Researches have found that there is a close link between local climate and the occurrence or severity of some diseases and other threats to human health. It is estimated that climate change contributes to 150,000 deaths and 5 million illnesses each year, and the World Health Organization estimates that a quarter of the world's disease burden is due to the contamination of air, water, soil and food.

In the last quarter of the 20th century, the average atmospheric temperature rose by about 1 degree Fahrenheit. By 2000, that increase was responsible for the annual loss of about 160,000 lives and the loss of 5.5 million years of healthy life, according to estimates by the World Health Organization. The toll is expected to double to about 300,000 lives and 11 million years of healthy life by 2020.

The biggest tolls were in Africa, on the Indian subcontinent, and in Southeast Asia. Most of the increased burden of death and disease were from malnutrition, diarrhea, malaria, heat waves, and floods. But those diseases will play a minor role, at best, in many regions that nevertheless will feel the effects of global warming.

Some of climate change’s impacts on health include: Increased frequencies of heat waves; more variable precipitation patterns compromising the supply of freshwater, higher  risks of water-borne diseases; and a rise in coastal flooding due to rising sea levels, etc.

Global Warming and Human Health

But even more subtle, gradual climatic changes can damage human health. During the past two decades, the prevalence of asthma in the United States has quadrupled, in part because of climate-related factors. For Caribbean islanders, respiratory irritants come in dust clouds that emanate from Africa's expanding deserts and are then swept across the Atlantic by trade winds, which have accelerated due to warmer ocean temperatures. Increased levels of plant pollen and soil fungi may also be involved. When ragweed is grown in conditions with twice the ambient level of carbon dioxide, the stalks sprout 10% taller than controls and produce 60% more pollen.

Some of the health effects may lie ahead if the increase in very extreme weather events continues. Abrupt change of temperatures leading to heat waves or cold spells have become widespread, causing indirectly fatal illnesses, such as heat stress or hypothermia, as well as increasing death rates from heart and respiratory diseases. Statistics on mortality and hospital admissions show that death rates increase during extremely hot days, particularly among very old and very young people living in cities.

Examples:

• The 2003 European heat wave — involving temperatures that were 18°F (10°C) above the 30-year average, with no relief at night — killed 21,000 to 35,000 people in five countries. Starting in August 2003, it caused more than 14,800 deaths in France. Belgium, the Czech Republic, Germany, Italy, Portugal, Spain, Switzerland, the Netherlands, and the UK all reported excess mortality during the same period, with total deaths in the range of 35,000. In France, deaths were massively reported for people aged 75 and over (60%).

• Heat waves were also reported in 2003 in Andhra Pradesh, India, and caused the deaths of 3,000 people
• In July 1995, a heat wave killed more than 700 people in the Chicago area alone. Studies based on these types of statistics estimate that in Atlanta, for example, even a warming of about two degrees (F) would increase heat-related deaths from 78 annually now to anywhere from 96 to 247 people per year.

Impacts of natural disasters

Over the period of 1995-2004, a total of 2,500 million people were affected by disasters, with losses of 890,000 dead and costs of US$ 570 billion. Most disasters (75%) are related to weather extremes that climate change is expected to exacerbate.

A massive increase in the frequency of occurrence of natural disasters such as floods, earthquakes, tsunamis, forest fires have been observed in last decades and have a direct impact in human 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. According to the Oxfam report (November 2007), the average number of natural disasters per year during early 1980s was about 120. Now, the number has increased to nearly 500.
Some examples:

• In October 1999, a cyclone in Orissa, India, caused 10,000 deaths. The total number of people affected was estimated at 10-15 million.
• In December 1999, floods in and around Caracas, Venezuela, killed approximately 30,000 people, many in shanty towns on exposed slopes.
• In 1998, Hurricane Mitch stalled over Central America and released six feet of rain, causing massive mudslides and claiming 11,000 lives
• In 2005, Hurricane Katrina left most of New Orleans under water.
• In 2006, Sichuan Province, China experienced its worst drought in modern times, with nearly 8 million people and over 7 million cattle facing water shortages

Resurgence of Infectious Diseases

Climate change accelerates the spread of disease primarily because warmer global temperatures enlarge the geographic range in which disease-carrying animals, insects and microorganisms--as well as the germs and viruses they carry--can survive.
Climate-sensitive diseases are among the largest global killers. Diarrhea, 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.

In addition to changing weather patterns, climatic conditions affect diseases transmitted via vectors such as mosquitoes (vector-borne disease) or through rodents (rodent-borne disease).

