Scientific Symposium Presentation Abstracts
September 19
Dr. Devra Lee Davis: Potential Effects of Weather Extremes and Climate Change on Human Health
Projected extremes and variations in weather associated with long term climate change pose a number of chronic and acute risks for human health. With sufficient advance warning and relatively gradual rates of change, public health systems, even in poorer developing countries, might adapt to such changes, limiting their adverse impacts. However, much of the evidence advanced by climate experts in this symposium suggests a bumpier ride. We may see an interplay of a variety of factors: significantly higher temperatures and, with them, more frequent and prolonged heat waves with associated deteriorations in air quality; more extreme weather events (storms, floods, and droughts); coastal inundation and salt water intrusion into fresh water supplies, water treatment plants, landfills, and hazardous waste facilities; and increased ranges for survival of pests and other disease vectors as a result of warmer winters and earlier onset of spring temperatures that allow more insects, rodents, and others to survive. Rapid rates of change will profoundly alter habitats, changing some forests to grasslands (with the pace of change often accelerated by wildfire), squeezing out many wetlands, and shifting distribution of flora and fauna. Granted, projecting the impacts of such changes on human health and well-being remains speculative; however, there are a number of reasons to anticipate some rude surprises.
The IPCC has tended to see the effect of climate change on agricultural production, if the warming were to occur at a moderate pace, as close to a wash in overall global food production, but with some regional variations that might harm food security. Generally the areas of the globe most vulnerable to climate-related stress on food production are poorer countries in lower latitudes. Not only do these nations have limited capacity to adapt, their food distribution systems are, as we have seen in several of the African famines, poorly positioned to disseminate food aid, even should it be sent. To date, the greatest human health disasters, on a par with global pandemics such as influenza, plague, or tuberculosis, have been famines. A huge wild card in the climate equation is the possibility of increased political and meteorological volatility as the world warms, causing more floods and droughts, and many agricultural regions having less soil moisture due to higher rates of evaporation. Thus, ethnic and tribal strife in Darfur has resulted from famines in the region. These famines have led to many thousands of deaths and environmental refugees, as well as persistent conflicts over the region’s sparse natural resources, in part exacerbated by desertification. The total world grain reserves amount to only about enough to cover 57 days of consumption. Should increased spatial and temporal volatility of seasonal climate regimes result in severely curtailed production in the same year in several food exporting regions (e.g., North America, Australia, and Argentina), this could not only raise food prices worldwide but also significantly raise the costs (and therefore reduce the likelihood) for rich nations to ship food to poorer nations that might also be affected. This year’s severe weather-related losses in the agricultural bastions of California and the Midwest underscore the potential fragility of global food security.
Important as food security is from a human health perspective, our remarks will focus chiefly on known and projected impacts on human health, including: heat stress and air pollution; changes in the frequency and intensity of extreme events such as flooding, droughts, and severe storms; changes in the incidence and magnitude of water and food- borne-diseases; changes in the incidence and distribution of diseases borne by insects, rodents, and other wildlife; and ripple effects of large-scale displacements of people and ecosystems projected as a result of extreme events and failure of food production. While the implications of climate change are global because, for example, one cannot readily build fences or barriers to ensure that diseases or pests will not cross national borders, this presentation focuses primarily on implications for North America and the Caribbean.
Although there remains considerable uncertainty on the magnitude and specific regional distribution of impacts, we can make some generalizations with a modest degree of confidence. These findings, together with those reported by IPCC and others, indicate that policies that will reduce greenhouse gas emissions in the long term can be expected to have significant benefits for public health and the environment in the near term.
Model projections clearly portend more frequent and more intense summertime hot spells, along with greater instability of weather patterns. As this past summer’s searing temperatures in the U.S. and Europe sadly confirmed, heat stress is likely to increase morbidity and mortality as well as lead to adverse impacts on food production. At the same time, adverse impacts from exposure to cold temperatures are likely to decline. With advance warning, societies can adapt in ways that can limit significant damage to public health, including increasing the availability of air conditioning (and providing funding for its use by the poor), and expanding the use of public facilities to shelter vulnerable populations. But crop losses, such as those that occurred in the nation’s breadbasket of California in July, cannot easily be averted. In Western Europe in 2004, over thirty thousand died from heat stress. In the U.S., our worst heat-related public health episode in recent years occurred in Chicago in July 1995, when over six hundred died from heat stress according to medical examiner reports. Yet deaths attributable to severe heat go well beyond numbers that a medical examiner will certify. Epidemiological evidence suggests that mortality and morbidity from cardiovascular and respiratory disease rises significantly during heat waves and deaths from these causes are consistently underestimated.
