warmer and sicker? Global warming and human health

Global warming will have many impacts on human health

In August 2003, Europe suffered its worst heatwave in recent memory. In France, temperatures peaked at about 40°C; unprepared for that kind of heat, many people – mostly the sick and elderly – succumbed. In all, nearly 15,000 deaths in France that summer were attributed to the high temperatures; across Europe, the scorching weather may have claimed as many as 35,000 lives.
According to the bulk of scientific opinion, the world is getting warmer. It is difficult, if not impossible, to prove the causes of this warming, but many scientists are convinced that increasing concentrations of greenhouse gases in the atmosphere are at least partly to blame.
How might the climate change?
Scientists use computer-based models to predict the effects on global climate of different levels of greenhouse gases in the atmosphere. According to the most recent projections of the Intergovernmental Panel on Climate Change (IPCC), the global mean temperature could increase by 1.4°C to 5.8°C between 1990 and 2100. The climatic effects of such a temperature increase might include:
Related site: Climate modelsDescribes a number of different computer climate models and how they are used.(Hadley Centre for Climate Prediction and Research, UK)
more frequent extreme high maximum temperatures and less frequent extreme low minimum temperatures;
an increase in the variability of climate, with changes to both the frequency and severity of extreme weather events;
alterations to the natural biological range of certain infectious diseases;
rising sea levels.
In Australia the climate is expected to become significantly warmer: by 2070 the annual average temperature is predicted to increase by 1°C to 6°C over most of Australia. The number of extreme rainfall events – such as those leading to flooding – is also expected to increase, even though most of the country is anticipated to become drier overall in the 21st century.
Heatwaves and cold snaps
Perhaps the most obvious impact of global warming will be the direct effects: a warmer planet will experience more extreme heatwaves. As seen in Europe in 2003, heatwaves often lead to an increase in the number of human deaths, particularly in temperate countries where people are often not accustomed to very hot weather and where houses and other infrastructure are not designed to cope with it. The sick and elderly are most vulnerable because their bodies are less able to increase cardiac output and sweat function for cooling purposes; they are often less able to afford cooling technologies.
It is difficult to predict the future effect on mortality levels, because as heatwaves become more frequent we can expect societies to adjust – technologically, behaviourally and physiologically. Technological adaptations such as the installation of effective air-conditioners and the construction of heat-minimising houses will happen more quickly among the rich, so heatwaves are likely to have a disproportionate effect in less-developed countries and in the poorer segments of rich societies.
In countries that currently experience extremely cold weather in winter, an increase in the mortality rate in summer might be offset by a decrease in winter mortality. Northern countries with severe winters have a higher mortality rate in winter because more sick and elderly people succumb in cold weather and because blizzards and extreme cold create dangerous conditions in which accidental deaths are more likely. Winters will tend to be milder under global warming, with the likely effect that winter death rates will decline.
Without measures to mitigate the effects of extreme heat, and with an increase in the proportion of older people in the population, we might therefore expect higher death tolls in Australia's future heatwaves. It is estimated that there are currently about 1100 heat-related deaths per year.
Extreme events and disasters
Most computer models generated by scientists indicate that the future climate will be more variable than in the past, and that droughts and floods will be more severe. Some of the health effects of weather-related disasters, in addition to the immediate death and injury to people and damage to property, include:
increases in psychological stress, depression, and feelings of isolation amongst people affected by natural disasters;
decreases in nutrition due to poorer agricultural yields caused, for example, by prolonged drought and problems of food distribution;
increases in disease transmission due to a breakdown in sewerage and garbage services. For example, cholera is one disease that thrives in such situations, particularly when flooding causes the contamination of drinking water by sewerage systems.
Australia's climate is naturally variable, although generally arid. In such a country, the implications of an even greater variation in rainfall is likely to be profound. Apart from the ecological and agricultural impacts, the availability of water may be reduced, with implications for human health. More frequent drought conditions would increase the risk of bushfires, which can kill people, release large quantities of particulate matter that can cause respiratory problems, and degrade water catchments.
Infectious diseases
Many infectious diseases are dependent on vector organisms, which are sensitive to environmental factors and therefore will be affected by global warming. Biological modelling under various climate scenarios suggests a widening of the potential transmission zone of some disease-causing pathogens and their vectors, such as mosquitoes (Box 1: Mosquito-borne disease).
Food- and water-borne diseases are also susceptible to climate change. Already, Australians suffer an estimated 4.6 million cases of diarrhoea or gastroenteritis each year, often caused by food contaminated with bacteria, parasites and (to a lesser extent) fungi and viruses. Food-poisoning bacteria grow best when the ambient temperature is in the range 35-37°C. Scientists speculate that if temperatures rise under global warming, the incidence of diseases caused by food-poisoning and by the contamination of drinking (and swimming) water could increase dramatically.
Rising sea levels
Scientists predict that sea levels will rise as the global temperature rises, due to the melting of land-based ice in the polar regions and glaciers, and the thermal expansion of the oceans. According to the most recent projections, sea levels could rise between 9 and 88 centimetres by the year 2100. A rise of this magnitude would have disastrous consequences for people living on low-lying islands, such as the Maldives group in the Indian Ocean and many South Pacific islands. Higher sea levels lead to coastal flooding and an increase in the frequency of extreme high water levels from storm surges. Related problems are the contamination of coastal freshwater supplies with encroaching sea water, and the degradation of fishing and agricultural areas.
The number of Australian fatalities from coastal flooding and storm surges has historically been low. It is currently estimated that 250 people each year experience coastal flooding due to storm surges, but this number could double by 2050. For the Pacific region as a whole, however, the number of people exposed to coastal flooding could be between 60,000 and 90,000 in an average year, a 50-fold increase on today's estimates.
Warmer and sicker?
Considerable uncertainty remains about how the climate may change and how such changes might affect human health. It seems likely, however, that people living in tropical and sub-tropical areas will be most affected. Affluent countries and social groups will be best able to adapt to climate change by reducing the impacts of natural disasters such as flooding, fire and drought, by maintaining high-quality health and emergency infrastructures, and by installing technologies that help ward off the worst climatic effects.
Neither uncertainty nor complacency should be allowed to prevent action to reduce greenhouse gas emissions. The risk to human (and ecological) well-being is too great, and prevention will be far better – and easier – than cure.
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