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Heat waves and cold winter weather killed roughly equal numbers of people in the U.S. between 1970 and 2004, according to a December 2008 article published by Kevin Bordenand 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."
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.
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.
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