The impacts of developmental, climatic and environmental scenarios on population health are important for health-system planning processes. Also, future trends in health are relevant to climate change because the health of populations is an important element of adaptive capacity.
The use of scenarios to explore future effects of climate change on population health is at an early stage of development (see Section 8.4.1). Published scenarios describe possible future pathways based on observed trends or explicit storylines, and have been developed for a variety of purposes, including the Millennium Ecosystem Assessment (Millennium Ecosystem Assessment, 2005), the IPCC Special Report on Emissions Scenarios (SRES, Nakicenovic and Swart, 2000), GEO3 (UNEP, 2002) and the World Water Report (United Nations World Water Assessment Programme, 2003; Ebi and Gamble, 2005). Examples of the many possible futures that have been described include possible changes in the patterns of infectious diseases, medical technology, and health and social inequalities (Olshansky et al., 1998; IPCC, 2000; Martens and Hilderink, 2001; Martens and Huynen, 2003). Infectious diseases could become more prominent if public-health systems unravel, or if new pathogens arise that are resistant to our current methods of disease control, leading to falling life expectancies and reduced economic productivity (Barrett et al., 1998). An age of expanded medical technology could result from increased economic growth and improvements in technology, which may to some extent offset deteriorations in the physical and social environment, but at the risk of widening current health inequalities (Martens and Hilderink, 2001). Alternatively, an age of sustained health could result from more wide-ranging investment in social and medical services, leading to a reduction in the incidence of disease, benefiting most segments of the population.
Common to these scenarios is a view that major risks to health will remain unless the poorest countries share in the growth and development experienced by richer parts of the world. It is envisaged also that greater mobility and more rapid spread of ideas and technology worldwide will bring a mix of positive and negative effects on health, and that a deliberate focus on sustainability will be required to reduce the impacts of human activity on climate, water and food resources (Goklany, 2002).
The health of populations is an important element of adaptive capacity. Where there is a heavy burden of disease and disability, the effects of climate change are likely to be more severe than otherwise. For example, in Africa and Asia the future course of the HIV/AIDS epidemic will significantly influence how well populations can cope with challenges such as the spread of climate-related infections (vector- or waterborne), food shortages, and increased frequency of storms, floods and droughts (Dixon et al., 2002).
The total number of people at risk, the age structure of the population, and the density of settlement are important variables in any projections of the effects of climate change. Many populations will age appreciably in the next 50 years. This is relevant to climate change because the elderly are more vulnerable than younger age groups to injury resulting from weather extremes such as heatwaves, storms and floods. It is assumed (with a high degree of confidence) that over the course of the 21st century the population will grow substantially in many of the poorest countries of the world, while numbers will remain much the same, or decline, in the high-income countries. The world population will increase from its current 6.4 billion to somewhat below 9 billion by the middle of the century (Lutz et al., 2000), but regional patterns will vary widely. For example, the population density of Europe is projected to fall from 32 to 27 people/km2, while that of Africa could rise from 26 to 60 people/km2 (Cohen, 2003). Currently, 70% of all episodes of clinical Plasmodium falciparum malaria worldwide occur in Africa, and that fraction will rise substantially in the future (World Bank et al., 2004). Also relevant to considerations of the impacts of climate change is urbanisation, because the effects of higher temperatures and altered patterns of rainfall are strongly modified by the local environment. For instance, during hot weather, temperatures tend to be higher in built-up areas, due to the urban heat-island effect. Almost all the growth in population in the next 50 years is expected to occur in cities (and in particular, cities in poor countries) (Cohen, 2003). These trends in population dominate calculations of the possible consequences of climate change. These are two examples: projections of the numbers of people affected by coastal flooding and the spread of malaria are more sensitive to assumptions about future population trajectories than to the choice of climate-change model (Nicholls, 2004; van Lieshout et al., 2004).
For much of the world's population, the ability to lead a healthy life is limited by the direct and indirect effects of poverty (World Bank et al., 2004). Although the percentage of people living on less than US$1/day has decreased in Asia and Latin America since 1990, in the sub-Saharan region 46% of the population is now living on less than US$1/day and little improvement is expected in the short and medium term. Poverty levels in Europe and Central Asia show few signs of improvement (World Bank, 2004; World Bank et al., 2004). Economic growth in the richest regions has outstripped advances in other parts of the world, meaning that global disparities in income have increased in the last 20 years (UNEP and WCMC, 2002).
In the future, vulnerability to climate will depend not only on the extent of socio-economic change, but also on how evenly the benefits and costs are distributed, and the manner in which change occurs (McKee and Suhrcke, 2005). Economic growth is double-sided. Growth entails social change, and while this change may be wealth-creating, it may also, in the short term at least, cause significant social stress and environmental damage. Rapid urbanisation (leading to plummeting population health) in western Europe in the 19th century, and extensive land clearance (causing widespread ecological damage) in South America and South-East Asia in the 20th century, are two examples of negative consequences of rapid economic growth (Szreter, 2004). Social disorder, conflict, and lack of effective civic institutions will also increase vulnerability to health risks resulting from climate change.
Health services provide a buffer against the hazards of climate variability and change. For instance, access to cheap, effective anti-malarials, insecticide-treated bed nets and indoor spray programmes will be important for future trends in malaria. Emergency medical services have a role (although not a predominant one) in limiting excess mortality due to heatwaves and other extreme climate events.
There are other determinants of vulnerability that relate to particular threats, or particular settings. Heatwaves, for example, are exacerbated by the urban heat-island effect, so that impacts of high temperatures will be modified by the size and design of future cities (Meehl and Tebaldi, 2004). The consequences of changes in food production due to climate change will depend on access to international markets and the conditions of trade. If these conditions exclude or penalise poor countries, then the risks of disease and ill-health due to malnutrition will be much higher than if a more inclusive economic order is achieved. Changes in land-use practices for the production of biofuels in place of grain and other food crops will have benefits for greenhouse gas emissions reductions, but the way in which the fuels are burnt is also important (see Section 8.7.1).
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