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Climate change and human health

Paper 2.1 - Part 1B - Human health issues


Anthony J McMichael

"Sustainability" and population health
Climate change and health
Infectious diseases
Implications for society at large

During 2006, Australians and people everywhere have begun to engage increasingly, and with growing concern, with the issue of human-induced global climate change. This is, by any criterion, an extraordinary phenomenon – in terms of its very nature (fancy the human enterprise now being of such dimensions that it is changing the way the planet works), its scale, the rate at which it is now evolving, and the diversity of its impacts.

Climate change is part of a larger syndrome. Contemporary phenomena such as global climate change, ecosystem disruption and the growth of massive modern metropolises pose great new challenges to our capacity to maintain satisfactory, health-supporting, environmental and social conditions. These environmental changes and their attendant health risks are of both unfamiliar scale and great complexity – and one aspect of unfamiliarity and complexity is that they are looming as major determinants offuturepopulation health. They are casting ever-longer shadows over future generations (McMichael, 2001; Raven, 2002; WHO, 2005).

Environmental health research has traditionally studied the effects of localised exposures to specific direct-acting agents (such as heavy metals, pesticides, ionising radiation, or microbes in local food or water). Today, though, we face a manifestly different category of environmental health hazard – one that occurs on a larger scale and impinges on health predominantly via systems-based changes to the environment (including climate, food-producing systems, freshwater circulation and ecosystem "goods and services").

The huge, escalating, global problem of climate change is now widely recognized. The basic science is no longer contentious. Climate scientists foresee a rise in average Earth’s surface temperature of several degrees centrigrade this century (Figure 1). Note also the rapid rise in temperature of approximately 0.5oC since 1975. More than four-fifths of this rise has been attributed to human actions that have affected atmospheric gaseous composition (IPCC 2001).

climat health fig 1Climate Health Fig1

Figure 1.
Estimated variations in Earth’s average global surface temperature since 1000 AD, with the modelled likely range of increased temperature over the coming century in response to the ongoing build-up of atmospheric greenhouse gas concentration. Graph based on data from Intergovernmental Panel on Climate Change (IPCC, 2001).

Increasingly, this forecast is looking like an under-estimate – climate conditions and related phenomena (extreme events, ice melting, ocean current shifts, etc.) are changing considerably faster than was foreseen a short decade ago.

These larger-scale systemic risks to population health are a fundamental signal that humankind is now on a non-sustainable path. The following sections explore: (i) the (fundamental) relationship of population health to sustainability; (ii) several aspects of the relationship between large-scale human-induced changes to natural environmental systems and human health; and (iii) identify some of the associated challenges to researchers.

"Sustainability" and population health

There is an urgent need for us to examine therationale, the purpose, of our striving for "sustainability". In my view, the primary (and unashamedly anthropocentric) reason for our concern about non-sustainability is because today’s generally adverse trends in environmental conditions, ecosystem functioning and non-renewable resource management have huge, and growing, implications for human wellbeing, health and (in some parts of the world already) survival. This is not to discount the important and moral obligation that we humans have to sustain as much of this planet’s (wonderful) natural systems for their own sake – but, frankly, it is also a matter of absolute self-interest for us, as a species.

The Wikipedia defines sustainability as a system that seeks to achieve "parallel care and respect for the ecosystem and the people within". As many scientists and other writers are now making clear, human society, its economy and the psychological and biological wellbeing and health of its members depend absolutely on the natural environment and its life-support systems. The Swedish Parliament has recognised this crucial linkage. Of the five fundamental principles underpinning their Environmental Objectives (1999), the first-listed is "Promoting human health".

This, unfortunately, is an all-too-rare example of enlightened understanding as to why human societies need, today, to take action to achieve environmental/ecological sustainability. Indeed, much of our day-to-day discussion of "sustainability" focuses on achieving a balance between environmental conditions, social conditions and economic productivity. This is now widely referred to, misleadingly, as "the triple bottom line". However, those three entities are actuallymeans; they are notends. Again: the true objective of our achieving a sustainable way-of-living is to ensure the continuation of good and equitable experience (both biological and social) for humans.

Human wellbeing and health ought, then, to be the central criterion of sustainability. "Ecological sustainability" is not just about maintaining the flows from and into the natural world that sustain the economic engine nor maintaining iconic species and iconic ecosystems. It is about maintaining the complex systems that support health and life.

Climate change and health

Human-induced climate change is just one part of a much larger problem of "global environmental change". The human enterprise, in aggregate, is now placing unprecedented pressures on the biosphere, and is beginning to cause systemic changes. As we destabilise the climate system and disrupt climate-sensitive natural systems, we inevitably incur adverse health risks – directly and indirectly, now and increasingly in future. Some health benefits may occur in some locations. However, any such benefits will be the (perhaps temporary) exceptions.

