Scientists have long theorized that climatic changes related to global warming could unleash outbreaks of diseases like malaria, dengue fever, cholera and heat stroke. But with the modest amount of warming experienced so far, they have been unable to produce much hard evidence.
Now the experts have a research gift from an unlikely benefactor: El Niņo. Combined with an underlying global warming trend, say scientists who track such things, El Niņo will probably make 1998 the warmest year in several centuries. And the heavy rains, droughts and other extreme weather fostered by El Niņo over the last year are precisely what many scientists expect will also result from a predicted warming of the globe in the decades ahead.
So, in the belief that 1997 and 1998 may provide a taste of what is to come, those who pay attention to the health effects of climate are now pulling together data about weather-related illness and death in recent months. Though most of the evidence is circumstantial, consisting of correlations and coincidences, it suggests that amid all the scientific smoke there is some fire.
These are some of the clues:
Heat and variations in rainfall affect transmission of the disease by mosquitoes.
In both cases the surge in cases was associated with heavy rainfall and floods linked to El Niņo.
The number of mice has increased, and the deaths of three people have been ascribed to the disease. Federal disease-control agents are still analyzing the problem.
While this and similar evidence is sifted and tested, scientists are also beginning to focus more closely on the health impact of extreme weather events quite apart from El Niņo, especially in the United States. One thing they are finding is that there is a statistically significant association in the United States between heavy rainstorms and outbreaks of waterborne diseases.
These include hepatitis, E. coli infections and the cryptosporidium parasite that infected 400,000 people and killed 50 to 100 in Milwaukee in 1993, as well as other gastrointestinal diseases caused by viruses, bacteria and parasites.
In East Africa, Model and Portent
f the relationship between heavy
rain and disease is a solid one and
climate experts are right, the country may be in for a surge of climatically induced waterborne illness.
Analyses of the last century's weather records by the National Climatic Data Center have found that extremely heavy rainstorms have become more common.
For example, single rainfalls on the order of eight and nine inches each deluged the Boston area in mid-June, the Ohio Valley in late June and south-central Tennessee in mid-July.
Climatologists say that this is just the kind of increase in extreme rainstorms that would be expected in a warming atmosphere, which holds more moisture and causes more water to evaporate from the oceans. The heavier rain that results is more likely to cause floods that not only kill people but also overwhelm drinking-water purification systems.
"If extreme weather events are part of a changing climate," said Dr. Paul R. Epstein, associate director of the Center for Health and the Global Environment at Harvard Medical School, "we've seen lots of evidence of the profound health effects associated with climate change this year."
East Africa, he said, especially illustrates the risk to health posed by climatic factors.
There, simultaneous outbreaks of cholera, malaria and Rift Valley fever followed heavy rains and flooding.
Dr. Epstein has long held the view that global warming will have a serious effect on health.
His group at Harvard has also made a compilation of possible health effects of El Niņo in the last year.
The view has provoked argument. In the always contentious debate over climate change and the uncertainties that attend it, it has been especially difficult to get a handle on the question of health and disease. The Intergovernmental Panel on Climate Change, an international group of scientists advising the United Nations, recognized this in its most recent report, issued in 1995.
The panel predicted that if emissions of heat-trapping industrial waste gases like carbon dioxide were not reduced, the average surface temperature of the earth would rise by about 3.5 degrees Fahrenheit by the end of the next century.
This is roughly half the amount of warming experienced since the depths of the last ice age. About a degree of warming has taken place in the last century, and the latest surface readings combined with tree-ring studies made by climatologists at the University of East Anglia in Britain indicate that 1997 was the warmest year in the last 1,000 and that 1998 will probably be warmer still.
Some recent events, the intergovernmental panel said in 1995, might plausibly be early signals of climate-induced changes in human health. The panel cited a surge in heat-related deaths in India in 1995 (there was a similar surge this year), changes in the ranges of some insect-borne diseases and the spread of cholera along coastal areas in some developing countries.
But the group also said it was "not possible to attribute particular, isolated events to a change in climate or weather pattern."
The problem, said Dr. Duane Gubler, director of the division of vector-borne infectious diseases at the Federal Centers for Disease Control and Prevention in Atlanta, "is that we may see some correlations" between climate and disease outbreaks, "but we don't know whether it's cause and effect." So, he said, "we're kind of cautious" in tying climatic change to changes in health.
More Than One Way to Kill a Mosquito
Not a few experts point out, for instance, that while a warming atmosphere would theoretically be expected to expand the range of mosquito-borne tropical diseases like malaria and dengue fever, reality is not so simple.
