FIGURE SA-1 People affected by hydrometeorological disaster (millions per year). SOURCE: Reproduced from United Nations Development Programme (2007) with permission of Palgrave Macmillan.
nessed, therefore, in order to assess changes in biological variables—including the geographic range and incidence of diseases—in relation to changes in temperature and precipitation (see Chapter 1). Information obtained from a variety of monitoring and mapping techniques can be integrated into geographic information systems (GISs) and used to identify and compare physical and biological phenomena. By enabling the overlay of multiple sets of data, GISs also provide contributions to descriptive and mathematical models that may be used to project the biological impacts of various climate change scenarios. Additional methods are used to analyze data gathered across scientific disciplines in order to reveal patterns and emerging trends associated with climate change, calculate rates of change (i.e., in the geographic range, prevalence, and incidence of infectious diseases), and compare these observations with predicted outcomes.
Many of the methodologies used to study the effects of climate change yield correlations, rather than proof of causation, Epstein acknowledged, but he argued that when observational data from multiple sources (1) match model projections,
(2) are consistent with each other, and (3) can be explained by plausible biological mechanisms, the preponderance of the evidence warrants further attention and exploration. Moreover, he added, models could be used to test such associations and their apparent underlying mechanisms (see Chapter 1).
In particular, Epstein identified three outcome variables as central to understanding the effect of climate change on the distribution of infectious diseases: shifts in altitude (and latitude), changes in seasonality, and responses to increased weather variability.
Shifts in altitude Many animal and plant species are adapted to specific habitats that occupy a narrow range along altitudinal and latitudinal climatic gradients.7 Increasing temperatures not only melt alpine glaciers and drive the upward migration of plant communities, but also enable insects and other species that serve as infectious disease vectors to occupy higher altitudes (Epstein et al., 1998).8 Such changes in conditions—which are conducive to changes in the ranges of disease agents and vectors—are occurring at high-altitude locations across the globe: in the Andes, the Sierra Nevada, the East African highlands, the European Alps, and the mountainous regions of India, Nepal, and Papua New Guinea, Epstein observed.
Seasonal shifts Climatic warming is expected to lengthen seasonal activity periods for mosquitoes and other insect vectors, thereby increasing opportunities for exposure to infectious diseases such as malaria (Tanser et al., 2003; van Lieshout et al., 2004). Ecological opportunists—including insects and rodents that serve as vectors of, and reservoirs for, infectious diseases—tend to proliferate rapidly in disturbed environments, while large predator species (infectious disease hosts) suffer under unstable environmental conditions, Epstein said.
Responses to increased weather variability Increased climate variability, along with habitat fragmentation and pollution, is likely to alter predator-prey relationships, which in turn influence infectious disease transmission dynamics. Such disequilibrium is thought to have precipitated the 1993 outbreak of a rodent-borne infection, hantavirus pulmonary syndrome, in the Four Corners region of the southwestern United States. That year, early, heavy rains ended an intense drought (during which predator populations declined) and provided new food for rodents, whose populations then expanded rapidly (Calisher et al., 2005; Patz et al., 1996).
7Plant and animal species first adapt to temperature changes by shifting their elevational ranges. A 1 km change in altitude is estimated to correspond to a geographic shift of 600 km north or south (Peters and Lovejoy, 1994). Highlands are considered sentinel regions for monitoring the biological response to global climate change.
8While some vectors may already be present at higher altitudes, higher temperatures may shorten the extrinsic incubation period, allowing the vector to transmit disease.
While it is anticipated that climate change will influence infectious disease emergence, several workshop participants emphasized that direct causal connections have yet to be established between climate change and infectious diseases, and that accurate predictions of infectious disease behavior cannot yet be made on the basis of climate projections alone.
"Climate change will affect the health of humans as well as the ecosystems and species on which we depend, and . . . these health impacts will have economic consequences," predicts a recent report published by the Center for Health and the Global Environment (2005), edited by Epstein and Evan Mills (see Chapter 1 for the executive summary of this report, Climate Change Futures: Health, Ecological and Economic Dimensions). The report highlights a broad range of known and anticipated health consequences of climate change for humans, animals, and plants. In addition to influencing the location and frequency of infectious disease emergence and outbreaks, these effects include increased pest damage of crop plants, which in turn could contribute to human malnutrition; greater concentrations of pollen and fungi in the air, raising the risk of allergic symptoms and asthma; and higher rates of injury and death due to weather disasters and fires. Indeed, as Epstein (2005) has concluded, "it would appear that we may be underestimating the breadth of biologic responses to changes in climate."
Figure SA-2 illustrates the multiple pathways by which variations in climate affect the health of humans, animals, and plants. Direct influences include long-term regional changes in average temperature and precipitation, as well as extreme weather events such as floods, droughts, or violent storms. Climate change may also exert health effects indirectly, by altering ecosystems in ways that, for example, affect the geographic distribution or transmission dynamics of infectious diseases.
Direct and Indirect Effects of Climate on Infectious Diseases
Climate exerts both direct and indirect influences on the transmission and geographic distribution of infectious diseases, such as those shown in Table SA-2 (NRC, 2001). Direct effects of climate on infectious disease occur through the following mechanisms:
• Pathogen replication rate. This is particularly true of vector-borne diseases of warm-blooded animals, due to the exposure of pathogens to ambient weather conditions for part of their life cycle.
• Pathogen dissemination. This occurs when floods contaminate drinking water reservoirs, resulting in diarrheal diseases, and also when dry winds distribute soil-borne pathogens.
Mitigation Policies for Reduction of Greenhouse Gas Emissions
Use of Renewable Energy Sources Forest Preservation
Population Density and Growth
Level of Technological Development
Standard of Living and Local Environmental Condition
Preexisting Health Status
Quality and Access to Health Care
Public Health Infrastructure
Weather Forecasting and Warning Systems
Emergency Management and Disaster Preparedness
Public Health Education and Prevention
Legislation and Administration
FIGURE SA-2 Potential health effects of climate variability and change.
SOURCE: Reprinted with permission from the American Medical Association from Haines and Pätz (2004). Copyright 2004. All rights reserved; adapted from Patz et al. (2000).
12 GLOBAL CLIMATE CHANGE AND EXTREME WEATHER EVENTS
Primarily tropical distribution,
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