Weather and mortality

Hurricanes, earthquakes, and tornadoes get the attention-grabbing headlines when a natural disaster kills people in the U.S. Yet heat waves, cold winter weather, severe thunderstorm winds, and flooding all killed more people in the U.S. between 1970 and 2004, according to a December 2008 article published by Kevin Borden and Susan Cutter of the University of South Carolina. Tornadoes and lightning were tied for fifth place, and Hurricanes and earthquakes tied for eighth place. However, had this study extended one more year into 2005, the roughly 1800 hurricane deaths from Hurricane Katrina would have vaulted hurricane deaths into third place, behind heat wave deaths and cold weather deaths. The study also showed that people living in rural areas were most likely to die from a natural disaster than those living in cities.


Figure 1. U.S. deaths due to natural hazards between 1970 and 2004 showed that weather associated with extremes of hot and cold weather, along with severe thunderstorm winds (the "Severe Weather" category), killed the most people. Image credit: Spatial patterns of natural hazards mortality in the United States, International Journal of Health Geographics. Authors: Kevin Borden and Susan Cutter of the University of South Carolina.

The authors used Spatial Hazard Event and Loss Database for the United States (SHELDUS)(available at http://www.sheldus.org). This database provides hazard loss information (economic losses and casualties) from 1960-2005 for eighteen different hazard types, and is primarily based on data from the NOAA/National Climatic Data Center publication, "Storm Data". The numbers have high uncertainty, and the authors conclude, "There is considerable debate about which natural hazard is the most "deadly". According to our results, the answer is heat. But this finding could be changed depending on the data source, or how hazards within a data source are grouped."


Figure 2. U.S. deaths due to natural hazards for the 10- and 30-year period ending in 2007, according to the National Oceanic and Atmospheric Administration (NOAA). Image credit: NOAA.

To illustrate, a 2008 study by Thacker et al. called, "Overview of deaths associated with natural events, United States, 1979-2004", concluded that cold deaths were twice as common as heat deaths in the U.S. However, they noted that the 1995 Chicago heat wave, which killed between 600 and 700 people by some estimates, was not properly represented in the data base used in their study. This data base attributed only 50 deaths in the entire state of Illinois to heat in 1995. The authors conclude that their data base "under-reports the actual number of deaths due to severe heat".

Another example: NOAA plots up annual natural hazard deaths from the same source ("Storm Data") as the first study I montioned. Their statistics for the ten-year period ending in 2007 show a much different picture (Figure 2). Heat deaths are a much more dominant source of mortality than cold and winter storm deaths, by a factor 3.5. The take-home message from all this is that heat- and cold-related extreme weather are probably the deadliest weather hazards in the U.S., but we really don't know the proportion of people killed by each. One can easily cherry pick the study of one's choice to show a desired result.

How global warming might affect heat and cold-related deaths
If the globe continues to warm up this century, as predicted by the Intergovernmental Panel on Climate Change (IPCC), heat-related deaths will increase and cold-related deaths will decrease (duh!). Unfortunately, that's about the most intelligent thing one can say about the matter. The 2007 IPCC report (section 8.4.1.3, Heat- and cold-related mortality), does not attempt to estimate the numbers, saying, "Additional research is needed to understand how the balance of heat-related and cold-related mortality could change under different socio-economic scenarios and climate projections."

This high uncertainty in future heat- and cold-related deaths does not stop advocates on either side of the global warming issue from cherry picking results from selected studies to support a particular point of view. For example, opinion columnist George Will stated in a recent Newsweek column: "In Europe, cold kills more than seven times as many as heat does. Worldwide, moderate warming will, on balance, save more lives than it will cost--by a 9-to-1 ratio in China and India. So, if substantially cutting carbon dioxide reverses warming, that will mean a large net loss of life globally." Will bases his arguments on Danish statistician Bjørn Lomborg's controversial 2007 book, "Cool It: The Skeptical Environmentalist's Guide to Global Warming." However, as pointed out by Danish biologist Kåre Fog, who has assembled a large web site dedicated to pointing out the errors in Lomborg's books, the huge number of excess deaths attributed to cold by Will and Lomborg are in large part because the death rate naturally rises in the winter: "Old and seriously sick people have less vitality in the dark season. It is too bold to say that the excess deaths during the dark part of the year are `deaths due to excess cold?. There is no evidence that a warmer climate will alter the seasonal variation. These people would soon die in any case, even if winters became warmer. Indeed, cold and warm climates, like Finland and Greece, have approximately the same seasonal variation in mortality." The IPCC underscores this problem, stating: "projections of cold-related deaths, and the potential for decreasing their numbers due to warmer winters, can be overestimated unless they take into account the effects of influenza and season".

Heat wave deaths are subject to a degree of uncertainty as well. It is somewhat of a subjective call if an elderly person who dies during a heat wave died primarily as a result of the heat, or of a pre-existing heart or respiratory condition. Complicating the diagnosis is the fact that air pollution is at its worst during heat waves, and can also be blamed as the cause of death in some cases. Different studies will use different criteria for classify deaths due to heat, pollution, or pre-existing medical conditions during a heat wave, leading to widely varying estimates of mortality. For example, the European heat wave of 2003 is blamed for 35,000, 52,000, or 70,000 deaths, depending upon the source. You're more likely to hear the higher 70,000 figure quoted by advocates of doing something about global warming, and the 35,000 figure quoted by those opposed.

The three 2008 studies for the U.S. show the ratio of cold deaths to heat deaths ranges from 2:1 to 1:3, which is very different from the 7:1 and 9:1 figures quoted by Will and Lomborg for Europe, India, and China. I don't trust any of these numbers, since heat and cold mortality statistics are highly uncertain and easy to cherry pick to show a desired result. It is rather unproductive to argue about how many people die due to heat and cold in the current climate or in a future climate. Excess heat deaths due to climate change should not get as much attention as the potential for death due to reduction in crop yields due to increased heat and drought, regional collapses of the oceanic food chain from the steady acidification of the oceans, and the wars these conditions might trigger.

For more information
For those interested, Kåre Fog also presents a list of the errors in Al Gore's book and movie, An Inconvenient Truth, and has a Comparison of error counts between Al Gore and Bjørn Lomborg. Lomborg has assembled a Short reply to Skeptical Questions to respond to some of Fog's criticisms, but does not answer Fog's criticism on cold deaths vs. heat deaths. Suffice to say, one should be wary of trusting climate change information from either source, or from opinion columnists, or from politicians. Blogs can also be a questionable source of climate change information, though I think wunderground Climate Change blogger Dr. Ricky Rood is one of the most knowledgeable and unbiased climate change experts in the world. Though imperfect, the best source of climate change information is the U.N. Intergovernmental Panel on Climate Change (IPCC). The level of scientific collaboration and peer review that went into that document is one of the most remarkable achievements in the history of science, and the IPCC was fully deserving of the Nobel Prize awarded to it last year. Blogs and books like Lomborg's and Gore's have not gone through peer-review by scientific experts on climate change, and will have far more errors, biases, and distortions of the truth than the IPCC reports.

Jeff Masters