"A brighter future." Politicians invoke it in their campaigns. Educators, environmentalists, and medical researchers everywhere strive to make real this brighter vision, the universal hope of modern civilization.
Some experts fear that tomorrow may be darkened by floods, pestilence, and human misery of nearly biblical proportions sweeping the planet's coastal areas. The water is toxic, the sewers are overwhelmed, the storms are violent, crops fail, the diseases of antiquity return, and familiar illnesses, such as cancer, become more widespread.
Public health experts worry that we will witness the dawn of such an age in the next century. The cause, they say, is greenhouse gases that are accumulating in the atmosphere as a result of human activities. If this continues, experts say, the world's climate system will shift, with potentially serious consequences for humanity.
"Recent increases in temperature and changes in climate variability in numerous parts of the world are regarded by many scientists as the first signals of such global change," say the directors of the World Health Organization, World Meteorological Organization, and United Nations Environment Program, in a seminal 262-page report, Climate Change and Human Health, published in 1996.
"Many diseases are extremely sensitive to climate," according to Jonathan Patz of the Johns Hopkins School of Hygiene and Public Health. "Now that the climatology community says that climate change is real, we know that there are going to be shifts in the distribution of many diseases."
Paul R. Epstein, a specialist in tropical public health at the Harvard School of Public Health in Boston, says: "I think climate change is a very big threat. It's a major wake-up call. Climate change is already a factor in terms of the distributions of malaria, dengue fever, and cholera." Gradual warming, the rise of minimum temperatures, and increases in extreme weather events all contribute to disease outbreaks and epidemics, he says.
The three agencies agree that climate change is one of the largest public health challenges for the next century. The Intergovernmental Panel on Climate Change (IPCC), a group of 2,500 scientists commissioned by the U.N., concurs, saying that "climate change is likely to have wide-ranging and mostly adverse impacts on human health, with significant loss of life."
Droughts and floods would claim many lives and changing sea levels would wipe out low-lying regions, including parts of the Atlantic coast, by the year 2100, given a rise of 3.6 degrees Farenheit in the average global surface temperature, according to these reports.
The people who live in poverty (more than one-fifth of the world's 5.6 billion persons, who already lack resources to prevent or treat illness) are the ones most likely to feel severe health effects from climate change, the experts note.
Alarm Over Old, New Diseases
The public health community already is alarmed by old diseases that are coming back, as well as new and emerging diseases that are having momentous consequences. Infectious and parasitic diseases remain the major causes of death in much of the world, where life expectancy is closely linked to per capita income; the poorest countries have almost half the life expectancy of the richest.
According to an international group of experts convened by the Pan American Health Organization (PAHO) in June 1995, a growing number of infectious diseases threaten to increase in the near future: both the "old" infectious diseases reemerging in many parts of the world, and "new" diseases emerging due to ecological and other factors.
Infectious diseases that affect humans can be caused by a virus, bacteria, or other organisms. In many cases, diseases are transmitted by "vectors," such as mosquitoes, flies, ticks, snails, or crustaceans, which "incubate" and then pass on the infective agent to humans. For many of these vectors with short lifespans 
(such as mosquitoes, sandflies, and blackflies) the incubation period is sensitive to temperature and humidity,
as well as rain and wind.
Other factors related to the ecology also play an important role. As people settle in new areas, such as the edges of forests, they are exposed to new disease-causing organisms. This seems to be the case in Africa with the Ebola virus, one of the deadliest known, which kills up to 80% of those infected through severe bleeding. Scientists are still hunting for the source of the mysterious virus, named for the river where it was first discovered 20 years ago.
Other factors are related to immediate changes in climate. Floods, for example, can drive rats out their underground warrens and cause outbreaks of potentially fatal leptospirosis, which is spread by rat urine. This happened in Nicaragua in 1995, when at least 16 people were killed by an unusual form of the bacterial disease, which broke out after heavy flooding in a rural area and affected about 1,850 people.
