In destabilizing the world's climate system and its dependent ecosystems, we are posing new and widespread risks to the health of human populations.

While most of our current environmental health problems occur on a local or regional level, many of the anticipated health effects of climate change would affect large populations in many regions.

The most direct health effects would be those caused by heatwaves, storms, and floods. These may increase in frequency and intensity as mean temperature and precipitation rise. However, the regional pattern of increases in these extreme events remains difficult to predict with existing climate models.

Stressful hot weather episodes are a known cause of short-term mortality excesses. An increase in the frequency and severity of heatwaves would obviously exacerbate such excesses.

The extent to which increases in heat-related deaths would be offset by a reduction in cold-related deaths is unresolved. On balance, evidence suggests that increases in "acute" mortality in response to more frequent hot weather episodes would outweigh decreases in winter-related mortality.

Current models indicate that, by around 2050, many major cities around the world could be experiencing up to several thousand extra heat-related deaths annually, independent of any increases due to population growth.

Climate change would exacerbate the production of some types of air pollutants and thus increase the incidence of associated health effects, such as cardiovascular and respiratory disorders.

A warmer and wetter climate in some regions could also result in higher airborne concentrations of various pollens and spores with likely effects on allergic disorders, such as hay fever and possibly asthma.

Climate change is also likely to bring about major regional shifts in rainfall patterns and, with them, increased frequency or severity of droughts, floods, and brushfires. In drought-prone regions, droughts could become longer lasting and more severe.

Climate change would tend to alter the geographic distribution (altitude and latitude) of disease vectors and to affect the behavior and seasonal activity of vectors and parasites. These effects could amplify the transmission of many vector-borne diseases in many parts of the world.

In the next century, climate change is expected to increase the global incidence of malaria by 50-80 million additional cases each year. The proportion of the world population exposed to the potential transmission of malaria is expected to increase from the current 45 percent to around 60 percent.

Increases in the incidence of non-vector-borne diseases, such as cholera, and other food-related and water-related infections, could also occur, particularly in tropical and sub-tropical regions, following changes in water distribution, temperature and the proliferation of microorganisms.

The predicted rise in sea level would have several adverse effects, apart from population displacement and the loss of agricultural land. These effects include saltwater intrusion and contamination of water supplies, changes in the distribution of vector-borne diseases, and increases in death and injury due to flooding.

Studies consistently predict that climate change will have significant regional impacts on agricultural yield. Some areas will gain; others will lose. Some mid-continental drying in temperate zones, such as the mid-west USA, southern Europe and Ukraine, may occur, but the most negative effects are foreseen at lower latitudes, in poorer tropical and semitropical countries, especially those reliant upon rain-fed, non-irrigated agriculture.

The responses of essential food crops to a change in climate will depend much on other ecological determinants such as soil erosion, saltwater intrusion or salination, the balance between pests and predators, and increases in ultraviolet radiation levels.

Additional adverse effects on health would result from the social-demographic disruptions caused by rising sea-level and from those caused by climate-related regional shortages in fresh water, food and other natural resources.

Many health consequences would arise via disturbances of complex biotic and other natural systems, reflecting the fundamental link between the integrity of natural ecosystems and the long-term health of human populations.

Stratospheric ozone depletion coincides with, but is separate from, troposphere-based climate change. Sustained depletion of ozone, resulting in increased exposure to ultraviolet radiation, would cause an increase in the incidence of skin cancer. It may also increase the incidence of ocular lesions, such as cataracts, and possibly cause weakening of the immune system, with a corresponding increased susceptibility to infections and altered responsiveness to vaccinations.

Human populations and communities vary markedly in their vulnerability to climatic change and in the resources available to them for protection and mitigation. Many of the anticipated consequences would be greatest in the world's poor and disadvantaged populations.

While there can as yet be no certainty in forecasts of the future health effects of climate change, the role of science in this context must be to assist adoption of precautionary policies that balance current social needs against serious future risks.