Conversely, the mortality risk from cold weather is expected to decline in northern latitudes.36 Currently, physiological and behavioural acclimatisation probably explains the gradient in the low-temperature threshold for increasing mortality, apparent from northern to southern Europe.12 But whether populations can offset temperature-related changes in mortality risks by acclimatisation (eg, through changes in building design12) is uncertain.
The accurate estimation of future deaths from floods and storms is impeded by the absence of empirically documented exposure-response relations. Further, the typical spatial scale of global climate models—even at the country level—is still too coarse for reliable projections of precipitation.45 Unless current deficiencies in watershed protection, infrastructure, and storm drainage systems are remedied, the risk of water-borne contamination events will probably increase.40
Infectious diseases
Climate change will affect the potential incidence, seasonal transmission, and geographic range of various vector-borne diseases. These diseases would include malaria, dengue fever, and yellow fever (all mosquito-borne), various types of viral encephalitis, schistosomiasis (water-snails), leishmaniasis (sand-flies: South America and Mediterranean coast), Lyme disease (ticks), and onchocerciasis (West African river blindness, spread by black flies).86
The formal modelling of the effects of climate change on vector-borne diseases has focused on malaria and dengue fever. Modelling of dengue fever is conceptually simpler than for malaria. Whereas malaria entails two main pathogen variants (falciparum and vivax) and relies on several dozen regionally dominant mosquito species, dengue fever transmission depends principally on one mosquito vector, Aedes aegyptii. Both statistical and biologically based (mathematical) models have been used to assess how a specified change in temperature and rainfall pattern would affect the potential for transmission of these and other vector-borne diseases.
Various research groups have published estimates of how climate change will affect future transmission of malaria.[87], [88], [89], [90], [91], [92] and [93] Biologically based models of climate-malaria futures depend on the documented mathematical relation between temperature and transmission, including a simple threshold for the effect of rainfall. Empirical statistical models can account for interactions between temperature and rainfall effects, but are affected by the uncertainty of modelled projections of future rainfall.92 Several models project a small geographical expansion of potential malaria transmission in the next few decades,[88] and [90] with some estimating more substantial changes later this century.[90], [91] and [93] In several studies that have modelled seasonal changes in transmission researchers estimate a substantial extension—such as a 16–28% increase in person-months of exposure to malaria in Africa by 2100.89
Three research groups have estimated how climate change will affect dengue fever. Early models were biologically based, driven mainly by the known effect of temperature on virus replication within the mosquito. Warmer temperatures (up to a threshold) shorten the time for mosquitoes to become infectious, increasing the probability of transmission.94 Studies with both biologically based94 and statistical models95 project substantial increases in the population at risk of dengue (eg, figure 4).
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Figure 4. Modelled estimates of the current (A) and future (2050) (B) geographic regions (shaded areas) suitable for maintenance of the dengue vector Ae aegyptii in Australia
Model based on baseline (1961–90) estimates of water vapour pressure estimates for current climate and for future climate, in settings of medium and high global emissions of greenhouse gases.35
Such modelling excludes many (often unforeseeable) non-climate aspects of the future world. Nevertheless, estimation of how the intrinsic probability of disease transmission would alter in response to climate change alone is informative—and accords with classic experimental science (see type (i) in Estimates of future health effects). Whether the change in disease transmission actually occurs also depends on non-climate factors; presence of vector and pathogen is prerequisite, as is vector access to non-immune people. The transmission of such diseases is also much affected by socioeconomic conditions and by the robustness of public health defences.[91], [140] and [141] For example, case surveillance and treatment in fringe areas, management of deforestation and surface water, and effective mosquito control programmes would tend to offset the increased risk due to climate change, whereas universally-funded bed-net campaigns would reduce infection rates. Future modelling will benefit by incorporation of those non-climate contextual changes that are reasonably foreseeable.
Other health effects
Beyond the specific and quantifiable risks to health are indirect and knock-on health effects due to the social, economic, and political disruptions of climate change, including effects on regional food yields and water supplies. Modelling of climate change effects on cereal grain yields indicates a future world of regional winners and losers, with a 5–10% increase in the global number of underfed people.97 The conflicts and the migrant and refugee flows likely to result from these wider-ranging effects would, typically, increase infectious diseases, malnutrition, mental health problems, and injury and violent death. Future assessments of the health effects of climate change should attempt order-of-magnitude estimates of these more diffuse risks to health.
The wider ramifications of climate change for health are well illustrated by a recent study of how ocean warming around the Faroe Islands will facilitate the methylation of (pollutant) mercury and its subsequent uptake by fish. Concentrations in cod and pilot whales would increase by an estimated 3–5% for a 1°C rise in water temperature.99 Eating methyl-mercury-contaminated fish impairs fetal-infant neurocognitive development.142 Further, ocean warming is already beginning to cause geographic shifts in fisheries.100 Climate change might also alter the timing and duration of pollen and spore seasons and the geographic range of these aeroallergens, affecting allergic disorders such as hay fever and asthma.48
The advent of changes in global climate signals that we are now living beyond Earth's capacity to absorb a major waste product: anthropogenic greenhouse gases. The resultant risks to health (and other environmental and societal outcomes) are anticipated to compound over time as climate change—along with other large-scale environmental and social changes—continues.
Research on climate, climate change, and health has focused largely on thermal stress, other extreme weather events, and infectious diseases. The wider spectrum of health risks should now be given more attention. With the adaptability of human culture, many communities will be able to buffer themselves (at least temporarily) against some of the effects of climate change. Buffering capacity, though, varies greatly between regions and communities, indicating differences in geography, technological resources, governance, and wealth.143 Research to characterise vulnerable groups is needed.
Knowledge of vulnerability allows an informed approach to development and evaluation of adaptive strategies to lessen those health risks. Although details144 are beyond our scope here, it is noteworthy that governments are now paying increasing attention to adaptation options. Researchers must engage, too, with the formulation, evaluation, and economic costing of adaptive strategies. Beyond structural, technological, procedural, and behavioural adaptations by at-risk communities are larger-scale technical possibilities—such as applying satellite data and computer modelling to natural disaster forecasting, and geographic information system modelling of the effect of changes in rainfall and vegetation on specific infectious diseases. Other generalised strategies include protection from coastal storm surges, improved sentinel case surveillance for infectious diseases, development of crops resistant to drought and disease, and most importantly, the fostering of renewable energy sources.
Conclusion
Research into the existence, future likelihood, and magnitude of health consequences of climate change represents an important input to international and national policy debates. Recognition of widespread health risks should widen these debates beyond the already important considerations of economic disruption, risks to infrastructure, loss of amenity, and threatened species. The evidence and anticipation of adverse health effects will indicate priorities for planned adaptive strategies, and crucially, will strengthen the case for pre-emptive policies. It will help us understand better the real meaning of sustainability.
Search strategy and selection criteria
We used keyword combinations to search MEDLINE and Science Citation Index databases for articles published in all languages during the years 1995–2005, including the search terms “climate”, “climate change”, “health”, “health effects”, “dengue”, “malaria”, “heat”, “heat waves”, “time-series”, “floods”, “extreme weather”, and “harmful algae”.
Conflict of interest statement
All the authors have been or are involved in the scientific review activities of the Intergovernmental Panel on Climate Change (IPCC).
