Climate Change and Human Health
CLIMATE CHANGE AND HUMAN HEALTH
Human societies over the ages have depleted natural resources and degraded their local environments. Populations have also modified their local climates by cutting down trees or building cities. It is now apparent that human activities are perturbing the climate system at the global scale. Climate change is likely to have wide-ranging and potentially serious health consequences. Some health impacts will result from direct-acting effects (e.g., heatwave-related deaths, weather disasters); others will result from disturbances to complex ecological processes (e.g., changes in patterns of infectious disease, in freshwater supplies, and in food production).
WHAT IS CLIMATE CHANGE?
Global climate change is caused by the accumulation of greenhouse gases in the lower atmosphere. The global concentration of these gases is increasing, mainly due to human activities, such as the combustion of fossil fuels (which release carbon dioxide) and deforestation (because forests remove carbon from the atmosphere). The atmospheric concentration of carbon dioxide, the main greenhouse gas, has increased by 30 percent since preindustrial times.
Projections of future climate change are derived from global climate model or general circulation model (GCM) experiments. Climatologists of the Intergovernmental Panel on Climate Change (IPCC) review the results of these experiments for global and regional assessments. It is estimated that global mean surface temperature will rise by 1.5° to 3.5° C by 2100. This rate of warming is significant. Large changes in precipitation, both increases and decreases, are forecast, largely in the tropics. Climate change is very likely to affect the frequency and intensity of weather events, such as storms and floods, around the world. Climate change will also cause sea level rise due to the thermal expansion of the oceans and the melting of the mountain glaciers. Global mean sea level is anticipated to rise by 15 to 95 centimeters by 2100. Sea level rise will increase vulnerability to coastal flooding and storm surges. The faster the climate change, the greater will be the risk of damage to the environment. Climatic zones (and thus ecosystems and agricultural zones) could shift toward the poles by 150 to 550 kilometers by 2100. Many ecosystems may decline or fragment, and individual species may become extinct. The IPCC Second Assessment report concludes that climate change has probably already begun.
IMPACTS ON HEALTH
To assess the potential impacts of climate change on health, it is necessary to consider both the sensitivity and vulnerability of populations for specific health outcomes to changes in temperature, rainfall, humidity, storminess, and so on. Vulnerability is a function both of the changes to exposure in climate and of the ability to adapt to that exposure (see Figure 1).
Science classically operates empirically, via observation, interpretation, and replication. However, having initiated a global experiment, it would not be advisable to wait decades for sufficient empirical evidence to describe the health consequences. Risk assessment must therefore be carried out in relation to future environmental scenarios. The traditional "top-down" approach is to
answer the question, "If climate changes like scenario X, then what will be the effect on specific health outcomes?" In contrast, "bottom-up" approaches begin with the question, "How much climate change can be tolerated?"
It is important to distinguish between "climate and health" relationships and "weather and health" relationships. Climate variability occurs on many time scales. Weather events occur at daily time scale and are associated with many health impacts (e.g., heatwaves and floods). Climate variability at other time scales also affects health. In particular, the El Niño Southern Oscillation has been shown to influence interannual variability in malaria, dengue, and other mosquito-borne diseases. Climate change is the long-term change in the average weather conditions for a particular location. Climate change will become apparent as a change in annual, seasonal, or monthly means. Thus, incremental climate change will be superimposed upon the natural variability of climate in time and space.
Natural Disasters. Climate change will increase the risk of both floods and droughts. Ninety percent of disaster victims worldwide live in developing countries, where poverty and population pressures force growing numbers of people to live in harm's way—on flood plains and on unstable hillsides. Unsafe buildings compound the risks. The vulnerability of those living in risk-prone areas is perhaps the single most important cause of disaster casualties and damage.
Water Quality and Quantity. Human health depends on an adequate supply of potable water. By reducing fresh water supplies, climate change may affect sanitation and lower the efficiency of local sewer systems, leading to increased concentrations of pathogens in raw water supplies. Climate change may also reduce the water available for drinking and washing. In developed countries, the anticipated increase in extreme rainfall events, which may be associated with the outbreaks of diarrheal diseases, may overwhelm the public water supply system. Flooding is likely to become more frequent with climate change and can affect health through the spread of disease. In vulnerable regions, the concentration of risks with both food and water insecurity can make the impact of even minor weather extremes (floods, droughts) severe for the households affected. The only way to reduce vulnerability is to build the infrastructure to remove solid waste and waste water and supply potable water. No sanitation technology is "safe" when covered by flood waters, as fecal matter
Food Security. Current assessments of the impact of climate change indicate that some regions are likely to benefit from increased agricultural productivity while others may suffer reductions, according to their location and dependence on the agricultural sector. The IPCC has reviewed the results of many modeling experiments that project future changes in crop yields under climate change. Climate change may increase yields of cereal grains at high and midlatitudes but may decrease yields at lower latitudes. The world's food system may be able to accommodate such regional variations at the global level, with production levels, prices, and the risk of hunger being relatively unaffected by the additional stress of climate change. However, populations in isolated areas with poor access to markets may still be vulnerable to locally important decreases or disruptions in food supply.
