Health in the Anthropocene Epoch—implications for epidemiology

Humanity has entered a new geological epoch during which dramatic environmental trends, on a range of scales from local to global, are transforming natural systems. The predominant influence of Homo sapiens on the global environment has led to the growing use of the term ‘Anthropocene Epoch’¹ to distinguish it from the ‘Holocene Epoch’, a climatically generally benign period which lasted around 11 500 years, during which humanity developed from hunter-gatherer and agrarian societies to increasingly urbanized communities. This transformation has been accompanied by pronounced increases in energy and freshwater use to drive economic growth and satisfy the growing demands of humanity for food and natural resources.^2,3 It has resulted in unprecedented human progress, including the ‘Escape from Poverty’⁴ of many millions of people, particularly in Asia and Latin America. Since 1900, the global average life expectancy has more than doubled and is now almost 70 years. All countries in the world have a higher life expectancy than the countries with the highest life expectancy in 1800.⁵

These striking advances have come at a cost, which has been borne by the Earth’s natural systems.² Global mean temperatures have increased by about 1°C since pre-industrial times as a result of emissions of greenhouse gases, biodiversity loss is about 100-fold pre-human rates, perturbation of nitrogen cycles is widespread and extensive degradation of marine ecosystems is occurring, together with ocean acidification and plastic pollution. Many complex systems can adapt to considerable change but then reach limits of adaptation which constitute a tipping point, after which, as a result of non-linear dynamics, sudden collapse occurs. The planetary boundaries framework aims to define a ‘safe operating space’ within which humanity can flourish,⁶ based on the assessment of nine biophysical processes essential for Earth systems’ functioning. A recent update of trends shows that two of them, biogeochemical flows and biosphere integrity, are in the high-risk zone, well beyond the proposed boundaries, and climate change and land system change are in the zone of uncertainty where risks are increasing. This analysis shows that we cannot merely extrapolate the trends of the recent past to understand and address the challenges of the future.

Estimates by WHO suggest that about 23% of the current global disease burden is related to environmental factors,⁷ including air and water pollution. The Lancet Commission on Pollution⁸ estimated that 9 million premature deaths in 2015 could be attributed to the effects of pollution, with the majority arising from ambient and household air pollution. The combustion of fossil fuels is a major contributor to both air pollution and climate change, and the effects of global environmental change are likely to become increasingly dominant over coming decades and centuries.

Environmental change can affect human health through three main types of pathways: (i) direct effects such as those due to increasing exposure to intense heat or changes in the frequency and intensity of extreme climate events; (ii) effects mediated through ecosystems, such as changes in the transmission of vector-borne diseases or zoonotic infections, due for example to land use change (or increased risk of undernutrition from reductions in crop yield due to climate change and other environmental stressors); and; (iii) socially mediated effects, for example as result of increased poverty or population displacement. Our understanding of the complex mechanisms underlying these pathways is still incomplete and is compounded by the methodological challenges of quantifying the magnitude of effects in the presence of multiple, potentially interacting, environmental and socioeconomic changes. An example is the effects of multiple environmental stressors, including climate change, water availability, air pollution and carbon dioxide levels on crop yields and nutritional quality of staple crops, vegetables, legumes,⁹ fruit, nuts and seeds. These could have profound but as yet incompletely understood effects on human health.

The Rockefeller Foundation/Lancet Commission on Planetary Health¹⁰ defined planetary health in summary as ‘the health of human civilisation and the state of natural systems on which it depends’. It outlined three challenges that would need to be addressed in order to reduce risks to health in the Anthropocene Epoch. First, there is an imperative to address conceptual or imagination challenges, such as a dominant economic system in which powerful interests do not pay the full economic costs of their activities (such as the costs to society of pollution and ill health). Second is the urgent requirement to address knowledge challenges resulting from both the failure to invest in rigorous research and the methodological challenges of undertaking the transdisciplinary research needed. These knowledge challenges can be broadly subdivided into three categories: (i) improved understanding and quantification of the effects of environmental change on health and of factors influencing vulnerability to such changes; (ii) development and evaluation of effective strategies to promote adaptation and resilience to environmental change; and (iii) development and evaluation of interventions, technologies and policies to mitigate environmental change by profoundly reducing the environmental footprint of societies while protecting and enhancing human health and development, capitalizing on the health co-benefits of low carbon development.¹¹ Advances in knowledge to safeguard humanity through the Anthropocene Epoch, as far as possible, will require transdisciplinary collaboration with epidemiology making a salient but not exclusive contribution. Epidemiology should play a central role, for example by improving exposure assessment and the quantification of disease burdens arising from environmental change and through rigorous evaluation of potential solutions, both adaptation and mitigation. An example of methodological innovation to assess adaptation to environmental change separated the contribution of pure adaptation to increasing temperatures and active changes in susceptibility (non-climate-driven mechanisms) to heat and cold. In order to do so, the investigators compared observed yearly-attributable fractions with predicted trends driven by either: (i) changes in exposure-response function (assuming a constant temperature distribution); or (ii) changes in temperature distribution (assuming constant exposure-response relationships). This comparison provides insights about the potential mechanisms and pace of adaptation.¹⁰

Systems approaches are needed in order to better understand complex interactions and feedbacks which may result in irreversible non-linear change in vital natural systems, such as the ‘state shifts’ that occur in some aquatic ecosystems as a result of changes in nutrient inputs, climate and fishing.¹¹ They can also illuminate the effects of policy choices, such as those to mitigate climate change, on national economies.¹² A study using optimization modelling, to assess the potential to reduce per person freshwater requirements for irrigation by dietary change in India, showed that diets meeting WHO guidelines could reduce freshwater requirements and greenhouse gas emissions while improving population health.¹³ In the case of potential solutions, unintended adverse consequences (such as the effects of some types of biofuels on food availability and security) may occur and systems approaches can help to anticipate and prevent these outcomes.¹⁴ Future studies will no doubt aim for more comprehensive approaches which integrate health, environmental and socioeconomic outcomes to understand trade-offs and synergies. In order to generate better data to facilitate the research needed and to hold decision makers to account, investment will be needed in linking health and environmental data at different scales using a range of sources, including demographic and health surveillance sites, cohort studies and remote sensing.¹⁵

The third category of challenges concerns the implementation of research findings in policy and practice.¹⁶ The need to address this challenge has been thrown into sharp relief by growing awareness that scientific evidence is increasingly contested by powerful stakeholders, emphasizing the need for effective governance to protect the health of current and future generations. A recent example is the reversal of Clean Air legislation in the USA.¹⁷ Barriers to implementation may range from inadequate efforts by researchers to engage well-intentioned decision makers in the co-creation of relevant knowledge, to organized efforts by vested interests to create disinformation and foment sufficient doubt to undermine political support for enforcement of evidence-based legislation. Designing research tailored to the needs of those who will implement the findings is a prerequisite for effective implementation, and the evaluation of credible strategies to accelerate uptake of research findings is an important component of the evolving research agenda for planetary health.

The Anthropocene Epoch poses challenges to our current research paradigms, suggesting the need to collaborate beyond traditional disciplinary silos to engage a range of sectors (including agriculture, energy, urban planning and design, transport etc.) in order to develop and implement solutions which protect the health of today’s population and future generations. Over 20 years ago, the late Tony McMichael presciently warned of the threats to health posed by overloading the Earth’s essential life support systems.¹⁸ It is now a matter of urgency that the epidemiological community responds to his clarion call by addressing the burgeoning challenges to health and sustainable development in the Anthropocene Epoch.