Climatic conditions that support the successful colonization of geographical locations by human societies also tend to support the populations of pests and pathogens associated with human diseases. Abrupt climate change can destabilize trends in the distribution of diseases in populations, as well as society's ability to cope with emerging pathogens and shifting demographic patterns.

The most important factor in the emergence and proliferation of pathogens is the availability of susceptible hosts. Therefore, many pathogens have coevolved, not only with human biological constraints against disease, but also with socially developed constraints such as climate-controlled domiciles and disinfection. In the event of abrupt climate change, leading to excessive fluctuations in extreme weather conditions, there occurs a selection process that affects the microbial diversity of ecosystems, with some organism declining, whereas other organisms increase in population density. In addition, long-term climate change may forge new interactions among different organisms. When these biodiversity changes coincide with increasingly dynamic relocation of people in response to climatic events, epidemics can result from the resurgence of old diseases, emergence of new diseases, or exacerbation of preexisting disease conditions.

Seasonal trends in morbidity and mortality have long been understood by human societies. Such understandings have formed the basis of preventive health care plans in many countries. For example, preparation for the influenza (flu) season means massive vaccination campaigns during the months of September, October, and November. Similarly, respiratory conditions with no clear involvement of pathogenic agents, such as asthma and allergies, are known to follow seasonal patterns. Humans have, more or less, adapted to such seasonal inconveniences until they become so extreme to the extent that population migrations can occur. Outbreaks of contagious disease associated with scarce water supplies can destabilize communities or force local extinctions in human habitats. However, it has not been straightforward to project health impacts as part of the consequences of anthropogenic climate change.

In 1990, a World Health Organization (WHO) task group issued one of the early reports on the potential health effects of climate change. The group based its assessments on the scenario that global average temperature could increase by 3œ Celsius by the year 2030; that sea level could rise by 0.10-0.32 meter; and that ultraviolet radiation, mainly UV-B, is expected to increase by a maximum of 20-25 percent in the same period. Based on these conditions, the task group anticipated both direct and indirect effects of climate change on human health.

The direct effects include those associated with thermal factors (heat disorders) and the effects of UV radiation on the incidence of skin cancer, immune response, eye function, and air quality. Indirect effects of climate change on human health are expected to include impacts on food production and nutrition, on wildlife and biodiversity, and on communicable diseases through effects on disease vectors and the incidence of infectious diseases that are not associated with specific vectors. In addition, indirect impacts of climate on health include the repercussions of human migration.

In 1997, responding to a request from the Subsidiary Body for Scientific and Technological Advice (SBSTA) of the United Nations Framework Convention on Climate Change (UNFCCC), Working Group II of the Intergovernmental Panel on Climate Change (IPCC) published a special report on the assessment of vulnerability in the regional impact of climate change. The Conference of the Parties (COP) to the UNFCCC needed information on the degree to which human conditions and the natural environment are vulnerable to the potential effects of climate change, but the regional assessment approach adopted by the IPCC revealed wide variation in the vulnerability of different populations, especially in the health sector. Different levels of vulnerability exist under similar climate and pathogen distribution patterns because of local economic, social, and political conditions, as well as the level of dependence on resources sensitive to climate variability. Therefore, instead of producing quantitative predictions of the impacts of climate change for each region, the IPCC took the approach of assessing regional sensitivities and vulnerabilities.

The adoption of "Weather, Climate, and Health" as the theme of the 1999 World Meteorological Day signified the convergence of global issue-framing strategies with the health impacts of climate change. This event emerged after more than a decade of policy formulation and scientific assessment activities by the WHO, the IPCC, and the World Meteorological Organization (WMO). Following the progress made by WHO researchers during the 1990's toward the development of quantitative methods for assessing the global burden of disease, it became possible to compare or project into the future the disease burden associated with specific risk factors such as climate change. Composite measures of disease burden such as disability-adjusted life years (DALYs) account for both mortality and morbidity, and are particularly suitable for evaluating risk factors with a broad range of disease end points. For example, in 2004, WHO estimated that global climate change accounts for approximately 5.5 million DALYs lost directly, but exacerbation of disease conditions associated with the creation of unsanitary conditions could result in a lot more DALYs lost. Not surprisingly, children younger than five years are particularly vulnerable.

To cap the evolution of health effects as a dominant frame of reference for the threats associated with climate change, in May 2008, the 193 member countries represented at the World Health Assembly adopted a resolution to protect public health from impending global climate change. This event signaled a much higher level of commitment from the health sector to strengthen the evidence for anthropogenic climate change and to better characterize the risks to public health at the regional and global levels.

References:

1)         Campbell-Lendrum, Diarmid, and Rosalie Woodruff. Climate Change: Quantifying the Health Impacts at National and Local Levels. Environmental Burden of Disease 14. Geneva: World Health Organization, 2007.

2)         Intergovernmental Panel on Climate Change. Climate Change, 2007--Impacts, Adaptation, and Vulnerability: Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Edited by Martin Parry et al. New York: Cambridge University Press, 2007.

3)         Ogunseitan, Oladele A. "Framing Environmental Change in Africa: Cross-Scale Institutional Constraints on Progressing from Rhetoric to Action Against Vulnerability." Global Environmental Change 13 (2003): 101-111.

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