Temperature, humidity, rainfall and other weather factors are indeed some of the main things that influence transmission of the diseases. In some instances, temperature increases the virulence of the disease itself, by prompting viruses to multiply more.
Heavier rainfall creates more standing water for some kinds of mosquitoes to breed in, and populations soar.
Drought encourages people to store water in open containers, with the same result.
But in the developed countries, experts like Dr. Gubler say, public health pluses like pest control and better living conditions (more tightly built houses, for example) have pretty much slammed the door on epidemics of mosquito-borne illness. Until early this century, malaria, dengue fever and yellow fever were common in the United States. Today, outbreaks of those diseases are small, isolated and rare.
Usually, says Dr. Gubler, the cases have arrived from somewhere else.
This public-health success, a number of experts say, would probably stop any climate-induced invasion by mosquito-borne diseases at the United States border -- as indeed might have happened already: dengue fever, resurgent in the Caribbean and Central America in recent years, has not become a major problem in the United States, even though, Dr. Gubler says, the temperature along the Gulf Coast is higher than in the Caribbean.
While Texas reported only a handful of cases in 1995 and again in 1997, said Dr. Gubler, there were thousands of cases a stone's throw away, in Mexico.
Dr. Gubler, Dr. Epstein and many others agree that developing countries, with less effective and in some cases nonexistent controls on mosquitoes, would be highly vulnerable to any climatically induced surge in diseases borne by vectors, or animals, usually insects, that carry a disease from a host to another animal.
They also agree that efforts to improve public health measures there and limit climatic change globally are necessary.
But the richer countries, including the United States, may not be immune to climate-sensitive health problems.
Two big cases in point are heat waves and waterborne diseases.
Many climate experts say that heat waves are likely to become more frequent and intense, and therefore more deadly. If they are right, what has happened in Texas this summer may be a foretaste. More than 100 people have died there in a heat wave that saw temperatures in Dallas rise above 100 degrees Fahrenheit for weeks on end.
It was even worse in India, where the most intense heat wave in 50 years sent the temperature soaring above 120 degrees in some places. Nearly 1,300 people died.
Skeptics point out that people in the southern United States have become acclimatized to heat, and argue that northerners will adapt if the climate warms.
But Dr. Laurence S. Kalkstein of the University of Delaware, who has long studied the subject, says that people become vulnerable to heat whenever it reaches a certain threshold above what they are used to -- even if they are used to high temperatures. The lethal Southern and Indian heat waves this year may provide some evidence of this.
The important unanswered question, Dr. Kalkstein said, is whether heat waves will get hotter with the general climate.
"If New York's climate becomes more like Jacksonville's and all the days stay about the same in the summer," he said, "I think New Yorkers will acclimatize." But if hotter heat waves are then superimposed on top of a generally warmer climate, "we are going to get more deaths," he said.
Skeptics also point out that cold-related deaths would probably drop in the winter.
But Dr. Kalkstein believes that heat-related deaths will more than offset this.
Many cold-season deaths, he explains, result from diseases like flu that are transmitted in confined spaces in buttoned-up winter buildings.
Climate change is not likely to alter this picture much, Dr. Kalkstein says.
Tracking Diseases Borne by Water
Experts are just beginning to get a firm handle on weather-related outbreaks of waterborne disease in the United States.
One problem is that heavy rainfalls tend to overwhelm sewage-treatment systems early in a flood, catching those who operate the systems unprepared.
As a result, disease organisms "ride the initial wave that comes downstream," in the first hour or two of a storm, infecting people before anything can be done, said Dr. Dennis Juranek, the associate director of the division of parasitic diseases at the Centers for Disease Control and Prevention. The difficulty is compounded by the fact that outbreaks of diarrheal diseases, the most common waterborne maladies, are not usually perceived as a public-health emergency -- even though they are sometimes fatal to people whose immune systems are depressed -- and therefore are probably underreported.
Diarrheal diseases do not "cause people to fall over or bleed out of the eyes or mouth," Dr. Juranek said, adding, "It has to be something big like Milwaukee" to gain attention. Even in Milwaukee, "we came within a hair's width" of failing to find out that the cryptosporidium outbreak of 1993 was waterborne.
Scientists at the University of South Florida, the University of Maryland and the National Climatic Data Center are taking some of the first steps in pinning down the cause and effect between climate and disease. They have plotted more than 300 outbreaks of waterborne disease over the last 50 years against the distribution of heavy rains.
Although the data are still being analyzed, "we can demonstrate increased probability of waterborne outbreaks when there are extreme events," said Dr. Joan Rose, a microbiologist at the University of South Florida.
In 1982-83, the last big El Niņo period before 1997-98, 60 to 70 percent of the outbreaks were associated with extreme rainfalls, she said.