Climate change and human health

Changes in climate can affect the number of people who die directly as the result of temperature extremes (either cold or hot) or violent weather. They can also increase the ranges of certain diseases and other health problems, which can sometimes lead to serious health effects.

The threat of severe climate change, driven in large part by the emission of greenhouse gases (GHGs) into the atmosphere as a result of human activities, has aroused worldwide concern about the potential effects of such change on human health. The United Nations’ Intergovernmental Panel on Climate Change (IPCC), in its 2014 report, assessed four potential scenarios for climate change throughout the twenty-first century based on future levels of GHG emissions, ranging from "stringent mitigation" of emissions to "very high GHG emissions." In all four scenarios, Earth's surface temperature was predicted to increase throughout the century, with the "stringent mitigation" scenario projected to result in a global surface temperature increase of between 0.3 and 1.7 degrees Celsius (relative to temperatures in 1986–2005) by the end of the twenty-first century, the "very high emissions" scenario projected to result in an increase between 2.6 and 4.8 degrees Celsius, and the other two scenarios falling somewhere in between.

In the twenty-first century, direct deaths from extremely cold or warm weather (e.g., hypothermia, heatstroke) are relatively rare in developed countries, but occasionally large numbers of people are killed or otherwise seriously affected by heart attacks caused by the weather (or by activity in response to the weather, such as people trying to shovel too much snow at once). The accelerated global warming trend that has been observed since the late twentieth century could affect the number of deaths directly related to weather, by increasing the frequency of extreme weather events. Climate changes can also alter the ranges of parasites and disease vectors, patterns of pollution, the availability of food and drinking water, and other factors that can significantly impact human health. Scientists agree that these risks make climate change a primary threat to humanity, especially for already vulnerable populations who may struggle with preexisting health concerns or lack of resources. Indeed, the World Health Organization (WHO) listed climate change as the single biggest threat to overall global health interests for the twenty-first century.

Direct Effects

Warmer temperatures tend to lead to more frequent deaths from excessive heat. Although this is mitigated somewhat in the case of warming due to GHG emissions by the fact that the greatest such warming occurs at night and in winter (and especially in the Arctic and Antarctic regions) when the temperatures otherwise would be cooler, researchers do warn that heat waves will likely become more common and more extreme in many areas as global warming increases. Overall warmer temperatures may reduce the numbers of deaths associated with extreme cold (including deaths due to cardiovascular and pulmonary diseases, such as influenza, that are worsened by exposure to cold). However, as the IPCC's Working Group II noted in its contribution to the 2014 report, “the influence of seasonal factors other than temperature on winter mortality suggests that the impacts on health of more frequent heat extremes greatly outweigh benefits of fewer cold days.” The group also pointed out that few studies had been conducted on temperature-related deaths in developing countries in tropical regions, where extreme cold is typically not a factor and where the health impact of extreme heat would likely be more significant than in more developed countries in temperate regions. The Fourth National Climate Assessment by the Global Change Research Program (USGCRP) echoes the 2014 report, citing increased mental health deterioration, increased potential for disease communicability, and damage from extreme weather conditions between 2014 and 2018.

The direct effects of rising temperatures are amplified in urban environments as a result of urban heat islands (UHIs), a fact that has become increasingly relevant as Earth’s population has continued to migrate from rural areas to more densely concentrated suburbs and cities. Cities are warmer than surrounding countryside because traditional roofs and paving surfaces absorb more solar heat than do dirt and vegetation, and because significant heat is generated by industry, power plants, residential heating, and air conditioning. According to 2024 estimates by the United States Environmental Protection Agency (EPA), an urban, highly populated city may have daytime temperatures 1-7 degrees Fahrenheit higher and nighttime temperatures 2-5 degrees Fahrenheit higher than areas such as forests or oceans. These areas, referred to as heat islands, put urban dwellers at a potentially heightened risk of heatstroke during extreme heat events.

Climate change can also alter rainfall patterns, causing some areas to be more subject to floods (which inflict heavy damage and can also lead indirectly to other health problems) or droughts (which can lead to crop failures and water shortages). Rising sea levels caused by climate change also pose a flood threat to low-lying coastal areas, including many of the world's largest cities. Many scientists believe such changes are inevitable if glaciers, permafrost, and sea ice continue to melt at a rapid pace. In addition to the potential direct deaths and injuries due to floods, rising sea levels would displace many significant populations, exacerbating existing health concerns and introducing new ones.

Other potential direct health impacts due to climate change include landslides, wildfires, greater exposure to both household and ambient air pollution—estimated by the WHO to be responsible for 7 million deaths per year in 2023—and increased exposure to ultraviolet radiation, potentially causing cancer and other adverse effects. Many experts further believe climate change and its related extreme weather events are likely to cause an increase in mental health problems among the most directly affected populations.

Food and Water

Climate changes can also affect food crops. Judging from past experience during the Medieval Warm Period (MWP), which lasted from about 950 to 1250 CE, this can lead to greater production in some places (partly from longer growing seasons as well as the fertilizing effect of increased carbon dioxide) and shortages in others (caused by droughts and floods rather than the temperature changes, though this can change what crops are grown in particular areas). These shortages can lead to malnutrition, deficiency diseases, and even famine, especially in less wealthy countries.

Drought also often forces people to work harder for water, expending labor that would otherwise be available for other needs. Available water may be tainted, which leads to increasing outbreaks of diseases such as dysentery, typhoid fever, and cholera, as well as aquatic parasites such as guinea worms. When water is scarce, cleaning and other sanitation practices suffer. Unclean bodies (especially hands) help spread diseases, and unclean clothes can carry and spread parasites such as lice and fleas.

In 2016, the World Bank published the report Shock Waves: Managing the Impacts of Climate Change on Poverty, which highlighted the ways in which the economic effects of climate change can be linked to health issues. It warned that unchecked climate change could help cause the population of global poor to grow sharply. Impacts on agriculture would generate poverty, accompanied by health problems such as stunted growth, malaria, and chronic diarrhea. Additionally, the Center for Disease Control (CDC), EPA, and USGCRP regularly updated and published their research concerning the links between climate change and human health through 2024.

Diseases

Scientists and public health organizations, such as the WHO, connect climate change to the spread of certain diseases. Climate change can affect the ranges of various life forms in many ways. Warmer weather, particularly if it is also wetter, tends to increase the number of insects; the mild winters created by GHG warming are especially important for those insects that are otherwise susceptible to freezing temperatures. Many of these are disease vectors, spreading serious diseases such as malaria, dengue fever, yellow fever, typhus, and the plague. Not only may these insects cover larger areas (and also spread to higher elevations), but warmer temperatures can also enable them to be active for a longer portion of the year. This is especially crucial for mosquitoes and ticks.

Beyond insects, other disease-vector species can also spread or interact more with humans due to climate change and its effects. For example, flooding can drive rodents, which help spread diseases such as the plague, from their burrows. It has been estimated that warmer temperatures (and potentially increasing poverty) could expand the incidence of schistosomiasis (caused by parasitic worms) into areas of Africa where the disease was previously not present. However, rates of transmission in the disease's normal range might be decreased as temperatures become intolerably high for the snails that host the worms.

Researchers also note that incidences of allergies could increase with climate change. In areas that become significantly wetter, increased molds can lead to increases in hay fever and asthma, which can be fatal. When carbon dioxide increases, crop yields can improve, but so can the growth of allergenic pollens such as ragweed. The spread of air pollution can also exacerbate allergies and respiratory illnesses.

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