9.1. Introduction and Scope
This chapter assesses how climatic changes and associated environmental and
social changes are likely to affect human population health. Such an assessment
necessarily takes account of the multivariate and interactive ecological framework
within which population health and disease are determined. This ecological perspective
recognizes that the foundations of long-term good health lie in the continued
stability and functioning of the biosphere's natural systemsoften
referred to as "life-support systems."
Deliberate modification of these ecological and physical systems by human societies
throughout history has conferred many social, economic, and public health benefits.
However, it also has often created new risks to health, such as via mobilization
of infectious agents, depletion of freshwater supplies, and reduced productivity
of agroecosystems (Hunter et al., 1993; Gubler, 1996). Consider, for
example, the chain of consequences from clearance of tropical forests. In the
first instance, it typically leads to a warmer and drier local climate. The
consequent drying of soil and loss of its organic structure predisposes the
area to increased water runoff during heavy rainfall. This, in turn, can endanger
human health via flooding, water contamination, impaired crop yields, and altered
patterns of vector-borne infectious diseases. Meanwhile, forest clearance also
contributes to the atmospheric buildup of carbon dioxide (CO2) and
hence to climate change and its health impacts.
Today, as the scale of human impact on the environment increases, a range of
population health impacts can be expected from these large-scale changes in
the Earth's life-support systems (Watson et al., 1998). That is the complex
context within which actual and potential health impacts of global climate change
must be assessed.
9.1.1. Summary of IPCC Second Assessment Report (1996):
Potential Health Impacts of Climate Change
The IPCC Second Assessment Report (McMichael et al., 1996a) relied on the relatively
limited scientific literature that had emerged during the late 1980s and early
1990s. Most published studies were on health impacts associated with climate
variability (e.g., El Niño) and extreme events (natural disasters and
heat waves). Predictive modeling of future health impacts was in an early developmental
The SAR noted the many inherent uncertainties in forecasting the potential
health impacts of climate change. This included recognition that various other
changes in social, economic, demographic, technological, and health care circumstances
would unfold over coming decades and that these developments would "condition"
the impact of climatic and environmental changes on human health. However, such
accompanying changes can be foreseen neither in detail nor far into the future.
The overall assessment was that the likely health impacts would be predominantly
adverse. Reflecting the published literature, most of the specific assessments
were nonquantitative and relied on expert judgment. They drew on reasoned extrapolations
from knowledge of health hazards posed by extreme weather events, increases
in temperature-dependent air pollution, summertime increases in certain types
of food poisoning, and the spectrum of public health consequences associated
with economic disruption and physical displacement of populations. It was noted
that the projected effects of climate change on agricultural, animal, and fishery
productivity could increase the prevalence of malnutrition and hunger in food-insecure
regions experiencing productivity downturns.
For two of the anticipated health impacts, the published literature available
by 1995 allowed a more quantitative approach. The relevant conclusions were
9.1.2. Population Health and its Significance as an Outcome
of Climate Change
- An increase in the frequency or severity of heat waves would cause a short-term
increase in (predominantly cardiorespiratory) deaths and illness. In some
very large cities (e.g., Atlanta, Shanghai) by about 2050, this would result
in up to several thousand extra heat-related deaths annually. This heat-related
mortality increase would be offset by fewer cold-related deaths in milder
winters, albeit to an extent that was not yet adequately estimated and likely
to vary between populations.
- Climate-induced changes in the geographic distribution and biological behavior
of vector organisms of vector-borne infectious diseases (e.g., malaria-transmitting
mosquitoes) and infective parasites would alterusually increasethe
potential transmission of such diseases. For example, simulations with global/regional
mathematical models indicated that, in the absence of demographic shifts,
the proportion of the world's population living within the potential
malaria transmission zone would increase from ~45% in the 1990s to ~60% by
2050. Some localized decreases in malaria transmissibility also may occur
in response to climate change.
This is the last of the sector-impact chapters in this volume. This is appropriate
because human population health is influenced by an extensive "upstream"
range of environmental and social conditions. Indeed, over time, the level of
health in a population reflects the quality of social and natural environments,
material standards of living, and the robustness of the public health and health
service infrastructure. Therefore, population health is an important integrating
index of the effects of climate change on ecosystems, biological processes,
physical environmental media, and the social-economic environment.
Two other points are important. First, the causation of most human diseases
is complex and multifactorial. Second, there is great heterogeneity in the types
of disease: acute and chronic; infectious and noninfectious; physical injury
and mental health disorders. These two considerations explain some of the difficulties
in fully understanding and quantifying the influences of climate on human health.
Profiles of health and disease vary greatly between regions and countries and
over time. Currently, noncommunicable diseases (including mental health disorders)
predominate in developed countries, with cardiovascular diseases and cancer
accounting for more than half of all deaths. In poorer countries, infectious
diseases (especially in childhood) remain important, even as noncommunicable
diseases increase in urbanizing populations that are exposed to changes in lifestyle
and environmental and occupational exposures. Globally, infectious diseases
remain a major cause of human morbidity and are responsible for approximately
one-third of all deaths (WHO, 1999a). Many of these water-, food-, and vector-borne
infectious diseases are sensitive to climate.