Public Health Emergency on Planet Earth: Insights from the Millennium Ecosystem Assessment

Human health is related to ecosystem functioning in complex ways. Human societies are fundamentally dependent upon ecosystem services—most obviously, the provision of food and clean water supplies. Less tangibly, but no less importantly, other aspects of ecosystem function that underpin human health include: soil formation; recycling of wastes; some degree of regulation of infectious diseases; and stabilization (buffering) of local environments against catastrophic physical disruption. These benefits that humankind derives from well functioning ecosystems have largely been taken for granted in the past. Increasingly, however, it is recognized that the human impacts on Earth’s natural systems threaten the ecosystem services on which we depend.

Three overlapping categories of causal mechanisms linking human well-being and ecosystem status can be described (Table 1).

Table 1. Ecosystem Changes and Human Well-being and Health

• 1. Local exposure → local health impact

  Local, short-term, often direct-acting effects on human health are the most familiar. This category includes, for example, direct health impacts of water quality and availability; local air pollution, or extreme climate events.

• 2. Ecosystem change → change in regional ecosystem service → regional health impact

  A second category of mechanism involves indirect effects on human health via changes in ecosystem services. These mechanisms are characteristically, though not exclusively, regional in scale. Examples include over-fishing, leading to collapse of fish populations; and the transfer of infectious diseases from farm animals to humans.

• 3. Social systems: population, consumption ↔ global ecosystems: ecosystem services

  The third category of impact involves the complex interaction of human societies and ecosystems at global scale, including feedbacks and implications for ecological sustainability. Global climate change is the best known example in this category; the (interlinked) sustainability of food and water supplies is another example with critical importance for human health.

The health impacts caused by local, direct mechanisms are relatively well understood and the best quantified (Table 2). Indirect effects on human health mediated via changes in ecosystems are less well studied and generally have not been quantified. A major reason for this is that these mechanisms are highly context-dependent, rendering them technically difficult to study.

Changes in the structure and functioning of ecosystems in the past half century have been more rapid than at any other time in human history. These changes have, in part, been essential to meet increasing human needs for food and water, but have not benefited all sections of society equally, and have been achieved at increasing cost to ecosystems. Affluent populations exert proportionately greater pressure on the environment but are less vulnerable to the consequences of ecosystem degradation (Corvalan et al., 2005).

Table 2. Health Impacts of Ecological Changes and Relevance to the Millennium Development Goals (MDGs)

Social inequalities within and between countries have generally increased over the past half century. On both global and national scales, poverty is associated consistently with exposure to health risks such as malnutrition, contaminated water supplies, and lack of sanitation, shelter, or other public services. In some regions, poverty, hunger, and ill health have been exacerbated by migration to urban slums or marginal drought-prone lands. Poor communities are more directly dependent on local ecosystem services and are highly vulnerable to ecosystem or economic changes, yet are less able to prioritize conservation of ecosystem services ahead of immediate needs. In these circumstances, further degradation of ecosystem services can trap poor communities in a downward spiral of poverty and poor health.

As part of the Millennium Ecosystem Assessment, scenario-based modeling of future global trends and developments to the 2050s was carried out. Under all scenarios considered, there was an increase in pressures on ecosystems, leading to continued loss of biodiversity and ecosystem degradation. In particular, demand for food was projected to increase by 70%–80% and demand for water by 30%–85%. Human societies cannot continue to increase per capita consumption, total population, and social inequality without risking irreversible damage to global ecosystems. More bluntly, the trajectory of human societies over the past 50 years, if extrapolated into the next 50, would be ecologically unsustainable. The effects on human health would be catastrophic.

This implies that measures to safeguard ecological sustainability are a critical aspect of public health policy. However, what role the health community can or should play in safeguarding ecological sustainability is still a matter for debate. Mitigation of ecosystem degradation and adaptation to past changes can sometimes go together. Avoiding, limiting, or remedying ecological damage is preferable in principle and is the only available approach in the case of certain poorly defined ecological thresholds. Examples include the minimum level of biodiversity required to maintain essential ecosystem services, or the maximum amount of greenhouse gases that can be emitted without causing irreversible destabilization of the climate system. Such global ecological thresholds are interdependent: for example, the sustainable level of greenhouse gas emissions will depend on the extent of biodiversity loss, while biodiversity is also sensitive to the degree of climate destabilization.

Provision of basic necessities of life is the first priority for public health; but if we degrade ecosystems in the process, this only postpones public health problems for future generations to manage. More equitable distribution of food and energy supplies has the potential to simultaneously improve public health and reduce pressure on ecosystems. For example, a reduction in the consumption of animal products and refined carbohydrates in rich countries would have benefits for human health and for ecosystems. Similarly, better transport systems and practices could reduce adverse health impacts (including injuries, obesity, cardiovascular and respiratory diseases) via improved safety and physical fitness, healthier balance of energy input and output at the individual level, and reduction in local and global air pollution.

In the future, achievement of the Millennium Development Goals (MDGs) will depend upon the sustainability of ecosystem services. Important links between MDGs and mechanisms of health impact are referenced in Table 2. The communities confronted with the greatest challenges in achieving the MDGs are those where needs for ecosystem services are already close to or beyond the rate of sustainable supply (dryland regions are a prime example here). On a global scale, ecologically unsustainable use of ecosystem services threatens improvements to water supplies, malnutrition, infectious diseases, maternal and child mortality, and poverty, all of which are necessary for the achievement of the MDGs.

There are unavoidable uncertainties about the exact magnitude, scope, and timescale of the health impacts of global change. This means that precautionary action is the only way to safeguard human health in the long term: anything less would be “playing games with our children’s future.”