High precipitation in the southwestern United States in 1993 increased deer mice populations and led
to an outbreak of hantavirus, transmitted by exposure to rodent feces. A previously unknown virus, Sin Nombre Virus (SNV), was identified as the cause of hantavirus pulmonary syndrome (HPS), an infection which primarily affected otherwise healthy young adults, many of whom were out hiking, and it killed almost half of those exposed.
Increases in diseases are not linked only to climate change. People travel widely, sometimes carrying infectious diseases, such as tuberculosis, which they may not even know they have. Wars, urban migration, and other social and political upheavals result in the mass movement of people and cause epidemics of cholera or dysentery due to crowded and unsanitary conditions. And poverty continues to be the largest factor. In the poorest nations, 100 of every 1,000 children born die before their first birthday, while in the most developed countries only seven of every 1,000 die. Poor children continue to face high risks of dying from diarrhea, pneumonia, or parasitic diseases, and more than 200 million of them suffer from malnutrition. This is likely to worsen as climate changes.
Sudden Climate Change?
Climate change may not always be slow in coming. According to PAHO Director Dr. George Alleyne, "Much of our thinking on disease related climate change seems to be based on the premise that it will occur gradually, if not silently. This may not be the case and it is quite possible that some of the effects may be massive and sudden."
Speaking at a Harvard University symposium held in September 1996, Dr. Alleyne noted that "If the predicted global climate change does become a reality, international and national public health will be presented with a major challenge which will be compounded by the lack of empirical data." He called for increased attention "to those essentially promotive and preventive services that are likely to bear the brunt of the responsibility for climate changes related to disease."
"Climate change that brings with it the kinds of diseases that we predict will again bear hardest on the poor and perhaps will be one other phenomenon that widens the health gap between groups of people," he concluded.
Old Diseases Come Back
Experts from PAHO and other organizations are disturbed by the reemergence of old diseases. One example is dengue. A potentially fatal illness also known as "breakbone fever" because of the severe joint pain it causes, dengue is widespread in Asia and parts of Africa, and is making a comeback in the Americas. It is carried by a mosquito, the Aedes aegypti, whose range is limited by cold weather. The mosquito cannot survive freezing temperatures, but dengue epidemics have been increasing in number and severity during the last 10 years in South America, following an absence of more than 50 years. Although worldwide in scope, the emergence of dengue and its more serious cousin, dengue hemorrhagic fever (DHF) as a major public health problem has been most dramatic in the Americas, where in 1995 there were severe epidemics and outbreaks with about 284,000 cases of dengue, including 7,850 cases of DHF and 106 deaths, the highest number since a severe outbreak in 1981.
Hot and Cold Weather Mosquitoes
Laboratory studies indicate that warmer temperatures aid dengue transmission by accelerating development of the mosquito larvae, thus extending the mosquito's range and lengthening the season of transmission. Yet another climate-related threat comes from the Asian tiger mosquito, Aedes albopictus, which transmits dengue and yellow fever, and is able to tolerate cold weather. It arrived in the U.S. in a shipment of tires in 1985, and is now in 24 U.S. states, Mexico, the Caribbean, and Brazil, and continues to spread.
Another major tropical disease carried by mosquitoes is malaria, which puts 2.4 billion people at risk, and infects 300 to 500 million people a year. Experts say it is highly likely to expand its distribution with climate change. Malaria now kills some 2 million people a year, including about a million children, largely in Africa. The malaria situation is already worsening due to combinations of social, economic, and ecological conditions, and its control is becoming increasingly difficult.
"Recent evidence of the responsiveness of malaria incidence to local climate change and perturbations includes marked increases in the incidence and distribution (particularly with reference to altitude) of malaria during atypically hot weather in Rwanda in 1987," and in other areas, the climate change report notes.
