuated the coastal area, thus Katrina will probably act as a warning message and wake-up call to coastal residents in the path of a major hurricane (see previous lecture page titled A Closer Look at Atlantic Hurricanes for more information). The aftermath of Katrina shows our vulnerability to hurricanes as many of the fastest growing areas of the United States are concentrated along coastlines that are at risk for hurricanes. High concentrations of people and property set the stage for potential hurricane disasters.
A Closer Look at Atlantic Hurricanes also describes the 20-30 cycle in Atlantic hurricanes. Hurricane damage in the United States varies with this cycle. The Atlantic has been in a period of high hurricane activity since 1995, and this is expected to continue for the next one to two decades. Several studies have indicated that the intensity of hurricanes has increased world-wide over the last 30 years. This may be the result of higher sea surface temperatures possibly caused by global warming. However, the link between hurricane intensity and global warming has not been scientifically proven, although an increase in worldwide hurricane intensity since 1970 has been labeled as "likely" in the IPCC report: Climate Change 2007 (summary for policymakers), wherin it is suggested to be a consequence of global warming.
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 (causing 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 (exceptly for the possibility that heavy rain events are becoming more common). This conclusion is echoed in the latest IPCC 2007 report (mentioned above). 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. Trends in flash flooding events were examined in a previous section.
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 may is probably nothing more than a natural cycle in weather patterns, but it is possibly some climate change related 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.
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 will likely 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. The hot summer of 2006 (in both the US and Europe) will have a high number of heat deaths, but final statistics are not available yet. The European heat wave in summer 2003 was particularly deadly Record Heat Wave in Europe takes 35000 lives.
There are large differences in the number of deaths attributed to exposure to extreme heat. The article above describes some of the reasons for the statistical discrepancies among published reports. The largest source of uncertainty is in classifying which deaths are caused by heat. Although overall mortalilty rates tend to increase sharply during heat waves, few deaths are directly attributable to heat stroke and there has been no consistent standard in reporting heat-related mortality. The majority of deaths are ascribed to causes not commonly considered to be weather related, such as circulatory and respiratory diseases. Studies that count all of the "excess deaths" (the number of deaths above the long-term average for a region) during a heat wave report significantly higher heat related deaths than those that only include heat stroke. Some authors also claim that there is a lag or delay between the end of the heat wave and the end of the related high mortality rates suggesting still higher death tolls related to extreme heat. The U.S. Centers for Disease Control consider heat related death and illness to be a significant problem and have been running a public campaign to alert people to the dangers of exposure to 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 is no scientific consensus indicating that the frequency or severity of heat waves has increased significantly in recent years. The table shown above shows no change in the frequency or intensity of heat waves over the United States, while the IPCC report: Climate Change 2007 (summary for policymakers) claims that it is "likely" that the frequency of heat waves has increased worldwide in the late 20th century. Again the claim is that this is related to global warming. Let's just say at the moment that trends in the frequency/intensity of heat waves are inconclusive. This does not mean that global warming has not (or will not) increase the number and severity of heat waves -- it just means that there has not been a proven relationship yet.