Severe Weather

Impacts of Extreme Events in The United States

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:

• NWS Severe Weather Statistics for 2003
• NWS Severe Weather Statistics for 2004
• NWS Severe Weather Statistics for 2005
• NWS Severe Weather Statistics for 2006

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 - 2007 by location
United States billion dollar weather disasters from 1980 - 2007 by year

Long Term Trends in Weather-Related Damages and Deaths

Consider the following question:

• Are there any significant trends in either loss of life or property damage related to severe weather events?

• How much of the trend can be attributed to changes in the frequency and/or intensity of extreme weather events and how much of the trend is related to societal changes?

Keep in mind that there are many different ways to estimate the impacts of severe weather and that different studies will produce different results. Thus, conclusions drawn from a single study can be considered somewhat subjective. 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 20-40 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.

As for death counts, there are no standard methods for determining which deaths should be attributed to a weather event, which leads to differences among studies. This is generally not a problem for things like hurricanes or tornadoes where cause of death is obviously attributable to the weather event, but what about something like a heat wave. If someone dies from a heart condition during the heat wave, was it because that person was stressed by the heat or was that person about to die anyway? Some studies will only count a heat death if the a coroner pronounces cause of death as heat stroke and not something like heart failure.

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, I will now provide a summary for trends in the US only based on my reading and interpretation of available information on trends in severe weather events and severe weather impacts. Please keep in mind that the information is surprisingly scarce and different sources are often contradictory.

It is very difficult for me to determine if there have been any significant trends is loss of life due to extreme weather events in recent years (say over the last couple of decades or so). For the most part, it appears that the number of deaths due to exteme weather events has not changed significantly over the last 20 years. Prior to the most recent 20 years in the United States, there was a continuing drop in the number of deaths caused by severe weather events, from highest in the early part of the 20th century to lowest in that latter part of the century. This was most likely the result of improving weather forecasts, weather warnings, and public education and preparedness. Over the last 20 years or so, improvements in forecasting and warning are happening at a slower rate. Thus, the United States may have reached a balance between an increasing population, which exposes more people to possible weather-realted deaths, and continuing modest improvements in weather forecasting and public preparation, which tends to reduce the number of deaths due to extreme weather.

Extreme Heat and Cold (the Silent Killers)

Perhaps because extreme heat and cold do not make spectactular news videos, many people are surprised when they hear that temperature extremes are the largest weather related cause of death. There are conflicting studies about whether heat waves or cold waves are the biggest killers, but most agree that on average exposure to extreme temperatures kill more people each year than hurricanes, lightning, tornadoes, blizzards, etc. combined. The World Meterological Organization has defined a heat wave as "5 or more consecutive days where the daily high temperature is 9°F or more above average (in the warmest season)." There does not seem to be a corresponding definition for a cold wave, but I guess it would be similar. This means that the conditions necessary for a heat wave or cold wave are different for different geographical areas. For example, a week of below freezing low temperatures in Florida would consitute a cold wave, whereas in Minnesota this would cause little difficulty. Conversely, a week of high temperatures above 100°F in Chicago is a heat wave, whereas in Phoenix, this is normal.

In the popular press, more attention has been paid to dangers of heat waves in comparison to the dangers of cold waves. In recent years there have been many articles and news stories stating that exposure to extreme heat kills more people than all other weather-related causes combined, including exposure to extreme cold. However, there have been studies by reputable medical researchers that conclude the opposite ... that exposure to extreme cold is responsible for more human deaths than exposure to extreme heat. For example, the following quote is taken from an article entitled The Impact of Global Warming on Health and Mortality published in the Southern Medical Journal: "Meanwhile, it must not be forgotten that cold weather in winter causes many more deaths than heat in summer, even in most subtropical regions, and measures to control cold-related deaths need to continue." This statement is partially based on a study of heat and cold related deaths in Europe, which found that over the 10 year period from 1993-2003, "Mean annual heat related mortalities were 304 in North Finland, 445 in Athens, and 40 in London. Cold related mortalities were 2457, 2533, and 3129 respectively." The rather unbalanced coverage by the popular press makes me wonder if it is somewhat driven by alarmist fears of global warming rather than ethically presenting the whole story. For example see It is Cold That Kills and European Heat Wave 2003: A Global Perspective.

Let's just say that we are unable to conclude whether heat waves or cold waves are responsible for more human deaths. In any case, there are large differences among various studies in the number of deaths attributed to extreme heat and cold. The largest source of uncertainty is in classifying which deaths are caused by exposure to extreme temperatures. Although overall mortalilty rates tend to increase sharply during both heat and cold waves, few deaths are directly attributable to either heat stroke or hypothermia and there has been no consistent standard in reporting temperature-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 or cold wave report significantly higher temperature-related deaths than those that only include heat stroke or hypothermia. For example, the National Weather Service statistics (shown above) will only count a death as heat or cold related if the official cause of death is reported to be due to heat stroke or hypothermia, which results in much lower estimates of temperature related deaths than studies that consider "excess deaths" during heat and cold waves. Some authors also claim that there is a lag or delay between the end of the heat or cold wave and the end of the related high mortality rates suggesting still higher death tolls related to exposure to extreme temperatures. The U.S. Centers for Disease Control consider heat and cold 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 temperatures.

Both heat and cold related deaths fluctuate quite a bit from year to year. Most deaths are associated with prolonged periods of hot or cold temperatures that occur sporatically. For very severe heat and cold waves death tolls due to exposure to extreme temperatures were estimated to be as high as several or even tens of thousands, while in more moderate years few are killed by exposure to extreme heat. Thus, most deaths occur in a small percentage of years with extreme heat or cold waves. For example, the European heat wave in summer 2003 was particularly deadly Record Heat Wave in Europe takes 35,000 lives. Soon after that, the cold winters in 2005 and 2006 were estimated to have killed 29,000 and 23,000 in England and Wales alone, Why more people die in winter. This last article also points out that countries that have the mildest winters tend to have higher cold weather-related deaths than countries with very cold winters.

Perhaps the most disconcerting part of the story is that the general public seems unaware that exposure to extreme temperature kills so many people. This was pointed out with regard to heat waves in a New York Times article Most Deadly of the Natural Disasters: The Heat Wave. Many public health officials believe that a large fraction of the excess deaths that occur during heat and cold waves are avoidable through public education and social services.

A few studies have concluded that the number of heat related deaths in the United States have been increasing in recent years. To be fair I should mention that other studies have concluded that there is no trend or that the number of heat related deaths have actually been decreasing in the United States. There are several recent societal changes that do put more people in the US at risk for 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. Some studies conclude that there have been no changes in the frequency or intensity of heat waves, 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. To be fair, however, we should also consider the possibility that global warming may also cause a reduction in the occurrence of killer cold waves as well, so maybe not all aspects of global warming would be negative.