WASHINGTON -- It's not just the heat, it's the humidity that's likely to cause much of the pain of global warming, meteorologists are realizing.
Across a swath of the United States, the heat index, a measure of discomfort that takes into account heat and humidity, is expected to soar over the next 50 to 60 years, forecasters predict in the federal government's first study to take increased humidity into account.
That could increase the yearly average number of heat-related deaths nationwide, now 1,200, to several thousand, one expert said.
Plugging humidity into their computer models of global warming, federal scientists forecast that after the next half-century, the average summer heat index is going to be near 100 degrees for much of the country.
What is the occasional extremely hot and sticky day will probably become the average summer day in the South and East, said three scientists at the National Oceanic Atmospheric Administration's Geophysical Fluid Dynamics Laboratory in Princeton, N.J.
"Things are going to be very hot and sticky, and that's going to be a problem with global warming," said research meteorologist Thomas Knutson, one of the study's co-authors.
The study concentrates on what the authors call the Southeast, but its warning applies from Texas to New York City, said Thomas Delworth, the lead author of the study.
For years, almost all scientists have agreed that an increase in man-made carbon dioxide is making the world hotter. Scientists disagree about how much hotter it will get, but the conventional wisdom is that average temperatures will rise 3 to 8 degrees as carbon dioxide levels double.
When humidity is factored in, that translates to a 7 degree to 10 degree increase in the heat index, Knutson and Delworth said.
In the Southeast, the heat index on an average summer day is about 87 degrees. On extreme days, it is about 100 degrees. In about 60 years, computer models say, the average day will feel more like 97 degrees and extreme days will be near 110, Delworth said.
The weather service puts out heat wave warnings when it appears the heat index will reach 105. Some cities, such as Philadelphia, often call a warning at 100 degrees, experts say.
Different regions react differently to the same heat and humidity. What is deadly in Philadelphia will not be much of a problem in Miami, where people are more acclimated to the heat, said Larry Kalkstein, a University of Delaware climatology professor who has researched heat stress and mortality.
As the heat index rises, Northern areas could, like the South, become more accustomed to heat and humidity, and heat deaths might decrease, Kalkstein said. But it's more likely that such deaths will increase by the thousands, he said, because the older houses in Northern and Midwestern cities are built to trap, not release, heat.