Climate and Health

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Science  16 Jul 1999:
Vol. 285, Issue 5426, pp. 347-348
DOI: 10.1126/science.285.5426.347

Extreme weather events—unusually heavy rainfall or long periods of drought—have a profound impact on public health, particularly in developing countries, and the aftershocks are rippling through economies worldwide (1). For example, Hurricane Mitch [HN1]—nourished by a warmed Caribbean—hit Central America in November 1998, killing more than 11,000 people and causing damage exceeding $5 billion. The intense precipitation and flooding associated with the hurricane spawned a cluster of disease outbreaks, including cholera, a waterborne disease (>30,000 cases), and malaria and dengue fever [HN2], transmitted by mosquitoes that flourish under these conditions (>30,000 and >1000 cases, respectively). If more frequent and intense extreme weather events continue to be a primary manifestation of climate change (2), then harnessing climate data to better forecast future disease outbreaks should enable preventive action to be taken.

In a report on page 397 of this issue, Linthicum et al. [HN3] (3) examine links between periods of heavy rainfall in East Africa between 1950 and 1998 and outbreaks of Rift Valley fever [HN4], a mosquito-borne viral disease that infects both domestic animals and humans (see the figure). In the Horn of Africa, records since 1950 indicate that Rift Valley fever outbreaks follow periods of intense precipitation. During the 1997–98 El Niño event, the Horn received up to 40 times the average rainfall, isolating villages, obliterating roads, and precipitating a cluster of diseases: tens of thousands of new cases of cholera and malaria and 89,000 cases (with nearly 1000 deaths) of Rift Valley fever (4). Linthicum and colleagues show that by tracking sea surface temperature anomalies of the Pacific and Indian oceans and combining these data with vegetation changes detected by remote sensing satellites [HN5] (3), they were able to forecast Rift Valley fever epidemics 5 months in advance of outbreaks. Such early warnings would give sufficient time for interventions, such as vaccination of livestock and treatment of mosquito breeding sites. Using climate data to project conditions that are conducive to disease outbreaks will be invaluable in combating the burden of extreme weather events on public health.

Predicting disease outbreaks.

The map shows regions of heavy rainfall and drought during 1997–98 and the associated clustering of outbreaks of emerging infectious diseases. Extreme weather events have resulted in a surge in epidemics, particularly in tropical regions. Using climate data to predict the arrival of conditions that are likely to favor disease outbreaks can facilitate public health interventions, such as vaccination and preparations at treatment facilities.


Connections between climate and disease are not new [HN6] (5). Climate constrains the range of many infectious diseases, and weather affects the timing and intensity of outbreaks (6). Reappraisal of these associations is aided by increased understanding of the Earth's climate system, in particular how land and sea surface temperatures and pressure gradients drive winds and weather. The atmosphere holds 6% more water vapor with each 1°C rise in temperature. The resulting increase in evaporation and greater residence time for water vapor in the atmosphere [HN7] (7) boosts humidity and heat indices, fuels storms, and reinforces the greenhouse effect (the trapping of heat by atmospheric gases such as carbon dioxide). An increase in the cloud cover blocks outgoing heat, contributing to disproportionate warming at night and during the winter (8)—conditions that are unhealthy for humans but advantageous for insects that transmit infectious diseases. A moisture-laden atmosphere also generates more tropical-like downpours that create breeding grounds for mosquitoes, propel rodents from burrows, and flush nutrients, chemicals, and microorganisms into waterways.

Sudden weather changes and sequential extremes can also yield surprises. Droughts suppress predators, whereas heavy rains boost food supplies—a synergy that can spark rodent population “explosions.” A large increase in the deer mouse population in the southwestern United States in 1993 resulted in the emergence of Hantavirus Pulmonary Syndrome [HN8] (9). In Honduras, drought-sustained wildfires consumed 11,000 km2 of forest during the summer preceding Mitch (4), widening the deforestation that magnified the flooding and devastation from the hurricane (1, 10).

