Theories of Earth's demise from the BA Festival of Science in Glasgow, September 3-7

Global climate change poses many health challenges for the world. Andy Haines says we are far from understanding the possible impacts:

The result of humankind's altering the Earth's environment on an unprecedented scale is, in effect, that a number of unplanned global experiments are taking place simultaneously.

Some of these changes - including changes to land use through, for example, deforestation, stratospheric ozone depletion and climate change - are likely to have substantial impacts on human health. But with many of these changes interrelated (deforestation can contribute to global warming) and with the consequences for human health also interacting in ways that are as yet imperfectly comprehended, understanding the health effects of these global changes can be complex, particularly in respect of climate change.

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Evidence that global warming is already occurring has been accumulating over recent years. Global mean surface temperature has risen by 0.6C (+/- 0.2C) between 1861 and 2000. This has been accompanied by a number of changes in the Earth's biophysical systems such as the thawing of permafrost and the widespread retreat of non-polar glaciers.

The recently published Third Assessment Report of the UN Intergovernmental Panel on Climate Change concludes that the observed climate change is attributable largely to the combustion of fossil fuels, which has raised concentrations of greenhouse gases. The report estimates that the global mean temperature for 1990-2100 will be in the range of 1.4C-5.8C - an increase much more rapid than any naturally occurring temperature rise that has been experienced by humans since the advent of agriculture about 10,000 years ago.

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It has long been known that climate and weather can influence disease processes directly and indirectly. As early as 400BC, Hippocrates related patterns of disease to seasonal weather changes when he wrote in Airs, Waters, and Places : "Whoever wishes to investigate medicine properly, should proceed thus: in the first place to consider the seasons of the year, and what effects each of them produces, for they are not all alike, but differ much from themselves in regard to their changes."

What will climate change mean for humans? Global climate change will likely lead to more frequent and intense heatwaves, which could increase heat-related deaths, particularly among the elderly in urban centres.

Although populations can acclimatise in part to hot environments by behavioural, physiological and technical adaptations, it is uncertain to what extent such acclimatisation will blunt the impacts of any future increases in heatwaves. In many temperate countries, death rates are substantially higher in winter than in summer. This is particularly pronounced in the United Kingdom.

This raises the prospect that climate change might result in decreased mortality in temperate countries due to milder winters. The balance between higher summer and lower winter death rates will probably vary worldwide.

Big impacts on human health are also likely to occur via changes in the frequency and intensity of extreme events such as floods, droughts and storms. Floods can have a range of impacts that extends beyond any immediate deaths and injuries to encompass increases in infectious diseases and in common mental disorders such as anxiety and depression brought about by economic losses, destruction of possessions or dislocation of communities. This may result in higher suicide rates.

Coastal flooding will increase as a result of sea-level rise and could cause major displacement of populations - particularly in countries such as Bangladesh, where many people live in low-lying deltaic regions.

On a global scale, some of the inter-annual variability in numbers of people affected by disasters is associated with the El Nino Southern Oscillation (Enso) phenomenon. This appears to have become more frequent, persistent and intense over the past 30 years compared with the previous century, perhaps because of climate change.

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Enso events are centred on the Pacific, but they strongly influence temperature and rainfall in parts of Africa, Asia and North and South America. The climatic changes in turn affect the incidence of a number of infectious diseases. Outbreaks of malaria in some highland and desert fringe areas, where it is normally too cold or too dry for malaria transmission, have been associated with Enso events. Forest fires associated with El Nino and with slash-and-burn methods of forest clearance have caused substantial increases in respiratory symptoms in parts of Brazil, Indonesia and Malaysia.

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It is clearly a complex task to estimate the potential impact of global climate change on human health over the next 100 years, and any estimates should be considered as only indicative of what might happen.

Climate impact studies often rely on global mathematical models that are linked to specific scenarios of future climate change. For example, global models have been developed for the mosquito-borne diseases malaria and dengue. But it has been difficult to translate these modelled outputs, with coarse geographical resolution, into robust estimates of numbers of cases of human disease - in part because of the importance of other determining factors such as the effectiveness of vector-control programmes.

Furthermore, climate change is likely to have important impacts on food production, particularly on the yields of cereal grains, which will probably rise at higher to middle latitudes but fall at lower latitudes. However, any benefits would likely decline and possibly become an overall net loss with more than a few degrees of warming.

Right now about 800 million people in developing countries do not have enough to eat. The potential for reduced production of food and increases in food prices must be a cause for concern, particularly as the poorest populations will be most affected.

Many potentially important impacts - especially those at the end of a long causal chain, such as increases in climate-related refugees and internally displaced persons or victims of conflicts triggered by disputes over diminishing freshwater and other natural resources - are not amenable to quantitative methods of impact assessment.

Despite the many uncertainties that exist, the overall impacts of global climate change are likely to be substantially negative, particularly for poor populations in countries of the South who lack adequate public health infrastructure. About 2 billion people do not have reliable access to electricity and suffer adverse health and economic effects as a result. It will be a huge political and technical challenge to provide their energy requirements through the increased use of renewable energy technologies without increasing greenhouse gas emissions.

Adoption of the Kyoto Protocol was a historic achievement, but it will do little to reduce the health effects of climate change. In addition to pressing for deeper cuts in greenhouse gas emissions, we will need to learn to adapt to changing climate as far as possible. This will require substantial resources, particularly directed towards more vulnerable populations in countries of the South who are responsible for only a small proportion of greenhouse gas emissions.

Because of the long residence time of carbon dioxide in the atmosphere, the profligate use of fossil fuels today will have consequences for the health of future generations over the next century and beyond.

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Andy Haines is dean of the London School of Hygiene and Tropical Medicine.