Climate change is expected to have wide-ranging consequences for human health. Public health depends on sufficient food, safe drinking water, secure shelter, good social conditions, and a suitable environmental and social setting for controlling infectious diseases. All of these factors can be affected by climate.

Heat waves are linked to cardiovascular, respiratory, and other diseases. Illness and deaths from these causes could be expected to increase, especially for the elderly and the urban poor. While the biggest rise in heat stress is expected in mid- and high latitude cities, milder winters in temperate climates would probably reduce cold-related deaths in some countries. A greater frequency of warm or hot weather, thermal inversions (a meteorological phenomenon that can delay the dispersal of pollutants), and wildfires may also worsen air quality in many cities.

By reducing fresh water supplies, climate change may affect water resources and sanitation. This in turn could reduce the water available for drinking and washing. It could also lower the efficiency of local sewer systems, leading to higher concentrations of bacteria and other micro-organisms in raw water supplies. Water scarcity may force people to use poorer quality sources of fresh water, such as rivers, which are often contaminated. All of these factors could result in an increased incidence of diarrhoeal diseases.

Any increase in the frequency or intensity of extreme weather events would pose a threat. Heat waves, flooding, storms, and drought can cause deaths and injuries, famine, the displacement of populations, disease outbreaks, and psychological disorders. While scientists are uncertain just how climate change will affect storm frequency, they do project that certain regions will experience increased flooding or drought. In addition, coastal flooding is expected to worsen due to sea-level rise unless sea defenses are upgraded.

Food security may be undermined in vulnerable regions. Local declines in food production would lead to more malnutrition and hunger, with long-term health consequences, particularly for children.

Higher temperatures may alter the geographical distribution of species that transmit disease. In a warmer world, mosquitoes, ticks, and rodents could expand their range to higher latitudes and higher altitudes. Climate change impacts models suggest that the largest changes in the potential for malaria transmission will occur at the fringes – in terms of both latitude and altitude – of the current malaria risk areas; generally, people in these border areas will not have developed immunity to the disease. The seasonal transmission and distribution of many other diseases that are transmitted by mosquitoes (dengue, yellow fever) and by ticks (Lyme disease, hantavirus pulmonary syndrome, tick-borne encephalitis) may also be affected by climate change. In addition, climate-induced changes in the formation and persistence of pollens, spores, and certain pollutants could promote more asthma, allergic disorders, and cardio-respiratory diseases.

Warmer seas could also influence the spread of disease. Studies using remote sensing have shown a correlation between cholera cases and sea surface temperature in the Bay of Bengal. There is also evidence of an association between El Niño (which warms the waters of the south-western Pacific) and epidemics of malaria and dengue. Enhanced production of aquatic pathogens and biotoxins may jeopardize the safety of seafood. Warmer waters would also increase the occurrence of toxic algal blooms.

People will have to adapt or intervene to minimize these enhanced health risks. Many effective measures are available. The most important, urgent, and cost-effective is to rebuild the public health infrastructure in countries where it has deteriorated in recent years. Many diseases and public health problems that may be exacerbated by climate change can be effectively prevented with adequate financial and human resources. Adaptation strategies can include infectious disease surveillance, sanitation programmes, disaster preparedness, improved water and pollution control, public education directed at personal behaviour, training of researchers and health professionals, and the introduction of protective technologies such as housing improvements, air conditioning, water purification, and vaccination.

Assessing the potential health effects of climate change involves many uncertainties. Researchers must consider not only future scenarios of climate change but many non-climate factors as well. For example, trends in socio-economic conditions can have a major affect on a population's vulnerability. Clearly, poorer communities will be more vulnerable to the health impacts of climate change than rich ones.