Understanding the evolution of such weather anomalies will require integrating data from the El Niño-Southern Oscillation [HN9] (ENSO; the anomalous warming or cooling of the eastern Pacific) with local sea surface temperatures (3) and, eventually, with decadal-to-centennial cycles in climate variability and human influences. Changes in atmospheric chemistry may have so altered Earth's heat budget that natural climate modes such as ENSO have been modified. Studies suggest that the ocean is becoming warmer at intermediate depths and around both poles (11). If the world's oceans are a long-term heat sink for this century's global warming [HN10], then this has profound implications for marine life and terrestrial weather patterns.

Historically, periods of accelerated social transition have often been accompanied by the reemergence of infectious diseases [HN11] (12). The current resurgence (13) can be attributed to changes at three levels—social (economic disparities and untoward practices such as excessive use of antibiotics and pesticides), ecological (habitat loss and simplification), and global (alterations in climate and stratospheric ozone). Ecological integrity is central to health. Habitat mosaics (for example, wetlands and forests) absorb floodwaters and support the genetic, species, and functional group diversity that ensures resilience to stress and resistance to pests and pathogens (14).

Extended droughts and thawing during warmer winters render northern forests vulnerable to infestations with insect pests. Parched agricultural lands attract aphids, locusts, and virus-bearing white flies, and fungi flourish after floods. Warming and “CO2 fertilization” causing enhanced plant growth may also encourage leaf-eating pests, favor weeds, and promote pollen production (15). Already, 35 to 42% of growing and stored crops are lost to pests, pathogens, and weeds, costing $244 billion worldwide annually (16). Increased climate variability could substantially alter future food security and global nutrition (17).

Emerging diseases associated with algal toxins, bacteria, and viruses are affecting a wide range of marine species: fish, shorebirds, and mammals (18). Of great concern are diseases that attack coral and sea grasses, essential habitats that sustain mobile aquatic species. Corals are already endangered: High sea surface temperatures in the 1990s have resulted in widespread bleaching [HN12], perhaps the most disturbing biological sign of global warming.

With disproportionate warming in the winter, at higher latitudes and high elevations (19), most summit glaciers are in retreat [HN13]. Polar researchers suspect that melting at the base of the Greenland ice sheet may be sculpting fault lines that could diminish its stability (20). Shrinking of Earth's ice cover (cryosphere) has implications for water (agriculture, hydropower, and health) and for climate stability. The impacts of warming and changing weather patterns on forests, agriculture, marine life [HN14], and water may hold the most profound consequences for global health.

The cost of extreme weather events and associated emerging infectious diseases is mounting. Trade, travel, and tourism can be affected. In 1998, livestock exports were blocked from East Africa because of Rift Valley fever, and Europe refused seafood imports because of cholera outbreaks in the same region. Indonesian forest fires (the extended drought compounding hazardous land-clearing practices) resulted in widespread acute and chronic respiratory illnesses and plummeting rice yields (1). The 1998 summer floods in China killed 3700 individuals, displaced 223 million people, and cost $30 billion. All told, weather-related losses—combining growth of coastal settlements, ecological vulnerabilities, and extreme weather—grew exponentially from the 1980s to the 1990s; losses of $89 billion in 1998 (11 months) eclipsed the losses of $55 billion for the entire decade of the 1980s (21).

There are several solutions to combating the increased burden of emerging infectious diseases. Greater surveillance of and response to outbreaks is essential (22). Health early-warning systems (23) based on climate forecasting and remote sensing, such as the system developed by Linthicum and colleagues for Rift Valley fever, can complement famine early-warning systems. Prevention is possible if the underlying social and environmental causes are addressed.

Contemporaneous changes in greenhouse gas concentrations, ozone levels, the cryosphere, ocean temperature, land use, and land cover challenge the stability of our epoch, the Holocene—a remarkable 10,000-year era that followed the retreat of the great ice sheets from temperate zones. High-resolution ice core records suggest that greater variance from climate norms may indicate greater climate instability (24), increasing the potential for rapid shifts between stable climate states (25).

