They advised Thursday that climate models show that global warming will increase air pollution and trigger more heat waves, floods and droughts, all of which will threaten human health.
"Climate change is a quintessential public health problem," said Michael McGeehin, director of the Division of Environmental Hazards and Health Effects at the Centers for Disease Control and Prevention, an agency of the federal government.
"Heat waves are a public health disaster. They kill, and they kill the most vulnerable members of our society," McGeehin warned. "The fact that climate change is going to increase the number and intensity of heat waves is something we need to prepare for."
McGeehin was one of several scientists who briefed reporters on a teleconference held by the nonprofit Union of Concerned Scientists.
Climate change models show that the kind of heat waves some parts of the country have been suffering through in recent weeks will occur more often and at closer intervals, and last longer, said David Easterling, a climatologist with the National Oceanic and Atmospheric Administration's National Climatic Data Center.
Dust storm in Queensland, Australia, September 2009. (Photo by Tom Fletcher)
"The current spate of heat waves could be a harbinger of things to come," he said, pointing out that from January through May, this year has been the hottest on record for global average temperatures.
Climate change could even make regions of the Earth uninhabitable, according to Matthew Huber, professor of earth and atmospheric sciences at Purdue University. His research on the effects of heat stress, reported in the Proceedings of the National Academy of Sciences, calculated the highest temperature-humidity combination that humans can withstand.
Huber's findings show that if emissions from burning fossil fuels continue unabated, extremely high temperature and humidity levels could make much of the world essentially uninhabitable for human beings.
Over the long term, perhaps 200 or 300 years, the planet could experience an increase of average global temperatures of 10 to 15 degrees Fahrenheit.
Under that scenario, much of the world, including Australia, many Mediterranean countries, and parts of Africa, Brazil, China, India and the United States, would be so hot and humid that people would not be able to survive outside during heatwaves for more than a few hours.
"We can still decide to try to avoid that" by dramatically reducing the heat-trapping emissions that cause global warming, Huber said. "And from our calculations, it is something we should try to avoid."
Jonathan Patz, director of global environmental health at the University of Wisconsin, Madison, said that while climate change is a health threat, tackling it is a major public health opportunity.
He pointed out that the World Health Organization reports about one million people annually die prematurely from air pollution. He says that cutting global warming emissions also would reduce certain kinds of pollution, especially ground-level ozone.
"If we can reduce air pollution," Patz said, "we can save lives."
Patz's latest research found that cutting down on the number short car trips and reducing the number of miles driven by about 20 percent would save hundreds of lives, avoid hundreds of thousands of hospital admissions, and save billions of dollars in healthcare costs in the Midwest alone.
If drivers got out of their cars and either walked or rode a bicycle, Patz added, "we could probably double those health care cost savings."
Climate scientist Brenda Ekwurzel with the Union of Concerned Scientists, who moderated the press briefing, noted that addressing climate change is not all about saving polar bears and other faraway creatures and habitats.
"More and more, studies demonstrate that the health care impact and health care costs related to climate change," she said, "are directly related to us."
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