Summers have gotten not just hotter but stickier in the United States over the past few decades, and that trend could pose health problems for old people if it continues, government scientists say.
Past studies have noted an increase in average summertime temperature. The new work also takes humidity into account. Humidity, which can make hot days more stressful on the body, was found to be rising by several percent per decade.
The researchers tracked "apparent temperature," a measure of both heat and humidity.
First, they came up with a definition of a hot and sticky day. They took 30 years of July and August data from 113 weather stations around the country, and for each station they calculated heat-and-humidity thresholds that had been reached only 15 percent of the time.
Then the researchers looked at data from each station for July and August from 1949 to 1995. They found a 67 percent increase in the number of days that exceeded the threshold for daily average apparent temperature.
They also found an 88 percent increase in the number of heat waves, or periods during which the threshold level was exceeded for at least three consecutive days. What's more, they found a 78 percent increase in how often nights were above the nighttime threshold. That was seen as particularly worrisome.
"If it's hot during the day and cools off at night, people can recover," said researcher Dian J. Gaffen of the National Oceanic and Atmospheric Administration. "But if it stays warm and humid at night, you are more likely to end up with public health problems, with people dying of heat."
Ms. Gaffen and NOAA colleague Rebecca J. Ross reported the findings in Thursday's issue of the journal Nature.
"If these climate trends continue, they may pose a public health problem, particularly as there are increasing numbers of elderly people, who are most vulnerable to heat-related sickness and mortality," they wrote.
The findings were seen by Kevin Trenberth, head of the climate analysis section at the National Center for Atmospheric Research in Boulder, Colo., as evidence of the effects of global warming.
But Ms. Gaffen said the study was not aimed at discovering the cause of the trends, so she can't attribute the findings to global warming.
The largest changes occurred in some of the most populated areas, particularly in the eastern and western thirds of the country.
Oregon State Climatologist George Taylor said the data are suspect because most of the weather stations cited were at airports, where increased urbanization may have pushed up temperatures over the years. Streets and buildings are known to retain heat longer than fields and forests.
The government researchers acknowledged that urbanization might play some role. But Ms. Gaffen said that didn't appear to be the whole explanation because similar trends appeared over large geographic areas.
By Jeff Barnard