The global scale, interconnectedness, and economic intensity of contemporary human activity are historically unprecedented,1 as are many of the consequent environmental and social changes. These global changes fundamentally influence patterns of human health, international health care, and public health activities.2 They constitute a syndrome, not a set of separate changes, that reflects the interrelated pressures, stresses, and tensions arising from an overly large world population, the pervasive and increasingly systemic environmental impact of many economic activities, urbanization, the spread of consumerism, and the widening gap between rich and poor both within and between countries.

In recent decades, international connectivity has increased on many fronts, including the flow of information, movements of people, trading patterns, the flow of capital, regulatory systems, and cultural diffusion. These exponential increases in demographic, economic, commercial, and environmental indexes have been labeled the Great Acceleration.3 Remarkably, the resultant environmental effects are now altering major components of the Earth system.4,5 The current geologic epoch is being called the Anthropocene (successor to the Holocene epoch)5,6 in recognition of the global force that Homo sapiens has become, pushing or distorting Earth’s great natural global systems beyond boundaries considered to be safe for continued human social and biologic well-being.4,7 The loss of biodiversity, the greatly amplified global circulation of bioactive nitrogen compounds, and human-induced climate change have already reached levels that are apparently unsafe.4

These changes pose fundamental threats to human well-being and health.4,7 For example, a positive relationship has been observed between regional trends in climate (rising temperatures and declining rainfall) and childhood stunting in Kenya since 1975, indicating that as projected warming and drying continue to occur along with population growth, food yields and nutritional health will be impaired.8 These human-induced climatic changes often act in concert with environmental, demographic, and social stressors that variously influence regional food yields, nutrition, and health. Furthermore, at the current level of global connectedness and interdependence, the environmental impact of human activity has a wider geographic range, although its influence may be offset somewhat by more effective global alerts and more rapid distribution of food aid. The extreme heat and wildfires in western Russia in the summer of 2010 destroyed one third of that country’s wheat yield, and the subsequent ban on exported grain contributed to a rise in the price of wheat worldwide, exacerbating hunger in Russia (where flour prices increased by 20%) and in low-income urban populations in countries such as Pakistan and Egypt.9,10 On the economic front, the recent global financial crisis has underscored the domino-like interdependence of national economies.

Effects of Globalization on Population Health

Global influences on population health such as those described above transcend the more specific, focused frame within which international health issues are addressed.2 The processes of global change are more systemic, involving disruption or depletion (not merely local pollution). Remediating or adapting to these changes requires an understanding of dynamic systems, their complexity and associated uncertainties, and coordinated policy responses across relevant sectors. The relationships between these pervasive processes of change and human health are shown in Figure 1Figure 1Influence on Human Health of Changes Related to Globalization..

Global Climate Change

Global climate change is part of the larger Anthropocene syndrome of human-induced global environmental changes. These include land degradation, ocean acidification, and disruptions and depletions of the stratospheric ozone concentration, soil fertility, fresh-water resources, biodiversity stocks and ecosystem functioning, and global nitrogen and phosphorus cycles.4 Greenhouse emissions from fossil fuel–based power generation and transport and from the agriculture and mining sectors increase the heat-retaining capacity of the lower atmosphere, resulting in global warming (see the interactive graphic at NEJM.org). In addition, deforestation and ocean saturation have added to greenhouse warming by reducing the capacity of terrestrial and marine environments to absorb extra carbon dioxide (the main greenhouse gas) from the atmosphere. Also contributing to such warming are any ongoing natural variations in climate caused by cosmologic and geologic influences.32

Most of the global warming since 1950 (an increase of 0.7°C) has been the result of human activity.32 Annual global emissions of carbon dioxide have increased over the past decade, as have the rates of sea-level rise, the loss of Arctic sea ice, and the number of extreme weather events.33 Without substantial and prompt international action to abate these emissions, average global temperatures (relative to the year 2000) are likely to rise by 1 to 2°C by 2050 and by 3 to 4°C by 2100, including increases of up to 6 to 7°C at high northern latitudes.33,34 Additional warming of another 0.7°C is locked in from the extra radiative energy already absorbed by the lower atmosphere and, in turn, by the oceans, though not yet manifested as surface warming. An average rise of 4°C would return Earth’s temperature to a level not experienced for 10 million to 20 million years.35

