9.2. Research into the Relationship between Climate Change and Health: Caveats
and Challenges
9.2.1. New Knowledge about Climate Change Impacts on Health
Since the SAR, much of the additional research on health impacts has examined
natural climate variability in relation to interannual variations in infectious
diseases—particularly vector-borne diseases—and the relationship between
daily weather and mortality in various urban populations. Predictive modeling
of the impact of climate scenarios on vector-borne disease transmissibility
has undergone further development. Meanwhile, however, data sets that allow
study of the effects of the health impacts of observed longer term trends in
climate remain sparse.
9.2.2. Characteristics and Methodological Difficulties
The research task of assessing the actual and potential health impacts of climate
change has several distinctive characteristics and poses four major challenges
to scientists:
- Anticipated anthropogenic climate change will be a gradual and long-term
process. This projected change in mean climate conditions is likely to be
accompanied by regional changes in the frequency of extreme events. Changes
in particular health outcomes already may be occurring or soon may begin to
occur, in response to recent and ongoing changes in world climate. Identification
of such health effects will require carefully planned epidemiological studies.
- In epidemiological studies (in which associations are observed with or without
knowledge of likely causal mechanisms), there often are difficulties in estimating
the role of climate per se as a cause of change in health status. Changes
in climate typically are accompanied by various other environmental changes.
Because most diseases have multiple contributory causes, it often is difficult
to attribute causation between climatic factors and other coexistent factors.
For example, in a particular place, clearing of forest for agriculture and
extension of irrigation may coincide with a rise in regional temperature.
Because all three factors could affect mosquito abundance, it is difficult
to apportion between them the causation of any observed subsequent increase
in mosquito-borne infection. This difficulty is well recognized by epidemiologists
as the "confounding" of effects.
- It is equally important to recognize that certain factors can modify the
vulnerability of a particular population to the health impacts of climate
change or variability. This type of effect-modification (or "interaction")
can be induced by endogenous characteristics of the population (such as nutritional
or immune status) or contextual circumstances that influence the "sensitivity"
of the population's response to the climate change (such as unplanned
urbanization, crowding, or access to air conditioning during heat waves).
Deliberate social, technological, or behavioral adaptations to reduce the
health impacts of climate change are an important category of effect-modifying
factor.
- Simulation of scenario-based health risks with predictive models entails
three challenges. These challenges relate to validity, uncertainty, and contextual
realism:
- Valid representation of the main environmental and biological relationships
and the interacting ecological and social processes that influence the
impact of those relationships on health is difficult. A balance must be
attained between complexity and simplicity.
- There are various sources of (largely unavoidable) uncertainty. There
is uncertainty attached to the input scenarios of climate change (and
of associated social, demographic, and economic trends). Subsequently,
there are three main types of uncertainties in the modeling process itself:
"normal" statistical variation (reflecting stochastic processes
of the real world); uncertainty about the correct or appropriate values
of key parameters in the model; and incomplete knowledge about the structural
relationships represented in the model.
- Climate change is not the sole global environmental change that affects
human health. Various large-scale environmental changes now impinge on
human population health simultaneously, and often interactively (Watson
et al., 1998). An obvious example is vector-borne infectious diseases,
which are affected by climatic conditions, population movement, forest
clearance and land-use patterns, freshwater surface configurations, human
population density, and the population density of insectivorous predators
(Gubler, 1998b). In accordance with point 2 above, each change in health
outcome must be appropriately apportioned between climate and other influences.