Temperature, Ticks,
and Kissing Bugs
Yellow fever, generally confined to tropical zones, is a disease that attacks the liver and has a mortality rate of up to 50%. Usually present in relatively low numbers in outlying rural areas of South America, it reemerged with alarming force in Peru in 1995. Although there is an effective vaccine, a massive urban outbreak would strain the delivery system and it is doubtful there would be enough vaccine. Urban yellow fever is transmitted by the same Aedes aegypti mosquito, and "climate warming may increase the currently rather low risk of urban epidemics of yellow fever in East Africa and the Americas," according to the report.
Chagas disease, a parasitic disease caused by the T. cruzi parasite and spread by blood-feeding "kissing bugs" is found from the southern U.S. to southern Argentina and Chile. Some 18 million people are currently infected and thousands die each year. "Higher temperatures could extend the geographical distribution of wild vectors of Chagas disease," the report notes.
Another harbinger of changes in disease patterns is the proliferation of South American arenaviruses, which shows how ecological changes, such as exploitation of new areas for human settlement and agriculture, will increase the likelihood that new infectious diseases will emerge. A new member of this group of rodent-borne viruses has been discovered on the average of every three years since the first was isolated in 1956. Five of them cause human disease and three viruses (Jun'n, Machupo, and Guanarito) are important regional health problems in Argentina, Bolivia, and Venezuela.
Another effect of climate change on health is a result of increases in air pollutants, both indoor pollutants from fires built in the homes of poor people and outdoor pollutants from the growing numbers of factories, automobiles, trucks, and buses. Pollutants are a principal cause of respiratory infections, especially in children, according to Dr. Yehuda Benguigui, PAHO's regional advisor on prevalent childhood illnesses. In almost all countries of the Americas, acute respiratory infections are the leading cause of pediatric outpatient visits, and account for an estimated 140,000 deaths among children. Experts say one key way to reduce these infections entails what might be called personal climate change: taking action against domestic air pollution, such as indoor cooking fires, by properly venting dwellings, and working to reduce outdoor pollutants by lobbying for cleaner factories and motor vehicles.
Research and Precautions
The experts conclude that expanded research and monitoring are needed to give early warning of potential health effects of climate change, and they recommend precautionary measures to reduce greenhouse gas emissions in agriculture, energy supply, forestry, industry, human settlements, and transportation.
"We must become familiar with the scale and complexity of the problem, its inherent uncertainties, and the attendant need for precautionary inferences and policies," say the heads of the three agencies. Also needed are "preventive approaches to reduce the pace and magnitude of climate change, in light of the possible irreversibility of some of the ensuing environmental/ecological changes and the range of potential adverse health effects that may result," as well as development of adaptive responses, "particularly those that would have immediate benefits for health."
In 1992 at the Rio Earth Summit, governments signed a treaty committing themselves to cutting greenhouse gas emissions to 1990 levels by the year 2000.
The insurance and banking industries face massive potential losses as well. More than 30 global insurance firms have signed an agreement with the U.N. Environment Program committing themselves to help reduce environmental risk and address climate change, fearing that major disasters caused by extreme climate events could bankrupt the industry. Bankers focusing on long-term trends, such as Union Bank of Switzerland Vice President Sven Hansen, say, "We have to be concerned and we must recognize that the financial markets will be affected by climate change."
Jeremy Leggett, a former professor at London's Imperial College and now a lobbyist for Greenpeace, has worked to convince insurance and banking sectors that they should divert some of their investment capital to solar and other renewable energies to cut back on the 6 billion tons of carbon dioxide and other greenhouse gases that are pumped into the atmosphere annually. Much of the US$ 1.4 trillion collected in premiums by the insurance industry each year is reinvested, he says. "A lot of it goes to fossil fuels, which only make things worse, and almost none to solar and other renewables. We'd like to reverse that."
Daniel Epstein is a Washington, D.C.-based journalist in charge of media relations for PAHO's Office of Public Information.
