Global warming and climate change may produce a wide variety of harmful impacts on human health. Although the magnitude and timing of climate change and the effects of human adaptation are not known with certainty, some inferences have been drawn from existing models. These models attempt to use historical scientific data and current global or regional trends to estimate a range of possible future outcomes. There remains significant disagreement in both scientific and political circles regarding trends in global warming, potential threats posed by change, and the need for mitigation effort. Five major potential outcomes of global warming and climate change associated with significant health consequences include increases in ambient temperature, weather extremes, shifts in infectious disease patterns, changes in air quality, and resource pressures associated with migration and adaptation.
Background
Public health practitioners and medical researchers in government, academia, and other agencies around the world are studying possible health impacts of global warming and associated climate change. There is a growing body of literature used to help understand current trends and extrapolate future trends, representing the work of experts from many disciplines, working to understand health effects. Increasingly, this research takes place through collaborative partnerships or organizations comprised of experts from across the globe. Two examples of these types of collaborative efforts are research conducted by the World Health Organization (WHO) and the Intergovernmental Panel on Climate Change (IPCC).
Both direct and indirect health effects can occur from transient heat increases, extreme weather events, water availability and quality, changes in air quality and atmospheric protection, infectious diseases, and the interplay of these factors. Although some impacts may be positive, such as the possibility that increased winter temperatures may result in lower seasonal morbidity and mortality in some areas, the overall impact of global warming and climate change on human health is expected to be negative. This imbalance is partly attributed to the likelihood that climate changes over the next several centuries will outpace local and regional capacity for adaptation in many areas.
There are many uncertainties and challenges in accurately predicting the health effects of global warming and climate change. Partly, this is because the overall outcomes depend extensively on other factors that impact human health and compound their effects. These factors include population expansion, energy production, pollution or misuse of land and water resources, deforestation, and urbanization. Another complication in predicting the health effects are the many ways in which health impacts may be offset by continuing scientific advances in medical care, pharmacology, and public health. This may prove especially important with improved access and resources in developing countries.
Effects of global warming and climate change are expected to vary by population socio-demographic characteristics and by geographic area. People in developing countries are particularly vulnerable, in part, because of the reduced resources for planning and responding to changes, often due to geographic location. People who live in areas that are prone to extreme weather events, such as coastlands and lowlands, are more likely to be adversely affected. In many areas, people who contribute the least to global warming and climate change (through energy consumption and waste production) will be the most vulnerable to its effects. In both developed and developing countries, the poor, elderly, very young, and chronically ill are more vulnerable than healthier or more affluent people. Finally, there are many areas that are currently only sparsely populated, due to marginal water and food resources. With relatively small changes in average annual temperature or shifts in rainfall patterns, many of these areas will no longer support sustained human activity. Other areas that are currently too cold or arid may become more hospitable, but migration requires tremendous amounts of resources, and often requires travel across semi-permeable political (or cultural) boundaries.
Temperature increases and extremes
Average temperature has increased globally over the past few decades and is expected to increase between 2.5-10.4 degrees F (1.4-5.8 degrees C) by the end of the 21st. century. An increase in mean temperature occurs in the context of fluctuating local, regional, seasonal, and other patterns. This translates to higher temperatures at the extreme, which in turn, are directly related to short-term human illness and death. For context, the estimated average temperature for the most recent ice age was approximately 7.2-9 degrees F (4-5 degrees C) below the mean temperature over the subsequent 10,000 years to the present. Research in this area has been especially controversial because of evidence of historic variation in temperature and difficulty establishing valid associations, in the context of global environmental complexity. Increasing mean temperature may be the most direct and measurable component of global warming and climate change, and health effects may be more related to extremes (both hot and cold) than to overall increased temperature.
Many scientists have proposed that the most significant impact of increasing mean temperatures will be the related disruption of regional and global weather patterns, driven, in part, by alterations in oceanic and atmospheric temperatures and currents. Relatively minor shifts in major currents, such as the Gulf Stream and El Nino, produce significant changes in the frequency and intensity of local storms and rainfall patterns.