Climate change and its environmental and social impacts poses a range of risks to health. The possibilities include impacts of heatwaves and other extreme weather events, changes to air quality, the geography and seasonality of various infectious diseases, effects on local food yields, freshwater supplies, the general vitality of ecosystems and the flow of "services" from them, and the underpinning of diverse livelihoods.

The risks to health arise via very diverse pathways. Consider the problems of climate change and the anticipated reduced rainfall in much of Sub-Saharan Africa. If farming families have no crops to sell because of drought, the risks from HIV and AIDS will rise yet further when women and children are forced to abandon drought-desiccated land and make their living, often as sex workers, in cities.

In Australia the main types of anticipated health impacts are:

  • The impacts of thermal stress (esp. heatwaves) on serious illness events and deaths
  • Physical and other hazards from extreme weather events (floods, fires, storms and droughts)
  • Changes in geographic range, season and incidence of various infectious diseases
  • Social and mental health problems in rural communities affected by falling farm yields
  • The diverse health consequences of increased immigration/refugee flows in Asia-Pacific region

In general, climate change will not cause exotic new health disorders. However, it will alter the probabilities of many familiar risks to health. Figure 2 shows three types of causal pathways: (1) direct; (2) those mediated by changes in natural systems; and (3) those due to social, economic and demographic disruption.

climat health fig 2Climate Health fig2

Figure 2.
Climate change and health: The three main pathways of risks to health.

Examples of the first two pathways are: (1) heatwave-related deaths and (2) the climatic influences on the occurrence of mosquito-borne infectious diseases.


As the world warms and as regional weather patterns become more variable, there will be more frequent and more intense heatwaves (and, probably, more variable patterns of storms, floods and cyclones). Heatwaves place great physiological stress on people, especially if the heat is sustained overnight (as in inner-urban areas especially) and if the event lasts for more than just several days. Most of the deaths occur in the frail, elderly, sick and very young.

The 2003 heatwave in Europe was a major killer, extreme in both its duration and temperature. It extended over much of western Europe. The 12-day heatwave caused an excess of approximately 35,000 deaths during the event. In Paris, the average daily temperature was about 12 degrees C above the usual August average for much of the heatwave. An estimated 900 excess deaths occurred in Paris during the event.

Infectious diseases

Many infectious diseases are sensitive to climatic conditions, particularly insect-borne infections and those spread person-to-person via contaminated food and water. Globally, malaria, dengue fever, cholera, and food-borne infections are of particular concern. There have been several recent scientific reports that suggest that recent climate change has already influenced some infectious diseases. These include the northwards extension of tick-borne encephalitis in Sweden over the past two decades, associated with warming winters; and the ascent of highland malaria to higher altitudes in some parts of eastern Africa.

The risks to population health from climate change pose a challenge for health-impact researchers., because it is difficult to tease out the various factors.

For example, malaria may have moved to higher altitudes in eastern Africa. Is this due to the warming that accompanied it – or is it due to changes in land-use, population movement, cessation of mosquito control programs, or the emergence of anti-malarial resistance? A tough question.

Of course, the causal pathways are not as simple as suggested by Figure 2. Many non-climate modulating influences affect climate-related health outcomes – such as material standard of living, population growth and demographic change, public-health infrastructure, access to healthcare and quality of healthcare. This raises the possibility of near-term adaptive strategies to lessen adverse health impacts. Adaptation includes actions, planned or unplanned, such as public education, the use of protective technologies, vaccination programs, disease surveillance, monitoring, use of climate forecasts and health-impact forecasts, and development of emergency-management and disaster-preparedness programs.

One further comment is very important, here. In addition to attempts at ‘holding-operation’ adaptation strategies, true primary prevention of the risks to health requires substantial abatement of global greenhouse gas emissions. While individuals, families and communities should contribute to the collective effort to abate emissions, the climate change problem is so large and systemic that only government-led mitigation, including via internationally agreed targets, emissions limitations and carbon trading, and penalties for non-compliance, will ensure that climate change is slowed and, hopefully, arrested.

Estimating Future Health Impacts in Australia

We have recently carried out a series of preliminary studies of climate-and-health relationships in Australia, and have used these, and other published results, to estimate likely future health impacts occurring in response to CSIRO’s climate change scenarios for 2020 and 2050.

The main findings were that the following changes in health risks would be likely to occur:

1. Increased summer-time deaths from heat extremes in all major cities.

These estimated increases will be a function of increased thermal stress affecting an ageing (more susceptible) population. On average, in each city there would be several hundred more deaths annually (assuming no increase in actual population size). In several countries (e.g. UK), estimates show that decreases in winter mortality due to milder winters may partly/fully compensate for increases in summer mortality. In Australia, heat-related deaths are unlikely to be much offset by cold-related averted deaths.

2. Southwards extensions of the mosquito-borne of dengue fever (see diagram below) and malaria.

In order to contain any such spread, public health surveillance and control measures would need to be extended into these at-risk regions. The size of the population at risk increases, and the costs of control would escalate commensurately.