Deadly diseases often associated with hot weather, like the West Nile virus, Cholera and Lyme disease, are spreading rapidly throughout North America and Europe because increased temperatures in these areas allow disease carriers like mosquitoes, ticks, and mice to thrive.

Extreme events--floods, storms, droughts, and uncontained fires--can be devastating for health. Floods spread bacteria, viruses, and chemical contaminants, foster the growth of fungi, and contribute to the breeding of insects. Prolonged droughts interrupted by heavy rains, favor population explosions of both insects and rodents. Extreme weather events have been accompanied by new appearances of harmful algal blooms in Asia and North America, and--in Latin America and Asia by outbreaks of malaria and various water-borne diseases, such as typhoid, hepatitis A, bacillary dysentery, and cholera.

Vector–borne Diseases:

Vector-borne diseases (VBD) are infections transmitted by the bite of infected arthropod species, such as mosquitoes, ticks, triatomine bugs, sandflies and blackflies.

Mosquitoes, which can carry many diseases, are very sensitive to temperature changes. Warming of their environment — within their viable range — boosts their rates of reproduction and the number of blood meals they take, prolongs their breeding season, and shortens the maturation period for the microbes they disperse. Mosquitoes and the diseases they carry—including malaria, dengue fever, Ross River virus, and West Nile virus—are especially sensitive to temperature changes and land elevation. Rates of insect biting and the maturation of microorganisms within them are temperature-dependent, and both rates increase when the air warms, enhancing the chances for disease transmission.

In highland regions, as permafrost thaws and glaciers retreat, mosquitoes and plant communities are migrating to higher ground. Both insects and insect-borne diseases (including malaria and dengue fever) are today being reported at higher elevations in Africa, Asia, and Latin America. Highland malaria is becoming a problem for rural areas in Papua New Guinea and for the highlands of Central Africa. Disease-carrying mosquitoes are spreading as climate shifts allow them to survive in formerly inhospitable areas.

Mosquitoes that can carry dengue fever viruses were previously limited to elevations of 3,300 feet but recently appeared at 7,200 feet in the Andes Mountains of Colombia. Malaria has been detected in new higher-elevation areas in Indonesia.

Mosquito resistance to insecticides and parasite resistance to many drugs are widespread, and there are no operational vaccines, nor any foreseen in the near future. Ecological changes, along with increased weather variability and a warming trend, appear to be playing increasing roles in the spread of this disease.

Mosquito populations, for example, are naturally controlled by reptiles, birds, spiders, ladybugs, and bats--as well as by pond fish that feed on mosquito larvae. Mosquitoes provide nourishment for these animals, but some carry malaria, yellow fever, dengue fever, and several types of encephalitis.

In the marine environment, fish, shellfish, and sea mammals consume algae that form the base of the marine food web. A reduction in these plankton feeders as a result of overfishing or disease may thus contribute to blooms of harmful algae. Plankton blooms can also harbor cholera and other bacteria, and threaten the health of swimmers, or those who consume affected fish and shellfish.

Some examples:

• Dengue fever (the world’s most important vector-borne viral disease)
• Cholera: its transmission occurs through the process of ingesting contaminated water or food. The last outbreak was reported in Iraq (August 2007)
• Chikungunya: Discovered in India in 1963, the virus caused the death of 237 people in la Reunion island where 1/3 of the island's population were infected
• Urban yellow fever: Found only in part of Africa and South America

Rodent-borne Diseases

Rodent-borne disease are carried by rats, mice, bats, or other rodents. There is good evidence that diseases transmitted by rodents sometimes increase during heavy rainfall and flooding because of altered patterns of human–pathogen–rodent contact. Floods are frequently followed by disease clusters: downpours can drive rodents from burrows, deposit mosquito-breeding sites, foster fungus growth in houses, and flush pathogens, nutrients, and chemicals into waterways.

Some examples:

• Weil's syndrome
• Plague
• Hantaviruses
• Lyme disease

Examples of how diverse environmental changes affect the occurrence of various infectious diseases in humans.

Source: Climate change and infectious diseases, World Health Organization

 

Additional Resources

Climate Change and Human health

Environmental Health Perspectives: Driven to extremes

IPCC Fourth Assessment (2007) Hemon and Jougla, 2004; Martinez-Navarro et al., 2004; Michelozzi et al., 2004; Vandentorren et al., 2004; Conti et al., 2005; Grize et al., 2005; Johnson et al., 2005.