WASHINGTON - The air conditioners that are cooling downtown office buildings are also making sweltering city streets even hotter.
Anyone who's felt the hot blast of an air conditioner's exhaust knows how that happens, but preliminary findings of new research suggest that the waste heat from air conditioners can add as much as 2 degrees to outdoor urban temperatures.
For cities already suffering elevated temperatures because of hot roofs, hot pavement and little greenery, using air conditioners makes matters worse, if only for those not cooled by them.
In what is shaping up to be one of the hottest years on record, "urban heat islands" are of growing concern. Even as air conditioning becomes nearly universal, U.S. heat-related fatalities are projected to rise with temperatures for decades to come.
Larry Kalkstein, associate director of the Center for Climatic Research at the University of Delaware, said increased use of air conditioners in downtown areas is linked to a "jump of a couple of degrees" in heat-island effects there, according to his study of Philadelphia, New York, St. Louis and Indianapolis.
Ed Dooley, spokesman for the Air Conditioning and Refrigeration Institute, an Arlington, Va.-based trade group, called Kalkstein's finding "a novel theory."
He added: "In truth, air conditioning is a lifesaver."
Kalkstein agreed. Air conditioning has reduced heat-related deaths in New York City by about 26 percent in recent decades, he said.
"The air conditioner is great and we all love it. In fact, I come home and kiss my thermostat," said Dale Quattrochi, a heat-effects specialist at the Marshall Space Flight Center in Huntsville, Ala.
"Everybody should be aware that there is a price," he continued. "That price is that there's more anthropogenic [man-made] heat."
An air-conditioner produces hot air as a byproduct of the cooling process. Its refrigerant - a liquid with a low boiling point - absorbs heat as it evaporates into gas. To return the gas to its liquid state, the air conditioner compresses the vapor, releasing heat outside the building, said Hashem Akbari, leader of the Heat Island Group at the Lawrence Berkeley National Lab in Berkeley, Calif.
10 degrees hotter
Cities can be up to 10 degrees hotter than nearby rural areas because dark roofs and asphalt absorb the sun's radiation rather than reflect it, and there is little vegetation to cool the air.
In addition, man-made heat - from air conditioners, cars and trucks, and electricity use - is generated in urban areas, Akbari said.
In summer, all that extra heat - as much as 25 times more than in suburbs - tends to get trapped close to the ground by high-pressure weather systems atop them.
The result can be a vicious cycle.
"It's hotter, so we use air conditioning, which makes it hotter, so we use more air conditioning," said J. Scott Greene, director of the Environmental Verification and Analysis Center at the University of Oklahoma.
To people accustomed to air conditioning, the outside heat feels even worse and that could cause health effects such as heat exhaustion and stroke, said NASA's Quattrochi.
Urban heat islands are of growing concern because the world is getting hotter. The first six months of this year averaged a record-high 47 degrees on land worldwide, according to the National Climatic Data Center in Asheville, N.C. That's more than two degrees hotter than the 123-year average and one tenth of a degree hotter than the previous record, set in 1998.
When ocean surface temperatures are included, the first half of this year was the second-hottest first half-year ever, one-tenth of a degree behind 1998.
If warming continues, as most climate scientists predict, heat-related deaths in America's largest cities are projected to more than double over the next 50 years, despite the northward spread of air conditioning, according to Kalkstein and Greene.
1,840 heat deaths
Today the 44 cities they studied report a total of about 1,840 heat-related deaths a year. They predict that the number will approach 3,000 by the year 2020. By 2050, they project, the toll will be more than 3,900 a year.
Philadelphia, which Kalkstein and others said has the nation's most effective program to combat heat deaths, would experience the second-highest increase. The researchers estimate a jump from 191 today to 214 in 2020 and 349 in 2050.
The Minneapolis-St. Paul area would have the nation's fifth-highest increase in heat deaths, according to Kalkstein's projections. Annual heat deaths would jump from 59 now to 129 in 2020 and 175 in 2050.
Kansas City would have the nation's eighth-highest increase in heat deaths, according to Kalkstein's projections. Annual heat deaths would rise from 49 today to 115 in 2020 to 127 in 2050.
Detroit would have the nation's tenth-highest increase in heat deaths, according to Kalkstein's projections. Annual heat deaths would rise from 110 today to 163 in 2020 to 180 in 2050.
Annual heat deaths in the Dallas-Fort Worth area would rise from 36 now to 51 in 2020 to 72 in 2050.
Heat deaths in South Florida would stay near zero in the future because of nearly universal air conditioning, according to Kalkstein's study.
Heat deaths in San Jose would stay near zero because of its closeness to the ocean and its housing stock, Kalkstein said.
"Heat waves kill more people than tornadoes or lightning or hail, easily," Greene said. "People don't realize it."