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.