While some studies have shown an association between short-term climate variability, such as that caused by El Nino, and a higher incidence of certain illnesses, numerous other factors must be considered to fully account for the spread of infectious diseases, said the committee that wrote the report.

"The potential exists for scientists one day to be able to predict the impact of global climate change on disease, but that day is not yet here," said committee chair Donald Burke, professor of international health and epidemiology, Johns Hopkins School of Public Health, Baltimore.

"To help scientists and policy-makers gain a better understanding, our committee recommends strong support from the federal government for interdisciplinary efforts to analyze climate- disease relationships.

"The critical capabilities that must be strengthened and brought together include epidemiological surveillance, field ecology, computer modeling and simulation, and evaluation of public health interventions."

Since the time of Hippocrates, scientists have understood that weather can influence where and when some epidemics occur. For instance, mosquito-borne diseases such as dengue, malaria, and yellow fever are generally associated with warm weather, while influenza epidemics usually occur during cold weather, and outbreaks of intestinal illnesses caused by cryptosporidiosis are linked to heavy rainfall.

In recent years, as research has confirmed the likelihood of a long-term global warming trend, questions have been posed as to what effect this might have on infectious disease patterns around the world, prompting numerous studies of climate-disease linkages.

Some of these studies have shown that climate variation from one season or year to the next can affect the life cycle of many pathogens and disease-carrying insects, potentially affecting the timing and intensity of disease outbreaks.

A number of computer models have been developed as well to simulate the effects of climate change on disease incidence, but estimates of the extent to which diseases will potentially spread have varied significantly among some of the models.

In addition, observational and modeling studies generally are not able to consider complex social factors -- such as sanitation and public health services, population density, and travel patterns -- that also play important roles in disease dynamics.

Because of this, the results of such studies must be interpreted with caution, especially when used to develop scenarios of the potential health effects of future climate change, the committee said.

Indeed, the highly uncertain impact of long-term climate change on the spread of disease and on the evolution and emergence of new pathogens stems largely from the confounding influences of human behavioral adaptations and public health interventions.

For instance, basic public health protections such as adequate housing and sanitation, as well as the availability of vaccines and drugs, can limit the geographic distribution of diseases regardless of climate.

One example of this is along the border between the United States and Mexico, where dengue fever outbreaks are common just south of the Rio Grande river in Mexico, but are rarely seen in neighboring regions just north of the river in the United States, mainly because of differences in socio-economic conditions.

The report also notes that there are potential pitfalls in extrapolating climate and disease relationships from one time scale to another. For example, the ecological effects of short-term climate events, such as El Nino, may be significantly different from the ecological effects and social adaptations expected under long-term climate change.

Research to understand the relationship between climate and infectious disease is in its infancy and needs to be strengthened, the committee said. Interdisciplinary research centers should be established to foster collaboration between scientists in fields such as epidemiology, climatology, and ecology.

Moreover, federal health agencies such as the Centers for Disease Control and Prevention and the National Institute of Allergy and Infectious Disease should become actively involved in the U.S. Global Change Research Program, an organization that coordinates climate research among federal agencies.

Recent technological advances, such as gene-sequencing techniques for studying the molecular biology of disease-causing pathogens and satellite-based remote sensing of ecological conditions, should be used to aid research efforts, the committee added.

In addition, to overcome the existing lack of high-quality epidemiological data for most diseases, a concerted global effort should be made to collect long-term disease surveillance information, along with the appropriate meteorological and ecological observations, and store it in centralized, accessible databases.

As the potential linkages between climate and disease become better understood, it may be possible to provide early warnings to help prevent disease outbreaks, the committee said.

To do this, climate forecasts will have to be complemented by ongoing meteorological, ecological, and epidemiological surveillance systems. Together, this information could be used to issue a "watch" for regions at risk and subsequent "warnings" as surveillance data confirm earlier projections.

The committee emphasized, however, that early warning systems should not take the place of proven public health measures since there will always be some element of unpredictability in climate variations and disease outbreaks. Officials should continue to place a high priority on reducing people's overall vulnerability to infectious disease through vaccination programs, mosquito-control efforts, and water-treatment systems, it said.

The report was sponsored by the U.S. Environmental Protection Agency, Centers for Disease Control and Prevention, National Science Foundation, NASA, National Oceanographic and Atmospheric Administration, U.S. Geological Survey, U.S. Global Change Research Program, and the Electric Power Research Institute. The National Research Council is the principal operating arm of the National Academy of Sciences and National Academy of Engineering. It is a private, nonprofit institution that provides scientific and technical advice under a congressional charter.

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