How It Affects Your Health

By Christine Gorman

It's a fair bet that global warming is going to lead to a rise in human sickness and death. But what form they will take is difficult to say. We can be pretty sure that as average temperatures climb, there will be more frequent and longer heat waves of the sort that contributed to the death of at least 20,000 Europeans in August 2003. Other predictions are more tenuous. For example, rising temperatures could--if rainfall and other conditions are right--result in larger mosquito populations at higher elevations in the tropics, which could in turn contribute to the spread of malaria, dengue and other insect-borne infections. Early indications are not encouraging. The World Health Organization (WHO) believes that even the modest increases in average temperature that have occurred since the 1970s have begun to take a toll. Climate change is responsible for at least 150,000 extra deaths a year--a figure that will double by 2030, according to WHO's conservative estimate. As with so many public-health issues, a disproportionate part of the burden appears to be falling on the poorest of the poor. That doesn't mean, however, that the comparatively wealthy--who account for more than their share of greenhouse-gas emissions--will escape harm.

A look at three key factors affected by warming offers a hint of things to come.

AIR We're used to thinking of industrial and traffic pollution as having a detrimental effect on air quality. But all other things being equal, rising temperature by itself increases the amount of ground-level ozone, a major constituent of smog. So many studies have linked higher ozone levels to death rates from heart and lung ailments that many cities issue smog alerts to warn those at risk to stay indoors. You can expect more and longer alerts.

It gets worse. Higher levels of carbon dioxide favor the growth of ragweed and other pollen producers over other plants, according to Dr. Paul Epstein at Harvard's Center for Health and the Global Environment. In addition, ragweed churns out more pollen as CO2 levels rise. Scientists have tied local spikes in asthma and allergy attacks to increases in molds and emissions from diesel engines. Apparently, the molds attach themselves to diesel particles, which deliver them more efficiently deep into the lungs. Add a plentiful helping of dust storms (from, for instance, the desertification of Mongolia or northern Africa) and a rise in drought-driven brushfires, and you have a made- to-order recipe for increasing respiratory distress worldwide.

WATER Residents of the U.S. Gulf Coast don't have to be reminded that water can be a killer. You can usually evacuate people ahead of a major storm, but you can't evacuate infrastructure. "Thirteen of the 20 largest cities in the world happen to be located at sea level," says Dr. Cindy Parker of the Johns Hopkins School of Public Health in Baltimore, Md. That means that where people are most at risk from floods, so are hospitals and water-treatment plants. As we have seen in New Orleans, the health effects of losing those facilities persist long after the water has receded.

Another predicted consequence of global warming is heavier downpours, leading to more floods. The immediate hazard is drowning, but the larger issue is water quality. To take just one example, more than 700 U.S. cities--most of them older communities in the Northeast, Northwest and Great Lakes area--have sewer systems that regularly overflow into water supplies during heavy rainstorms, mixing dirty and clean water and sometimes requiring mandatory boiling to make contaminated tap water safe. A heavy rainfall preceded the majority of waterborne-disease outbreaks in the U.S. over the past 60 years, says Dr. Jonathan Patz of the University of Wisconsin at Madison.

Ocean-water patterns also play a role in human health. Mercedes Pascual and her colleagues at the University of Michigan have been poring over more than a century's worth of data on cholera outbreaks in Bangladesh and tying them to detailed temperature reports of the surface waters of the Pacific Ocean. True, Bangladesh isn't anywhere near the Pacific, but the researchers are using the temperature data as an indication of a larger weather pattern called the El Nino/ Southern Oscillation, or ENSO. What they have found is that the severity of an epidemic is linked to water temperature--but only in years of higher-than-normal temperatures on the ocean's surface. More alarming: as the ENSO pattern has become more pronounced since the 1970s, the association with cholera has become even stronger.

INSECTS The news here is not all bad. Ticks, for example, may not be able to survive hotter temperatures in the southwestern U.S. And global warming is unlikely to have much of an effect on malaria, as long as you focus on lowland areas (because those regions already have so many mosquitoes). That picture may change, however, as you move upward in elevation. Malaria has seen a dramatic upswing since the 1970s in highland cities like Nairobi (around 5,500 ft. above sea level). How much of that can be tied to temperature increases--as opposed to population movement, lapses in mosquito control or the spread of drug-resistant parasites--is a matter of debate. But because each year there are at least 300 million cases accounting for more than 1 million deaths, even a small uptick in the spread or severity of malaria could be devastating.

The tricky thing about all those predictions is that you can't point to any outbreak or any individual's death and say, "This occurred because of climate change." But we know that good public health relies on a long list of factors--the availability of doctors and nurses, effective medicines, clean water, proper sanitation--and that even today, millions of people die every year of what should be preventable diseases. With global warming, you can expect the death toll to be even higher.