In the 1980s, health considerations helped forge international agreements banning ozone-depleting chemicals and atmospheric nuclear testing. Today, concerns for our health in the face of global change necessitate ecological restoration and development of nonpolluting energy sources.

Clean energy helps to stabilize the climate and also can be used to power health facilities, pump water for irrigation, and purify it for consumption. Renewable and energy-efficient technologies may become the new engine of economic growth, driving improvements in public health. Ultimately, we must shed inherited economic burdens and adopt new financial mechanisms—incentives, subsidies, and funds—to reverse environmental assaults on public health, preserve the global commons, and achieve healthy, clean, and equitable development in the coming century.

HyperNotes Related Resources on the World Wide Web

General Hypernotes

Planet Earth and the New Geosciences, a Web textbook from the University of Pittsburgh, includes units titled “The climate puzzle: The Atmosphere” and “The Climate puzzle: Climates of the Earth.”

A presentation on Global Warming and Climate Change is offered by the U.S. Global Change Research Information Office.

A report on the science of climate change titled Climate Change: State of Knowledge from the office of the White House Initiative on Climate Change includes a chapter on vulnerabilities and potential consequences.

G. Lash, Department of Geosciences, Fredonia State University College, Fredonia, NY, provides lecture notes for a course on catastrophic weather and climatic change.

E. Takle, Department of Geological and Atmospheric Sciences, Iowa State University, offers lecture notes for an Internet course on global change.

The Department of Atmospheric Sciences, University of Illinois, provides online guides to meteorology and remote sensing as part of its Weather World 2010 Web site.

The National Climatic Data Center (NCDC) of the U.S. National Oceanic and Atmospheric Administration (NOAA) offers a page about climatic extremes and weather events.

An essay by A. Krupnick titled “Climate change, health risks and economics” is available from Resources for the Future in its Weathervane section on global climate policy.

The Physicians for Social Responsibility maintain a Global Climate Change and Human Health Web site.

The Center for Health and the Global Environment at Harvard Medical School offers a course on human health and global environmental change; course lecture notes for the previous year are also available.

Consequences (vol. 3, no. 2, 1997) had an article by P. Epstein titled “Climate, ecology, and human health.” An online archive of the issues of Consequences, a journal about the nature and consequences of environmental change, is provided by the U.S. Global Change Research Information Office.

CIESIN (Center for International Earth Science Information Network) at Columbia University presents a guide to human health and global environmental change. Included is an overview with links to documents that provide more information. Other thematic guides on the human dimensions of global environmental change are offered by CIESIN.

A chapter titled “Linking environment and health” is included in World Resources 1998-99:Environmental Changes and Human Health, a publication of the World Resources Institute. The chapter titled “Changing environments, changing health” includes a section on climate change and health.

The WWF-World Wide Fund For Nature provides a Climate Change Web site. The section on climate impacts offers a presentation by P. Epstein titled “Global warming: Health and disease,” as well as reports on climate change and health and southern Africa and climate change.

The United Nations Framework Convention on Climate Change Web site provides a Climate Change Information Kit, which includes a fact sheet on the consequences of climate change to human health.

The El Niño - Southern Oscillation (ENSO) Home Page from the NOAA Office of Global Programs provides extensive information and links to Internet resources. Included are a section on how changes in the ENSO system affect regions of the world and a section on ENSO effects on ecosystems and society.

The International Research Institute for Climate Prediction has as its mission to assess and develop climate forecasts, and to foster the application of such climate forecasts to the explicit benefit of societies. The Experimental Climate Forecast Division offers presentations on El Niño and ENSO-Related Impacts.

The mission of NOAA's Climate Prediction Center is to maintain a continuous watch on short-term climate fluctuations and to diagnose and predict them in order to provide assistance with such climate-related problems as food supply, energy allocation, and water resources. A discussion of typical impacts of warm (ENSO) and cold episodes and a report on the impact of the 1997-98 El Niño are provided.

Our Planet, published monthly by the United Nations Environmental Programme, had issues devoted to climate change and climate and action.