Heat Waves and Milder Winters. Heat stress is a direct result of exposure to high temperatures. Stressful hot weather episodes (heat waves) cause deaths in the elderly, as well as heat related illnesses such as heat stroke and heat exhaustion. A change in world climate, including an increase in the frequency and severity of heat waves, would affect the quality of life in many urban centers. Heat waves are responsible for a significant proportion of disease-related mortality in developed counties such as the United States and Australia, where the impact of weather disasters has been significantly reduced. Milder winters under climate change would reduce the excess morbidity and mortality, such as the United Kingdom, the beneficial impact may outweigh the detrimental.
Air Pollution. The air is full of particles and gases that may affect human health, such as pollen, fungal spores, and pollutants from fossil fuel emissions. Weather conditions influence air pollution via pollutant (or pollutant precursor) transport and/or formation. Exposures to air pollutants have serious public health consequences. Climate change, by changing pollen production, may affect timing and duration of seasonal allergies.
Social Dislocation. The growth in the number of refugees and displaced persons has increased markedly. Refugees represent a very vulnerable population with significant health problems. Large-scale migration is likely in response to flooding, drought, and other natural disasters. Both the local ecological disturbance caused by the extreme event and the circumstances of population displacement and resettlement would affect the risk of infectious disease outbreaks. Even displacement due to long-term cumulative environmental deterioration, including sea level rise, is associated with such health impacts.
Infectious Diseases. Vector-borne diseases are transmitted by insects (e.g., mosquitoes) and ticks that are sensitive to temperature, humidity, and rainfall. Climate change may alter the distribution of important vector species, and this may increase the risk of introducing disease into new areas. Temperature can also influence the reproduction and survival of the infective agent within the vector, thereby further influencing disease transmission in areas where the vector is already present. However, the ecology and transmission dynamics of vector-borne diseases are complex. The climate factors that could critically influence transmission need to be identified before the potential impact of a changing climate can be assessed.
Malaria is on the increase in the world at large, but particularly in Africa. In several locations around the world, malaria is reported in the twenty-first century at higher altitudes than in preceding decades, such as on the mountain plateaus in Kenya. The reason for such increases has not yet been confirmed but include population movement and the breakdown in control measures. Climate change may contribute to the spread of this major disease in the future in highlands and other vulnerable areas. Climate change impact models suggest that the largest changes in the potential for disease transmission will occur at the fringes—in terms of both latitude and altitude—of the potential malaria risk areas. The season transmission and distribution of many diseases that are transmitted by mosquitoes (dengue, yellow fever), sandflies (leishmaniasis), and ticks (Lyme disease, tick-borne encephalitis) may also be increased or decreased by climate change.
ADAPTATION AND MITIGATION
There are two responses to global climate change:
- Mitigation. Intervention or policies to reduce the emissions or enhance the sinks of greenhouse gases. The current international legal mechanism for countries to reduce their emissions is the United Nations Framework Convention on Climate Change (UNFCCC).
- Adaption. Responses to the changing climate (e.g., acclimatization in humans) and policies to minimize the predicted impacts of climate change (e.g., building better coastal defenses).
The key determinants of health—as well as the solutions—lie primarily outside the direct control of the health sector. They are rooted in areas such as sanitation and water supply, education, agriculture, trade, transport, development and housing. Unless these issues are addressed, it can be difficult to make improvements in population health and reduce vulnerability to the health impacts of climate change.
R. SARI KOVATS
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McMichael, A. J., and Haines, A. (1997). "Global Climate Change: The Potential Effects on Health." British Medical Journal 315:805–809.
Patz, J. A.; McGeehin, M. A.; Bernard, S. M.; Ebi, K. L.; Epstein, P. R.; Grambsch, A.; Gubler, D. J.; and Reiter, P. (2000). "The Potential Health Impacts of Climate Variability and Change for the United States: Executive Summary of the Report of the Health Sector of the United States National Assessment." Environmental Health Perspectives 108:367–376.
Watson, R.; Zinyowera, M. C.; Moss, R. H.; and Dokken, D., eds. (1996). "Climate Change 1995. Impacts, Adaptations, and Mitigation of Climate Change: Scientific and Technical Analyses." Second Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge: Cambridge University Press.