It is expected that there will be both direct (or more immediate) impacts, as well as indirect impacts, of temperature changes and extremes. The most direct and observable effects are heat strokes and deaths related to heat extremes. Indirect effects are more debatable and difficult to attribute to any one cause, but one possibility is decreased health related to decreased physical activity, and more time spent indoors in air-conditioned environments. Some research has also shown associations between higher temperatures and violence in urban areas.
In summer 2003, a two-week heat wave in Europe killed tens of thousands of people. It was estimated that summer temperatures averaged 6.3 degrees F (3.5 degrees C) above baseline, perhaps the hottest season in several hundred years. Lessons learned include highlighting the vulnerability of elderly and health-impaired individuals, and the disparate impact of temperature extremes on poor people and resource-poor communities. This event was particularly important for future planning, because it highlighted the continuing inadequacy of mitigation plans and available resources in developed countries. Another compounding factor is the increased ambient temperature of urban areas, which can result in a significant contribution to morbidity and mortality during peak heat events.
Increases in average ambient temperature are also expected to contribute to melting of ice caps and glaciers, which currently reflect large amounts of incident radiation (albedo), potentially leading to further global warming and climate change that may negatively impact health at regional and global levels.
Extreme weather events
Flooding, drought, and environmental degradation associated with climate change may lead to population displacement and more environmental refugees.
While some inferences have been drawn from existing models, more studies are needed for a better understanding of the health impacts related to global warming and climate change. Very little is known about its long-term consequences on health.
Sea-level rise is likely to affect low-lying coastal populations, especially in countries where economic means do not allow construction of sea defenses and other counter-measures. Human populations have tended to concentrate in major cities over the past 150 years, and many of the largest population centers are located on low-lying oceanfront terrain, often with an associated major river delta.
As with heat changes and extremes, there are expected to be both direct and indirect health effects of extreme weather events. Direct health impacts include injury and mortality associated with these events. There are potentially many more indirect effects, such as increases in vector-borne diseases associated with flood waters, the spread of infectious diseases in sewage runoff and poor drinking water in the aftermath of these events, and increases in mental illnesses due to community displacement, loss of property or lives, or anxiety. In late August 2005, hurricane Katrina formed over the Bahamas and crossed the Gulf of Mexico, where it strengthened before reaching landfall along the Gulf Coast, including the New Orleans metropolitan area. Because New Orleans had been built in low-lying areas (in some cases below sea level) at the Mississippi delta, it was particularly vulnerable to the breaching of its protective levees. Nearly all of the city levees broke in at least one place, flooding large parts of the city and completely disrupting the community support system. It is estimated that over 1,800 people died, and over one million were displaced by the storm. Two years later, it was estimated that approximately one-third of the population had not returned to the area and both public and private services to remaining citizens had not returned to pre-hurricane levels.
Regarding drought, changing water patterns may put severe strain on human populations that do not have access to resources for diversion of rivers or development of major reservoirs. In areas that do have resources, the environmental impact of drought may be significantly exacerbated by human activity.
Infectious diseases
Perhaps the earliest measurable changes in human population health related to climate change are relatively rapid shifting of patterns in bacterial, viral, fungal, and parasitic diseases. As living organisms strive to adapt to changes in temperature and weather over time, pursuing both immediate survival and longer-term access to resources, shifts occur in the geographic range of plants, animals dependent on plants, animals dependent on animals, and associated disease patterns. For example, vector-borne diseases are caused by microorganisms that are transmitted by spending part of their life cycle in an animal that then interacts with humans. Diseases may appear in previously unaffected areas in one of several ways, including migration of the vector to newly-warmer or wetter ecosystems.