The Famine Early Warning System (FEWS), a project of the U.S. Agency for International Development, is an information system designed to help prevent famine in Sub-Saharan Africa. FEWS specialists in the U.S. and Africa assess remotely sensed data and ground-based meteorological, crop, and rangeland conditions for early indications of potential famine areas. Data from the FEWS archive are available through the U.S. Geologic Survey's Africa Data Dissemination Service.

The DisasterRelief Web site offers a 25 June 1999 article by C. Long titled “International Red Cross predicts more global super disasters” about the World Disasters Report 1999. The International Federation of Red Cross and Red Crescent Societies provides summaries of the chapters of the report with links to Internet resources included.

Numbered Hypernotes

1. NOAA NCDC provides a report on Hurricane Mitch titled “Mitch: The deadliest Atlantic hurricane since 1780.” The U.S. Agency for International Development issued a fact sheet on 19 November 1998 about the impact of Hurricane Mitch on Central American countries. The World Health Organization (WHO) provides a news release from the Pan American Health Organization about disease outbreaks attributed to Hurricane Mitch. The U.S. Geological Survey's Center for Integration of Natural Disaster Information provides information about Hurricane Mitch on its Disaster in Central America Web page.

2. The WHO Communicable Disease Surveillance and Response (CSR) division provides a page of information about cholera, including a basic fact sheet. Information on malaria and dengue prevention and control is provided by WHO's Division of Control of Tropical Diseases. A fact sheet on dengue is available from WHO CSR. The U.S. Centers for Disease Control and Prevention (CDC) provides information about malaria, cholera, and dengue fever. The International Research Institute for Climate Prediction provides information about the correlation between malaria and El Niño in Colombia. The Guide to Nursing offers a feature about El Niño and dengue fever.

3. K. J. Linthicum is in the Division of Preventive Medicine, Walter Reed Army Institute of Research, Washington, DC.

4. The National Geographic Society presents a map of Africa with links to information and maps for each country; the map of Kenya shows the location of the Great Rift Valley. An illustrated section on the African rift is included in the tectonic landforms chapter of Geomorphology from Space, available from the NASA Goddard Distributed Active Archive Center. An educational module on Rift Valley fever is presented by the NASA Classroom of the Future project; background information on Kenya and the Rift Valley, and a section on the use of remote sensing are included. The CDC's Special Pathogens Branch offers a fact sheet about Rift Valley fever. WHO provides a fact sheet on Rift Valley fever. WHO CSR provides a disease outbreak report about Rift Valley fever in Kenya and Somalia; an outbreak news index for Rift Valley fever and other hemorrhagic fevers is available.

5. The USAID Famine Early Warning System offers a presentation on tracking El Niño; information on satellite vegetation analysis and sea surface temperature indicators is available on the Eastern Africa and the Horn Satellite Imagery section of the Weather Analysis page. The Greater Horn of Africa Web site from U.S. Geological Survey's EROS Data Center provides information on climate variability and greenness in that East African region. N. Short's Remote Sensing Tutorial, made available by NASA Goddard Space Flight Center, includes sections on the AVHRR Vegetation Index of Africa, oceanographic observations, and the Rift Valley of Kenya. NOAA's National Environmental Satellite, Data, and Information Service offers images of sea surface temperatures. Sea surface temperature is discussed in a presentation on El Niño from the Earth Space Research Group, University of California, Santa Barbara. CIESEN offers a guide to the use of satellite remote sensing to study the human dimensions of global environmental change.

6. WHO offers a presentation on climate and health. The Global Warming Web site from the U.S. Environmental Protection Agency has a section on health concerns. Summary information from the 1996 WHO report Climate Change and Human Health is available from the U.S. Global Change Research Program. The ENSO Experiment, sponsored by the NOAA Office of Global Programs, is an interdisciplinary research effort to study the human health impacts of the current ENSO event and to explore the potential for applying forecast information in the public health arena. The El Niño - Southern Oscillation (ENSO) Learning Module, developed by the Department of Geology, University of Florida, includes a section on ENSO and infectious disease.

