Tree-munching beetles, malaria-carrying mosquitoes and deer ticks that spread Lyme disease are three living signs that climate change is likely to exact a heavy toll on human health.

These pests and others are expanding their ranges in a warming world, which means people who never had to worry about them will have to start. And they are hardly the only health threats from global warming.

The Lancet medical journal declared in a May 16 commentary: "Climate change is the biggest global health threat of the 21st century."

Individual threats range from the simple to the very complex, the Lancet said, reporting on a year-long study conducted with University College London.

As the global mean temperature rises, expect more heat waves. The United Nations Intergovernmental Panel on Climate Change projects 25 percent more heat waves in Chicago by the year 2100; Los Angeles will likely have a four-to-eightfold increase in the number of heat-wave days by century's end.

These "direct temperature effects" will hit the most vulnerable people hardest, according to the U.S. Environmental Protection Agency, especially those with heart problems and asthma, the elderly, the very young and the homeless.

The EPA has declared that carbon dioxide, a powerful greenhouse gas. is a danger to human health and welfare, clearing the way for possible regulation of emissions.

At the same time, the U.S. Congress is working on a bill that would cap emissions and issue permits that could be traded between companies that spew more than the limit and those that emit less.


People who live within 60 miles of a shoreline, or about one-third of the world's population, could be affected if sea levels rise as expected over the coming decades, possibly more than 3 feet (1 meter) by 2100. Flooded homes and crops could make environmental refugees of a billion people.

As it becomes hotter, the air can hold more moisture, helping certain disease-carriers, such as the ticks that spread Lyme disease, thrive, the EPA said.

A changing climate could increase the risk of mosquito-borne diseases like malaria, dengue fever, yellow fever and various viral causes of encephalitis. Algae blooms in water could be more frequent, increasing the risk of diseases like cholera. Respiratory problems may be aggravated by warming-induced increases in smog.

Other less obvious dangers are also potentially devastating.

Pine bark beetles, which devour trees in western North America will be able to produce more generations each year, instead of subsiding during winter months.

They leave standing dead timber, ideal fuel for wildfires from Arizona to Alaska, said Paul Epstein of the Center for Health and the Global Environment at Harvard University.


Other insects are spreading in the United States, and while immediate protection is possible, the only real solution is to curb climate change, Epstein said in a telephone interview.

"You can tuck your pants into your socks and be very vigilant, but ultimately, if we don't stabilize the climate, it's going to continue to increase ... infectious diseases," Epstein said.

Carbon dioxide emissions, from coal-fired power plants, steel mills and petroleum-fueled cars, trucks and boats, among other sources, do more than modify climate, Epstein said. They also stimulate ragweed, some pollen-bearing trees and fungi, extending the spring and fall allergy and asthma seasons.

It is hard to quantify the potential financial cost of U.S. climate-change-related health problems, said Dr. Chris Portier of the U.S. National Institute of Environmental Health Sciences.

Some costs might actually decline if programs are put in place to cut greenhouse emissions from fossil fuels, which would also reduce some types of toxic air and water pollution.

Without cuts in carbon dioxide emissions, that pollution will remain, and the other unhealthy effects of climate change will continue, including more severe floods, droughts, heat waves and storms.

"You'll get more extreme weather events that will occur more frequently ... and so it just piles on in terms of the human health effects," Portier said. "And the cost will be tremendous, there's little doubt of that."