At the dawn of the twenty-first century, the population of the United States as a whole is one of the healthiest in the world. The socioeconomic development of the last century and a half both allowed for a vast improvement in sanitation and nutrition, and provided resources for the development and maintenance of a generally effective public health system. While current health concerns in this country revolve largely around lifestyle factors such as diet, alcohol use, and physical inactivity, climate change raises the possibility that environmental factors including higher temperatures and increased occurrence of infectious diseases — could become a growing problem.

“Human Health and Global Climate Change” is the sixth in a series of Pew Center reports evaluating the potential impacts of climate change on the U.S. environment and society. The report finds that, in general, the United States should have sufficient resources to limit climate change impacts on human health over this century. At the same time, because the linkages between climate and human health are often complex and not well defined, it is difficult to predict exactly how climate change will impact human health in the United States. Nevertheless, there are some important findings worthy of our attention:

• Higher temperatures are likely to negatively affect health by exacerbating air pollution and increasing the occurrence of heat waves. The elderly, infirm, and poor are most at risk because these conditions can exacerbate pre-existing disease. Lack of access to air conditioning increases the risk of heat-related illness.
• While there is some indication that changing climatic conditions may increase the risk of vector- and water-borne diseases, sanitation and public health system infrastructures in the United States should prevent these diseases from becoming widespread. To prevent such outbreaks, it is vital that we take steps to maintain and strengthen these infrastructures, including increased surveillance and vector control. At the same time, global health impacts from infectious diseases will almost certainly be greater, as many countries lack either the resources and/or infrastructures to protect their populations.
• Uncertainty about adverse health effects should not be interpreted as certainty of no adverse health effects. Moreover, the potential for unexpected events — e.g., sudden changes in climate or the emergence of new diseases cannot — be ruled out.

The authors and the Pew Center gratefully acknowledge Drs. Kris Ebi, Duane Gubler, and Jonathan Patz for their review of previous drafts of this report. This report also benefited from comments received at the Pew Center’s July 2000 Workshop on the Environmental Impacts of Climate Change. The Pew Center would also like to thank Joel Smith and Brian Hurd of Stratus Consulting for their management of this Environmental Impacts Series.

Executive Summary

The population of the United States is among the healthiest in the world, although there are disparities in life expectancy, infant mortality, and other indices of health among different groups within the U.S. population. The main determinants of disease-related mortality in the United States today are lifestyle factors — tobacco use, alcohol use, dietary intake of calories and fats, sexual behavior, and physical inactivity. The national level of economic and social development in this country has generally provided resources to address critical health determinants such as nutrition, sanitation, and housing quality. In addition, the United States devotes a large amount of resources to health care and maintains a relatively effective public health infrastructure.

This report on the effects of climate change on human health in the United States finds that the complexity of the pathways by which climate affects health represents a major obstacle to predicting how, when, where, and to what extent global climate change may influence human well-being. Some linkages are strong and clearly defined, whereas other important connections are made difficult to define by being variable, region-specific, or mediated through multiple intervening steps.

Mortality from heat waves has been predicted to increase under most scenarios of climate change. The degree to which heat-related mortality rates increase will be determined by the ability to implement early warning systems and other interventions that focus on at-risk populations, as well as by the frequency of extreme heat waves and the changes in daytime temperature variation under future climate regimes. It is less clear whether warmer winter temperatures will result in a significant decline in wintertime mortality from cardiovascular disease.

If extreme precipitation events become more frequent, and sanitation and water-treatment infrastructure is not maintained or improved, an increase in water-borne infections may result. People are also at risk of injury or death from exposure to extreme climate events such as floods, hurricanes, and tornadoes. The public health burden of such events, however, partly depends on the ability to anticipate them, and the education and emergency response planning that may reduce impacts. In addition, current climate models are not able to confidently predict the future frequency of such events, although there has been a trend toward heavier precipitation events during the twentieth century.

Global climate change may affect human respiratory health by changing levels of air pollutants and pollens. For the United States, impacts of climate change on tropospheric, i.e., ground-level, ozone are both more certain and likely to be more important than impacts on other air pollutants. This is due to the importance of temperature in the formation of ozone as well as the large areas of the country currently affected by ozone levels exceeding national standards. Nonetheless, to date, no published studies have modeled the health impacts in the United States due to climate change effects on air pollutants.

In the United States, improved housing, sanitation, and public health interventions have controlled most of the infectious disease risks that are felt to be most climate sensitive (e.g., dengue, malaria, cholera). Of greatest concern are insect vector-borne infections that may increase as the result of changing climate. However, the multiple determinants of vector-borne disease risk and the complexity of transmission dynamics make estimating future patterns of disease difficult. In addition to climate, the risk of many vector-borne diseases is linked to lifestyle, hygiene, housing construction, trash removal, and a host of other socially- and economically-based factors. Thus, infectious disease risk may increase or decrease with climate change, depending upon the interplay of the above factors within a specific region.

For the United States, the success of public health interventions in eradicating malaria and other vector-borne diseases early in the twentieth century underscores the importance of continued public health surveillance and prevention in protecting the U.S. population from any climate-induced enhancement in vector-borne disease transmission. Maintenance and strengthening of public health infrastructure, especially surveillance and vector control, will be critical to preventing significant outbreaks in the future. Inclusion of public health and climate change experts in planning regarding land-use and utility infrastructure will also help assure maximal protection of public health during this upcoming period of climate change.

It is critical to keep in mind that uncertainty regarding adverse health outcomes is not the same as the certainty of no adverse outcomes. Given the potential scope and irreversibility of ecosystem changes and consequent effects on human health and society, traditional public health values would urge prudent action to prevent such changes. The possibility of relatively sudden but unpredictable consequences further raises the value of climate change mitigation for health concerns.