Dengue Fever: Estimated geographic region suitable for maintenance of Ae. aegypti, under alternative climate scenarious for 2050

climat health fig 3a

climat health fig 3b

climat health fig 3c


NCEPH/CSIRO/BoM/UnivOtago, 2003

3. Increased risk of diarrhoeal disease, in summer, and especially in remote/rural communities. For example, we estimated a 15% increase in diarrhoeal hospitalisations in Aboriginal children living around Alice Springs by 2020.

4. Increased mortality and injury risk from inland floods, with increased health impacts expected in parts of Queensland, NSW and Victoria, and reduced impacts in SA and WA.

Note that the above estimation of future health impacts is based on climate change scenarios limited to changes inmeanclimate conditions. Increasingly it appears likely that changes in climate variability that will accompany climate change, especially changes in frequency, intensity and location of extreme events, will have greater health and other impacts than will changes in mean conditions.

Environmental refugees

As demographic and environmental conditions deteriorate in vulnerable regions, there will be increased immigration and environmental refugee pressures on neighbouring (especially richer) countries. This will be accompanied by the diverse adverse health consequences that typically affect displaced persons – and, perhaps, their hosts.

The UN’s latest assessment of this knock-on effect of climate change and other large-scale environmental changes is rather alarming. Within the next two decades there could be as many as 50 million people displaced by environmental deterioration. Sea level rise, floods, desertification and land degradation have already contributed to large out-migrations. In the Pacific, with rising seas, the small population of the Carteret Atoll has recently been relocated, and the island population of Tuvalu is planning to do likewise. A recent report by the CSIRO (2006) has underscored the rapidly increasing likelihood and substantial scale of this international moral, economic and geopolitical problem – of an increase in the numbers of displaced and refugee persons.

Implications for society at large

Globally, the emerging situation poses more than an environmental problem; it poses a political and moral problem in that the great majority of the greenhouse gas emissions to date have come from today’s developed countries, whereas most of the adverse health risks, by dint of population location and vulnerability, occur in developing countries. Sub-Saharan Africa, hugely dependent on rain-fed agriculture and already widely impoverished, under-fed and ravaged by infectious diseases (especially HIV/AIDS, tuberculosis, malaria and child diarrhoea), is especially vulnerable to the environmental and social stresses of climate change (especially the expected decline in rainfall).

True primary prevention must depend on a massive international effort to curtail greenhouse gas emissions and, hence, climate change. This looms as a huge and unprecedented international challenge – and, as yet, few countries are making a serious effort to help slow the process. Australia has been one of the slowest of the developed countries to make a commitment to genuine internationalism in this matter.

No matter what degree of international commitment is achieved over the coming decade, the world is already committed to at least another half-to-one degree centigrade rise in average surface temperature. This reflects the enormous inertia in the climate system and the associated slow redistribution of excess energy (heat) throughout the world’s oceans. Therefore, every country needs now to also take secondary preventive action – that is, to identify the main health risks confronting their population, the most vulnerable sections of the population, and the appropriate interventions to reduce those health risks.

The threat of serious illness and death due to extremes of heat can be reduced by simple, immediate measures – such as early warnings for such events (by meteorology agencies), special provision for (or attention to) the elderly and sick, and public education about clothing, staying indoors and household ventilation. However, to render communities more ‘heatwave-proof’ in the longer term will require wider-ranging and farsighted changes to urban layout, transport systems, housing design, and the nurturing of green space.

Whether in Australia or elsewhere, it will be important for government and community to think in imaginative and multi-sectoral fashion about how to modulate living environments, physical infrastructure, ecosystem management, social institutions and public information flows in order to achieve more resilience against the shocks and stresses of climate change.

The task, and indeed the moral responsibility, of our generation is to achieve mid-course corrections that will steer the biosphere back to a life-supporting state able to sustain the wellbeing and health of future human generations.


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Tony McMichael is Director of the National Centre for Epidemiology and Population Health at the Australian National University, Canberra. Originally from Adelaide, he was during 1994-2001 Professor of Epidemiology at the London School of Hygiene and Tropical Medicine, UK. His main epidemiological research interest is in studying environmental influences on human health. Since 1993 he has played a central role in the scientific assessment of health risks for the UN's Intergovernmental Panel on Climate Change (IPCC), and has played a corresponding role in the recently-completed international Millennium Ecosystem Assessment Project (2001-2005). He has been an advisor to WHO, the UN Environment Program, and the World Bank on health risks from environmental exposures and global environmental changes. Within the Earth System Science Partnership of the International Council of Science he co-chairs the newly-launched international research network on Global Environmental Change and Health. He has authored several books, including most recently Human Frontiers, Environments and Disease: Past Patterns, Uncertain Futures (Cambridge University Press, 2001), and (as senior editor) Climate Change and Human Health: Risks and Responses (WHO/UNEP/WMO, 2003).


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