Weather extremes tied to overall climate change will impact mortality and morbidity, including hospital admissions and sick days away from work due to air pollution. We also need to recognize that the combustion of fossil fuels that generates greenhouse gases and contributes to climate warming is also the principal source of air pollution. Even though the U.S. has one of the most stringent sets of air pollution control regulations on the planet, it is estimated that at least 30,000 Americans die each year from exposure to air pollution, especially fine particulate matter. While control strategies to lower greenhouse gas emissions are likely to produce lowered mortality from air pollution, the warming itself may have several adverse effects. Although the situation will vary across air quality regions, warming will change and generally accelerate the photochemical reactions associated with ground level ozone. This may produce increased mortality and other adverse health effects. It also may exact a large economic penalty, as many of the gains in ozone attainment won at great economic cost will be counteracted by an acceleration of ozone photochemistry. Another probable adverse effect that is likely to increase mortality and morbidity is an interaction between temperature stress and air pollution. Higher ozone levels and temperature stress may place those with cardiovascular and respiratory disease at added risk.
Climate-related effects on water and food-borne diseases are likely to be a much more significant factor in developing countries than in the U.S. The ENSO cycle already is associated with large fluctuations in the incidence of cholera in Peru. Although climate change may affect the overall availability of water in the U.S., it does not seem likely to compromise the safety of drinking water. Higher temperatures may, however, heighten risks from salmonella in the U.S., and hotter and less mixed estuaries may lead to more frequent contamination of fish and shell food. Effects on the food production and distribution system and water supply systems in the U.S. are likely to have a much greater effect on cost than on human health. Exceptions to this generalization can occur as a result of major storm disasters such as Hurricane Katrina, when populations are ripped away from water and health support infrastructure, and when greater incidence of heavy precipitation causes storm runoff to overload systems where this water is passed through sewage treatment plants.
An increase in temperatures, including warmer temperatures in the winter when many pest populations are typically curtailed, is expected to exacerbate the proliferation of pests such as insects and rodents. Climate warming, particularly if it occurs rapidly, is likely to change ecosystems, thereby affecting predator-prey relationships and allowing some populations of disease-bearing pests to proliferate. The hanta virus outbreak in the southwestern U.S. a few years ago was a manifestation of this risk. Warming and associated changes in water availability, evaporation, soil moisture, and habitat may also change the areas in which diseases such as malaria, West Nile Virus, and dengue can thrive. While the U.S. public health infrastructure is sufficiently strong that these disease risks should be manageable, except when there have been massive disruptions such as severe hurricanes or floods, Americans traveling overseas are likely to experience less adequate controls.
Large-scale climatic disruptions in nearby nations, such as Mexico or Caribbean island nations, may also have spillover effects on the health system in the U.S. This might occur in several ways—a rapid surge in illegal migration as people flee storm-ravaged or sun-parched regions to earn a living, or simply an outbreak of disease in fetid urban slums that spreads across borders. New immigrants, especially those who come here illegally, may have a higher incidence of some diseases, such as tuberculosis, that are rare today in the U.S. It is very hard to make projections of what the potential impacts are likely to be. Public health officials will, however, need to be on the lookout for a wider variety of diseases as climate changes and the mobility of people and other disease-carriers increases.
A well-planned response to managing the risks to human health from large-scale climate change will have two dimensions. The first goal is slowing the rates of change in order to increase time for measured preparation. Here we depend on the wisdom and will of world leaders in laying out a policy framework and the innovation of industry in achieving a viable economic path to stabilizing the atmospheric concentration of greenhouse gases. Second, we in the public health community need to develop better monitoring systems to assess emerging risks that may increase as the climate warms and advise governments at all levels on how to minimize the chance of epidemics and small outbreaks, along with other short-term impacts of weather extremes.
