Effects of climate change on human health

The direct, indirect and social dynamic effects of climate change on health and wellbeing produce the following health impacts: cardiovascular diseases, respiratory diseases, infectious diseases, undernutrition, mental illness, allergies, injuries and poisoning.[19]^{: Figure 2}

Health and health care provision can also be impacted by the collapse of health systems due to climate-induced disasters such as flooding. Therefore, building health systems that are climate resilient is a priority.[20][4]^: 15

A sustained wet-bulb temperature exceeding 35 °C is a threshold at which the resilience of human systems is no longer able to adequately cool the skin.[29][30] One study has concluded that even young, healthy people may be unable to maintain their core temperature within survivable limits at wet bulb temperatures above 31 °C.[31]

Another impact that the warming global temperature has had is on the frequency and severity of heat waves. The effects of heatwaves on human health are generally worse in urban areas, due to the "heat island" effect. The heat island effect is when urban areas experience much higher temperatures that surrounding rural environments. This is caused by the extensive areas of treeless asphalt, along with many large heat-retaining buildings that physically block cooling breezes.[32]

The human response to heat stress can be hyperthermia, heat stroke and other harmful effects. Heat illness can relate to many of the organs and systems including: brain, heart, kidneys, liver, etc.[33] Heat waves have also resulted in epidemics of chronic kidney disease (CKD). Recent studies have shown that prolonged heat exposure, physical exertion, and dehydration are sufficient factors to developing CKD.[34] These cases are occurring across the world congruently with heat stress nephropathy.

A 2015 report revealed that the risk of dying from chronic lung disease during a heat wave was 1.8-8.2% higher compared to average summer temperatures. Bodily stress from heat also causes fluid loss, which disrupts pulmonary perfusion. In combination with higher pollutant concentrations, this leads to bronchial inflammation. A 2016 study found in people with moderate to severe chronic obstructive pulmonary disease (COPD), increased indoor temperatures resulted in worsening breathlessness, cough, and sputum production. A 2009 study, conducted in New York, found a 7.6% increase in hospitalization rate for COPD patients for every 1 °C increase in temperatures above 29 °C.[32]

The human body requires evaporative cooling to prevent overheating, even with a low activity level. With excessive ambient heat and humidity during heatwaves, adequate evaporative cooling might be compromised. Even under ideal conditions, sustained exposure to a wet-bulb temperature exceeding about 35 °C (95 °F) is fatal. As of 2020, only two weather stations had recorded 35 °C wet-bulb temperatures, and only very briefly, but the frequency and duration of these events is expected to rise with ongoing climate change.[35][36][37] Elderly populations and those with co-morbidities are at a significantly increased health risk from increased heat.[32]

Exposure to extreme heat "poses an acute health hazard, with individuals older than 65 years, populations in urban environments, and people with health conditions being particularly at risk".[4]

A study that investigated 13,115 cities found that extreme heat exposure of a wet bulb globe temperature above 30 °C tripled between 1983 and 2016. It increased by ~50% when the population growth in these cities is not taken into account. Urban areas and living spaces are often significantly warmer than surrounding rural areas, partly due to the urban heat island effect.[38][39]

Global warming is projected to substantially erode sleep worldwide, especially for residents from lower-income countries. The greatest increases of ambient temperatures were recorded at night.[40][41]

Health experts warn that "exposure to extreme heat increases the risk of death from cardiovascular, cerebrovascular, and respiratory conditions and all-cause mortality. Heat-related deaths in people older than 65 years reached a record high of an estimated 345 000 deaths in 2019".[4]^: 9

Increasing access to indoor cooling (air conditioning) will help prevent heat-related mortality but current air conditioning technology is generally unsustainable as it contributes to greenhouse gas emissions, air pollution, peak electricity demand, and urban heat islands.[4]^: 17

High temperatures can reduce the frequency and duration of physical activity as well as the desire to engage in exercise.[4]^: 7

The higher temperatures may have a substantial effect on human physiology and mental health. These effects may also be indirect: for example during the past four decades "the number of hours in which temperatures were too high for safe outdoor exercise" increased by an average loss of 3.7 hours for people in developing countries (low HDI country group).[4]

High temperatures can affect people's ability to work.[4]^: 8 Occupational heat exposure affects especially laborers in the agricultural sector of developing countries: It has been estimated that "295 billion hours of potential work were lost due to extreme heat exposure in 2020, with 79% of all losses in developing countries occurring in the agricultural sector."[4]^: 8

Heat stress can lead to labor force productivity to decrease as well as participation because employees' health may be weaker due to heat related health problems.[42] A study by NOAA from 2013 concluded that heat stress will reduce labor capacity considerably under current emissions scenarios.[43]

Floods have short and long term negative implications to peoples' health and well-being. Short term implications include mortalities, injuries and diseases, while long term implications include non-communicable diseases and psychosocial health aspects.[46]