Rainfall patterns will also change, with rainfall increasing in some regions and seasons and decreasing in others. Modeling consistently projects an increase in regional aridity, and in the geographic range and severity of droughts, during this century.36 The frequency, and perhaps intensity, of extreme weather events is also expected to increase in most regions — and may well have already begun to do so.37

Effects of Climate Change on Human Health

The complex nature of climate change and its environmental and social manifestations results in diverse risks to human health.17,18,38 A three-way classification of these risks and causal pathways is shown in Table 1Table 1Categories of Climate-Change Risks to Health, According to Causal Pathway..39

Our current, rather skewed knowledge of climate–health relationships has come from epidemiologic studies of health risks in relation to differences and extremes in temperature and from quasicyclical climatic events such as the El Niño–Southern Oscillation phenomenon. However, most of the health risks will arise from climatic influences on environmental systems and social conditions that affect food yields, water supplies, the stability of infectious disease patterns, and the integrity of natural and human-built protection against natural disasters (including forest cover, windbreaks, mangroves, vulnerable constructed seawalls, and urban water-drainage systems) and from the adverse health consequences of social disruption, displacement of communities, and conflict situations.18,38

Key examples of these types of causal paths are shown in Figure 2Figure 2Processes and Pathways through Which Climate Change Influences Human Health.. Many of the indirect effects of climate change will be simultaneously influenced by other global changes and sociodemographic pressures that act in conjunction with climate change. Food yields and, hence, nutritional status reflect changes not only in climate40 but also in water supplies, soil fertility, nitrogen levels, biodiversity (e.g., pollinators and pest predators), and the health and vitality of farm workers.

It is not surprising that the health effects of climate change will be predominantly adverse.38 After all, human biology, domesticated food sources, and culture in general have evolved over many millennia within the usual prevailing climate. Furthermore, populations everywhere will be vulnerable to increasingly severe extreme weather events.

Some beneficial health effects are expected to occur, at least in the earlier stages of climate change.38 In some temperate zones, milder winters may lead to fewer wintertime deaths from myocardial infarction and stroke, and in some low-latitude regions, hotter and drier conditions may reduce mosquito survival and, hence, mosquito-borne infection.

Populations living in diverse social, economic, and physical conditions will be affected differently by climate changes.18,38 Low-income and remote populations are more vulnerable to physical hazards, undernutrition, diarrheal and other infectious diseases, and the health consequences of displacement. Populations on low-lying islands and in coastal areas, such as Bangladesh, are vulnerable to increased storm surges and flooding as the sea level rises. In Arctic circumpolar regions, communities may undergo enforced changes in diet as land and marine animal populations migrate or decline and as access to traditional food sources becomes physically more difficult.41

The likely future effects of climate change on various health outcomes have been modeled with the use of plausible scenarios of future climate change that have been agreed on internationally. For example, in temperate countries, as summers become hotter and heat waves more severe, modeling indicates that, from around mid-century, additional heat-related deaths will progressively overwhelm the number of deaths averted as a result of milder winters.42,43 Such estimates of the extreme effects of weather will improve as the modeling of changes in climatic variability under climate-change conditions improves and as researchers take better account of physiological, behavioral, and technological adaptation by populations over time.

In China, the modeling of medium-scenario warming indicates that the transmission zone of freshwater snail–mediated schistosomiasis will extend northward, putting another 20 million people at risk by 2050.44 Such model-based estimation of the direction and approximate extent of likely change in health risks is an important resource for decision-making about both climate-change abatement and localized adaptation.

Meanwhile, an important research task is to identify ongoing changes in health risks and outcomes that can be reasonably attributed to recent climate change. Given the multivariate causation of most human health outcomes, attribution is rarely simple.45 Nevertheless, over the past decade, observed changes in some health outcomes, viewed collectively, suggest a climate signal (Table 2Table 2Indicators of Early Health Effects of Climate Change.).18,38,46

Health Risks and Benefits of Climate-Change Mitigation and Adaptation

The mitigation of climate change is a crucial first-order task for the world. However, while governments continue to wrestle with this unprecedented, complex political and ethical task, the more immediate challenge for the health sector is to identify the main regional health threats posed by climate change and ensure the development of risk-lessening adaptations.