7. The Science for Everyone Web site from the American Geophysical Union (AGU) provides a special conference report titled “Water vapor in the climate system.”

8. The CDC's Special Pathogens Branch offers a presentation “All about Hantavirus.” A discussion of whether El Niño could cause an outbreak of Hantavirus is included. The April-June 1997 issue of Emerging Infectious Diseases had an article by C. Schmaljohn and B. Hjelle titled “Hantaviruses: A global disease problem.”

9. U.S. National Oceanic and Atmospheric Administration (NOAA) presents an El Niño Page. The El Niño Scenario from the Earth Space Research Group, University of California, Santa Barbara, provides an illustrated hyperlinked introduction to El Niño, including sections on its mechanisms and effects. NOAA's Pacific Marine Environmental Laboratory provides an El Niño Theme Page; a presentation on the impacts of El Niño and the benefits of El Niño prediction is included. The Center for Ocean-Atmospheric Prediction Studies, Florida State University, Tallahassee, provides an extensive collection of links to Internet resources on El Niño, as well as information on their ENSO research. A presentation titled “Understanding ENSO and forecasting drought” is available from the National Drought Mitigation Center at the University of Nebraska, Lincoln. The NOAA Climate Diagnostics Center provides an El Niño/Southern Oscillation (ENSO) Information Web page. A report prepared by the University Corporation for Atmospheric Research titled El Niño and Climate Prediction is made available by the Department of Atmospheric Sciences, University of Washington.

10. AGU's Science for Everyone Web site provides an article by C. Sabine, D. Wallace, and F. Millero titled “Keeping our cool: Does the ocean dampen the greenhouse effect?”

11. Emerging Infectious Diseases is a journal published by the CDC's National Center for Infectious Diseases (NCID); the NCID provides links to Internet resources on emerging infectious diseases. An article by S. Morse titled “Factors in the emergence of infectious diseases” appeared in the January-March 1995 issue of Emerging Infectious Diseases. An essay by A. Jacobson titled “Emerging and re-emerging viruses” is provided by the Institute for Molecular Virology, University of Wisconsin.

12. NOAA News provides a news story titled “El Niño-related drought and heatwave over Australia causing coral bleaching at the Great Barrier Reef.” Status of Coral Reefs of the World: 1998, a report from the Australian Institute of Marine Science (AIMS), includes a chapter on the worldwide 1997-1998 mass coral bleaching event. Reefs at Risk, a 1993 publication about coral reefs, human use, and climate change, is made available by AIMS, as are links to further information on corals and coral bleaching. T. Turner, Division of Science and Mathematics, University of the Virgin Islands, provides an introduction to coral reefs that includes information on coral bleaching and other threats to reefs. The International Coral Reef Initiative, a partnership among nations and organizations seeking to implement international conventions and agreements for the benefit of coral reefs and related ecosystems, provides links to information on coral reefs. NOAA's National Environmental Satellite, Data, and Information Service offers information on global coral bleaching hotspots. The WWF offers a presentation on coral reefs and climate change.

13. Scientific American presents an exploration feature titled “Going, Going—Gone?” about the melting of glaciers. The August 1998 Scientific American Presents had an article by D. Schneider titled “The rising seas.” The Earth Surface Dynamics component of the U.S.G.S. Global Change Research Program provides a fact sheet on the satellite image atlas of glaciers of the world.

14. WWF offers a report by N. Dudley and A. Markham titled “Global warming: Impacts on forests.” CIESIN presents a guide to agriculture and global environmental change. The Guide to Agriculture presents a feature titled “El Niño threatens global food supplies” and a follow-up titled “El Niño redux.” An overview titled “The effects of El Niño on marine life” by A. Forrester is a available in the Hot Topics Series offered by Cambridge Scientific Abstracts; citations and Web resources are included.

15. P. R. Epstein is at the Center for Health and the Global Environment, Harvard Medical School, Boston.

References and Notes

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