The evidence for anthropogenic climate change is now clear and convincing. The Earth's surface has warmed by more than 0.8 [degrees]C over the past century, and by approximately 0.6 [degrees]C in the past three decades. (1) This warming has been linked to more extreme weather conditions such as intense floods and droughts, heavier and more frequent storms, and a possible increase in frequency and intensity of the El Ninio Southern Oscillation. These changes are largely caused by human activities, mainly the burning of fossil fuels releasing carbon dioxide (C[O.sub.2]) that traps heat within the atmosphere. These C[O.sub.2] emissions continue to rise, and climate models project the average surface temperature will rise by 1.1 [degrees]C to 6.4[degrees]C over the 21st century. (2)
Since 1990, WHO has published a series of reports on climate change and has participated in review processes such as the Intergovernmental Panel on Climate Change. These activities have outlined four key characteristics of the health risks generated by a warming and a more variable climate. First, these hazards are diverse, global and probably irreversible over human time scales. They range from increased risks of extreme weather, such as fatal heat waves, floods and storms, to less dramatic but potentially more serious effects on infectious disease dynamics, shifts to long-term drought conditions in many regions, melting of glaciers that supply freshwater to large population centres, and sea level increases leading to salination of sources of agriculture and drinking water. Second, the health impacts of climate change are potentially huge. Many of the most important global killers are highly sensitive to climatic conditions. Malaria, diarrhoea and protein-energy malnutrition together cause more than 3 million deaths each year. (3) Third, these risks are inequitable, in that the greenhouse gases that cause climate change originate mainly from developed countries, but the health risks are concentrated in the poorest nations, which have contributed least to the problem. (4) Finally, many of the projected impacts on health are avoidable, through a combination of public health interventions in the short term, support for adaptation measures in health-related sectors such as agriculture and water management, and a long-term strategy to reduce human impacts on climate.
Rethinking health in a changing environment
Threats to health are often cited to justify actions to mitigate or adapt to climate change; for example, United Nations Secretary-General Kofi Annan's 2006 speech in Nairobi. (5) For all the attention that these issues receive, however, a comprehensive strategy to support a public health response is conspicuously lacking. This strategy is urgently needed, both because the health community has a duty to counter emerging threats, and because increased attention to climate change offers opportunities to focus on the most disadvantaged populations' current needs. For example, the international community has agreed on the principle of a global climate change adaptation fund, funded by a 2% levy on a multibillion-dollar Clean Development Mechanism. (6) However, this opportunity to strengthen public health can be seized only if the health sector knows what it should do differently because of climate change.
The development of a comprehensive strategy will take time, but some essential principles are already clear. A global problem requires a strategy of international dimensions that can translate into regional and local actions. Just as climate change's underlying causes are global, its health implications do not respect national boundaries. Impacts in one location, such as infectious disease epidemics or population displacements caused by droughts or rising sea levels, quickly spread across national borders. Coordinated investments in preventive measures therefore contribute to the "global public good" of reducing the risk of health emergencies. …