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Harmful Heat Is More Frequent, Especially at Night, Study Finds
by William K. Stevens, New York Times, December 10, 1998

Extreme summer heat and humidity of the kind most threatening to health have become more frequent in the United States over the last half century, two Federal researchers say. According to their study, in today's issue of the journal Nature, the frequency of extremely hot, humid days and of heat waves lasting several days increased substantially from 1949 to 1995. In terms of the threat to health, however, another finding was especially significant: The increase in heat stress was greater at night than in the daytime. Many experts say that extremely high temperature and humidity over successive nights is a crucial factor in heat-related deaths.

When nearly 600 people died in a heat wave in Chicago in 1995, for example, many of the deaths were attributed to an unremitting combination of record high nighttime heat and humidity that allowed no respite from daytime heat stress. Some studies have shown that extreme summer heat has more impact on people's health than any other kind of severe weather and that the elderly are most vulnerable. "If these climate trends continue, they may pose a public health problem, particularly as there are increasing numbers of elderly people," wrote the authors of the study, Dr. Dian J. Gaffen and Rebecca J. Ross, research meteorologists at the National Oceanic and Atmospheric Administration's Air Resources Laboratory in Silver Spring, Md.

The apparent trend toward more extreme heat generally coincides with a warming of the earth's surface in recent decades, globally and in the United States. The dominant view among scientists is that at least some of this warming has been caused by emissions of heat-trapping gases like carbon dioxide, which is emitted by the burning of coal, oil, natural gas and wood. The earth's average surface temperature, which has risen 1 degree in the last century and is projected to rise 2 to 6 degrees over the next century, further increasing humidity and making heat waves more frequent and intense. By comparison, surface temperature has risen 5 to 9 degrees since the depth of the ice age 18,000 to 20,000 years ago.

While the increasing frequency of extreme heat and humidity cannot be directly linked to the warming trend, Dr. Gaffen said it was "consistent with what is predicted in the global warming scenario." Thomas R. Karl, director of the National Climatic Data Center, agreed. "I think these are the types of conditions that will become more frequent and more intense when they do occur" if the climate warms as projected, Karl said. "We've already seen examples of that this past year," he added, referring to last summer's heat wave in Texas. The projected increases in global temperatures, he said, "would make this rather commonplace." By and large, Karl said, the Gaffen-Ross findings "seem to be consistent with what we know from other analyses." He said that if data through 1998 were included in the study, "I'm sure they would find even stronger trends."

Worldwide, the last two years have been the two warmest on record. Dr. Gaffen and Ms. Ross had for some time been studying global distributions of temperature and water vapor, and they found an increasing trend of rising humidity accompanying rising temperatures. So, Dr. Gaffen said, they decided to see whether this meant that extremes of heat and humidity were also increasing. They defined "extreme" as the highest 15 percent of temperature and humidity measurements, as recorded every three hours around the clock in July and August from 1961 through 1990. This threshold, they report, is "closely correlated" with levels above which, other researchers say, mortality increases sharply. Dr. Gaffen and Ms. Ross analyzed records at 113 Government weather stations around the country, mainly at airports, in search of information on two variables: the combination of temperature and humidity that meteorologists call "apparent temperature," and temperature alone.

Apparent temperature is a measure of the combined impact of heat and humidity on people and the sensation that results. It is increasing faster than temperature alone, the researchers said, because humidity is increasing faster. This would be expected in a warmer climate, because more water evaporates and the atmosphere can hold more. The researchers computed their thresholds of extremity for the average daily temperature, the daily maximum temperature and the daily minimum temperature for July and August at each weather station. They found that from 1949 to 1995, the annual frequency of days on which the thresholds were surpassed increased at all but 16 of the 113 stations. The increases were largest for the minimum "apparent temperature," which basically translates to a measure of combined nighttime heat and humidity.