As headlines any day of the week can tell us, climate change is no longer a black cloud looming on the distant horizon. Rather, it is swirling into our backyards and cities, rolling waves along our coastlines, and shimmering in the heat not only of our deserts but our farmlands as well. And although science has settled that climate change is occurring and that human activities are in large part the cause, what this will mean for persons, communities, and human populations over the decades to come remains mostly shrouded in a cloud of scientific fog. A new report, however, suggests that a fresh wind of epidemiologic research, both directly and in collaboration with other critical disciplines, could help to dispel this fog, and in the process reveal a silver lining of new understanding of human responses and vulnerability to an extensive range of environmental and public health problems.
The new report,1 part of the US Global Change Research Program’s sustained National Climate Assessment process, is the result of the efforts of more than 100 experts to synthesize the most current existing scientific literature on the relationship between climate change and health outcomes across a comprehensive set of climate-driven exposure pathways. The report also presents, for the first time, quantitative estimates of future health risks and changes in exposures for five different health impact areas. The nearly 3-year process of creating the report included multiple stages of both public and federal agency review, as well as review by a panel of experts assembled by the National Academy of Sciences.
Framed by an introduction that provides context for existing climate and health trends and a closing discussion focused on populations of concern, the report examines seven different types of health impacts: effects of temperature extremes (particularly heat), poor air quality, extreme events (and resulting disasters), vector-borne diseases, water-related illness, food safety and nutrition, and mental health impacts. Unlike previous assessments, the report breaks new ground in emphasizing mental health impacts and food quality and nutrition.
While the report stresses that all Americans are exposed to climate change risks, it also emphasizes that there are important differences among people and communities in vulnerability. The report defines vulnerability as a combined measure of a person or population’s exposure to a climate stressor, their sensitivity, or the degree to which they are affected, and their adaptive capacity, or ability to respond to such stressors. Hence, vulnerability is influenced by a variety of factors (social, genetic, behavioral) through changes in relative exposure, sensitivity, and adaptive capacity to climate change. The report also defines the related concept of resilience as the long-term ability to prepare for, recover from, and adapt to climate impacts. As epidemiologists are acutely aware, vulnerability and resilience are closely associated with social determinants of health, such as income, education, age, and other factors. Risks are compounded when these determinants interact or occur in clusters in people or populations to exacerbate the effects of environmental health exposures. For this reason, the report devotes a lengthy initial discussion of social determinants, and integrates these considerations throughout the individual topic chapters.
Although the report was not intended to be a research needs assessment, each chapter identifies examples of research data or methods gaps that may warrant further traditional epidemiological investigation or possibly collaborative multidisciplinary approaches. Taken together, these discussions offer a rich vein of inquiry for epidemiologists to mine. In this article, we draw strongly, although not verbatim, from the needs identified by the assessment authors to illustrate potential opportunities for epidemiological input to a number of research areas. The vision of the NIEHS is to provide global leadership for innovative research that improves public health by preventing disease and disability. This commentary represents the authors’ thoughts on potential areas where epidemiologic inquiry and research on climate change and public health may intersect to offer new knowledge that informs solution to this global issue.
The assessment finds that even small differences from seasonal average temperatures result in illness and death. And that “because small temperature differences occur much more frequently than large temperature differences, not accounting for the effect of these small differences would lead to underestimating the future impact of climate change.” This finding prompts multiple potential follow-on epidemiologic research questions such as: How do temperature-related health impacts vary by different populations (e.g., outdoor workers, children, elderly)? How do genetic and social factors contribute to vulnerability to temperature-related mortality and morbidity? Or how can we better predict health impacts of (unprecedented) extreme heat events?
Another example involves air pollution. The assessment finds that “Unless offset by additional emissions reductions, these climate-driven increases in ozone will cause premature deaths, hospital visits, lost school days, and acute respiratory symptoms.” Questions to be explored around this finding could include: How do exposures to extremes of temperature and relative humidity, aeroallergens, and air pollution interact to affect human health? The chapter also explores indoor air issues related to climate change as an emerging issue. Here social determinants of health, which have a strong influence over the quality of housing and ventilation systems, come more clearly into play. As climate change results in greater extremes of precipitation and episodes of flooding, important questions include: What are the impacts of mold or other biologic contaminants on indoor air quality? And what are the associated health effects?
In the discussion of effects of climate change on vector-borne diseases, the literature suggests that climate change affects pathogens and transmission pathways for many infectious diseases. In many cases, however, only shifts in the geographic range of pathogens or vectors can be estimated. The authors emphasize the importance of human behaviors in leading to contact with vectors and pathogens, as well as the challenges of modeling and predicting the changes in such behaviors in concert with changes in disease transmission. Areas where epidemiology may be able to provide additional insight include: How do climate and nonclimate factors (socioeconomic or human behavioral factors) interact to influence the disease burden of vector-borne disease? What socioeconomic or human behavioral factors are most important for adaptive capacity to protect health in the future?
The area of climate change impacts on mental health is one that is emerging, but likely to become more prominent in the scientific discussion, given the high prevalence of mental health conditions in the US general population. The assessment cites that as of 2013, “there were an estimated 43.8 million adults ages 18 or older in the United States who had any mental illness in the past year, representing 18.5% of all adults in the United States.”2 Mental health conditions may increase a person’s vulnerability to a variety of climate change exposures such as extreme weather and heat. The report found that “Specific groups of people are at higher risk for distress and other adverse mental health consequences from exposure to climate- or weather-related disasters. These groups include children, the elderly, women (especially pregnant and post partum women), people with preexisting mental illness, the economically disadvantaged, the homeless, and first responders.” Just a sample of research questions that epidemiology studies could help to answer around this issue includes: What factors predict adverse psychological outcomes after extreme events? What factors facilitate recovery and post traumatic growth following extreme events? Or how do other health risks from climate change (extreme heat, poor air quality, diminished food safety, increased vector-borne risk) increase a person’s vulnerability to adverse mental health impacts?
Perhaps of greatest interest for epidemiologists might be the issues raised in the “Populations of Concern” chapter of the report, which identifies a number of groups who may be particularly vulnerable to the impacts of climate change or least able to recover or adapt to its impact on their health. Some of these include children and pregnant women, elderly, low-socioeconomic status communities, indigenous peoples, certain occupational groups, and others. The report makes the point that, just as risk factors associated with social determinants of health may cluster in individuals or specific populations, so also may increased vulnerability and probability of climate change exposures. Research on the relative contributions and causal mechanisms that link vulnerability factors—genetic, physiological, social, and behavioral—to risks of climate change health effects will enable understanding that can improve the health of not only those groups studied but also of populations generally.
Epidemiology, as the study of populations’ health, is in a unique position to address many of the research opportunities identified in this assessment. Insights into interactions between climate stressors and other risk factors observed in the present and recent past can inform both near-term interventions for extremes of weather and climate variability and also simulations and predictive modeling of conditions into the future. And even for epidemiologic research not focused on climate change, it is important to understand if or how climate change may impact the health outcomes and risk factors we study. In doing so, epidemiologists can help our society to better prepare for and adapt to the impacts of climate change, actions that can produce major health benefits independent of effects on the environment. Increasing climate change resilience can increase public health resilience, thereby easing the burden on our public health systems, improving the quality of people’s health and lives, and ensuring clearer skies in our future and those of generations to come.