From the tropics to the arctic, climate and weather have powerful direct and indirect impacts on human life. While people adapt to the conditions in which they live, and human physiology can handle substantial variation in weather, there are limits.
Marked short-term fluctuations in weather can have serious effects on health:
These effects can be significant. Abnormally high temperatures in Europe in the summer of 2003 were associated with over 35 000 more deaths than the equivalent period in previous years.
Other weather extremes, such as heavy rains, floods, and hurricanes, also have severe impacts on health. Approximately 600 000 deaths occurred worldwide as a result of weather-related natural disasters in the 1990s; some 95% of these were in poor countries. Some examples:
In addition to changing weather patterns, climatic conditions affect diseases transmitted through water, and via vectors such as mosquitoes. Climate-sensitive diseases are among the largest global killers. Diarrhoea, malaria and protein-energy malnutrition alone caused more than 3.3 million deaths globally in 2002, with 29% of these deaths occurring in the Region of Africa.
About two thirds of solar energy reaching Earth is absorbed by, and heats, the Earth's surface. The heat radiates back to the atmosphere, where some of it is trapped by greenhouse gases, such as carbon dioxide. Without this 'greenhouse effect', the average surface temperature would make the planet uninhabitable for human populations.
Over the last 50 years, human activities - particularly the burning of fossil fuels - have released sufficient quantities of CO2 and other greenhouse gases to affect the global climate. The atmospheric concentration of carbon dioxide has increased by more than 30% since pre-industrial times, trapping more heat in the lower atmosphere.
According to the Fourth Assessment Report (2007) of the Intergovernmental Panel on Climate Change (IPCC), the effects include:
Global emissions of carbon dioxide are still increasing. Estimates of future population growth and energy use are used as inputs to global climate models, in order to project future climate change. Reviewing outputs from a range of such models, the IPCC has made the following projections for the next century:
Many countries are working to reduce greenhouse gas emissions under the United Nations Framework Convention on Climate Change. Unfortunately, even with these efforts, past and expected future trends in development and energy use mean that the world will still face significant changes in climate and a rise in sea levels in the coming decades.
To a large extent, public health depends on safe drinking water, sufficient food, secure shelter, and good social conditions. A changing climate is likely to affect all of these conditions. Reviews of the likely impacts of climate change by the IPCC suggest that a warming climate is likely to bring some localized benefits, such as decreased winter deaths in temperate climates, and increases in food production in some, particularly high latitude, regions. Public health services and high living standards would protect some populations from specific impacts; for example it is unlikely that climate change would cause malaria to become re-established in northern Europe or North America. Overall, however, the health effects of a rapidly changing climate are likely to be overwhelmingly negative, particularly in the poorest communities, which have contributed least to greenhouse gas emissions. Some of the health effects include:
Measurement of health effects from climate change can only be very approximate. Nevertheless, a WHO quantitative assessment, taking into account only a subset of the possible health impacts, concluded that the effects of the climate change that has occurred since the mid-1970s may have caused over 150 000 deaths in 2000. It also concluded that these impacts are likely to increase in the future.
WHO co-ordinates reviews of the scientific evidence on the links between climate, climate change and health, including supporting the IPCC assessment process. Based on these assessments, WHO considers that rapid climate change poses substantial risks to human health, particularly among the poorest populations. The organization therefore supports actions to reduce human influence on the global climate.
Carefully planned mitigation policies bring direct health benefits. For example, well-designed urban transport systems can reduce greenhouse gas emissions, while simultaneously reducing the major health impacts of urban air pollution and physical inactivity, which kill millions each year. Housing with efficient insulation can cut energy consumption and associated greenhouse gas emissions, reduce deaths from both cold and heat, and in poor countries, reduce the need for burning of biomass fuels and the impacts of indoor air pollution. WHO is stepping up its efforts to support healthy development, which reduces current environmental risks to health, and at the same time helps to reduce our impact on the global climate.
WHO also recognizes that, given past emissions of greenhouse gases, the world will continue to be faced with a warming and more variable climate for at least several decades. WHO's work in supporting programmes to combat infectious disease, improve water and sanitation services and respond to natural disasters already helps to reduce health vulnerability to future climate change. The organization has also carried out workshops in the most vulnerable countries, to raise awareness of the health implications of climate change and related weather patterns. WHO is increasingly working with partner agencies in the UN to support countries in strengthening the key components of health systems, such as disease surveillance and response, and health action in emergencies, that are most needed to protect public health from the impacts of climate change.