In its quest for a meal, the mosquito must penetrate the skin of animals and access blood, in the process providing access to organisms harbored in its mouthparts. Malaria (Plasmodium genus, Anopheles genus mosquito) and dengue fever (dengue virus, Aedes genus mosquito) are two major diseases transmitted in this way. Malaria is endemic to tropical areas, largely because the lifecycle of the Anopheles mosquito depends on warm, humid climate and pools of warm standing water for maturation of eggs; the malarial parasite itself does not reproduce in colder temperatures. Global and regional rises in average temperature are expected to extend the range into higher altitudes and more northern latitudes. In Central and South America, researchers have documented an increase in malaria associated with warmer temperatures that occur during El Nino affected periods.
Challenges in measuring regional and global infectious disease changes as they relate to temperature and climate alterations are compounded by many other (sometimes interrelated) human activities. For example, diseases with rapid human-to-human transmission may quickly jump large geographic obstacles such as oceans and mountain ranges through rapid transport capability and increases in overall mobility (travel) of human sub-populations.
Water-borne (especially diarrheal) infectious diseases could increase due to changes in temperature and rainfall patterns. Increasing coastal water temperatures may give rise to more frequent toxic algal blooms. Food-borne diseases, such as salmonellosis, also increase in incidence in warmer months, and may have a significant impact in many areas.
Air quality
Weather and climate changes can affect the mix and level of contaminants in the air. This has important consequences for health, according to a growing body of research showing associations between poor air quality and adverse health effects. In particular, cardiovascular conditions such as cardiac arrhythmia, and respiratory conditions, such as asthma, have been associated with poor air quality. For asthma, temperature increases can affect the levels of aeroal-lergens, such as pollen and mold, which can exacerbate symptoms in allergy and asthma sufferers. The combination of heat and urban pollutants interact to form the urban heat island effect. This refers to the increase in temperatures in urban areas, compared to surrounding rural areas, as a result of less tree coverage, and more use of heat-absorbing materials, such as rooftops and roadways in urban areas. Increases in temperature cause an increase in ground level ozone, a principal component of urban smog.
Atlanta, Georgia is a comparatively well-studied example of a large urban area affected by ground-level ozone and smog pollution. The increases in congestion, vehicle traffic, drought, and development of the built environment over the past several decades have interacted to increase temperatures, ozone, and smog in the city. In the summer months, Atlanta often exceeds air quality standards for ozone and particu-late matter set by the U.S. Environmental Protection Agency. The Georgia Environmental Protection Division reports daily air quality levels in metro Atlanta and issues Smog Alert warnings for days when measured pollutants are projected to exceed a certain level. The purpose of alerts is to warn people with respiratory conditions and other vulnerabilities to limit exposure. In 2006, metropolitan Atlanta was issued 42 smog alert days, as well as 236 days of moderate-risk air pollution warnings for vulnerable groups.
Adaptation and mitigation
Although it is difficult to determine the extent of the health impacts of global warming and climate change, both currently and in the future, there are several adaptation and mitigation measures that can be taken to lessen potential impacts. On the adaptation side, these include improved design of homes, buildings, and other land and water structures to withstand extreme weather events. In some cases, relocation of populations away from low-lying flood plains and other geographic areas that are especially susceptible to extreme weather events should also be considered.
On the mitigation side, more health studies are needed for a better understanding of the health impacts of global warming and climate change. Although research on health effects has increased in the 21st century, very little is known about the contribution to global warming and climate change to ill health beyond immediate and observable impacts. A related need is for better disease and environmental surveillance data on an ongoing basis, particularly in developing countries. Many developing countries lack even basic environmental-monitoring capabilities or disease-tracking systems. Another need is for the implementation or improvement of disaster response planning, including educational intervention and outreach. Each of these efforts requires financial resources. Finally, research and innovation on ways to reduce the human contributions to global warming will play an important role in adaptation and mitigation strategies. A notable area is the reduction of fossil fuel use and increasing the availability and use of renewable energy technologies. The relationship between the human population and the environment is dynamic, so if there is a change in the environment, it will require an adaptation across a broad and diverse range of resources and local and environmental challenges.