Stronger hurricanes create more opportunities for vectors to breed and infectious diseases to flourish.[51][52] Extreme weather also means stronger winds. These winds can carry vectors tens of thousands of kilometers, resulting in an introduction of new infectious agents to regions that have never seen them before, making the humans in these regions even more susceptible.[51]

Another result of hurricanes is increased rainwater, which promotes flooding. Hurricanes result in ruptured pollen grains, which releases respirable aeroallergens. Thunderstorms cause a concentration of pollen grains at the ground level, which causes an increase in the release of allergenic particles in the atmosphere due to rupture by osmotic shock. Around 20–30 minutes after a thunderstorm, there is an increased risk for people with pollen allergies to experience severe asthmatic exacerbations, due to high concentration inhalation of allergenic peptides.[32]

Many of the consequences of droughts have impacts on human health. This can be through destruction of food supply (loss of crop yields), malnutrition and with this, dozens of associated diseases and health problems. Immune function decreases, so mortality rates due to infectious and other diseases climb. For those whose incomes were affected by droughts (namely agriculturalists and pastoralists), and for those who can barely afford the increased food prices, the cost to see a doctor or visit a clinic can simply be out of reach. Without treatment, some of these diseases can hinder one's ability to work, decreasing future opportunities for income and perpetuating the vicious cycle of poverty.[62]

Air pollution from a surface fire in the western desert of Utah (wildfires become more frequent and intense due to climate change).

Climate change increases wildfire potential and activity.[64] Climate change leads to a warmer ground temperature and its effects include earlier snowmelt dates, drier than expected vegetation, increased number of potential fire days, increased occurrence of summer droughts, and a prolonged dry season.[65]

Warming spring and summer temperatures increase flammability of materials that make up the forest floors.[65] Warmer temperatures cause dehydration of these materials, which prevents rain from soaking up and dampening fires. Furthermore, pollution from wildfires can exacerbate climate change by releasing atmospheric aerosols, which modify cloud and precipitation patterns.

Wood smoke from wildfires produces particulate matter that has damaging effects to human health.[66] The primary pollutants in wood smoke are carbon monoxide and nitric oxide.[65] Through the destruction of forests and human-designed infrastructure, wildfire smoke releases other toxic and carcinogenic compounds, such as formaldehyde and hydrocarbons.[44] These pollutants damage human health by evading the mucociliary clearance system and depositing in the upper respiratory tract, where they exert toxic effects.[65]

The health effects of wildfire smoke exposure include exacerbation and development of respiratory illness such as asthma and chronic obstructive pulmonary disorder; increased risk of lung cancer, mesothelioma and tuberculosis; increased airway hyper-responsiveness; changes in levels of inflammatory mediators and coagulation factors; and respiratory tract infection.[66] It may also have intrauterine effects on fetal development, resulting in low birth weight newborns.[67] Because wildfire smoke travels and is often not isolated to a single geographic region, the health effects are widespread among populations.[44]

Mosquito-borne diseases are probably the greatest threat to humans as they include malaria, elephantiasis, Rift Valley fever, yellow fever, and dengue fever.[71][72][73] Studies are showing higher prevalence of these diseases in areas that have experienced extreme flooding and drought.[71][72] Flooding creates more standing water for mosquitoes to breed; as well, shown that these vectors are able to feed more and grow faster in warmer climates.[51] As the climate warms over the oceans and coastal regions, warmer temperatures are also creeping up to higher elevations allowing mosquitoes to survive in areas they had never been able to before.[51] As the climate continues to warm there is a risk that malaria will make a return to the developed world.[51]

Increased heat, generated by the buildup of carbon, has been found to help disease-carrying organisms such as mosquitos thrive by producing stable environments for them. A research organization known as Climate Central states "The land area of the U.S. most suitable for Aedes albopictus mosquitoes is projected to increase from 5 percent to about 50 percent by 2100, putting 60 percent of the northeastern U.S.' population at risk for the diseases carried by this mosquito, including West Nile virus, dengue and Zika."[74] An outbreak of diseases like the West Nile and Zika virus could trigger a crisis since it would cause severe illness in people as well as birth defects in infants. Less advanced countries would be especially affected as they may have very limited resources to combat an infestation.

The area of coastline with suitable conditions for vibrio bacteria has increased due changes in sea surface temperature and sea surface salinity caused by climate change. These pathogens can cause gastroenteritis, cholera, wound infections, and sepsis. It has been observed that in the period of 2011–21, the "area of coastline suitable for Vibrio bacterial transmission has increased by 35% in the Baltics, 25% in the Atlantic Northeast, and 4% in the Pacific Northwest.[4]^: 12

The warming oceans are leading to more frequent cholera.[51][73][75] As the nitrogen and phosphorus levels in the oceans increase, the cholera bacteria that lives within zooplankton emerge from their dormant state.[75] The changing winds and changing ocean currents push the zooplankton toward the coastline, carrying the cholera bacteria, which then contaminate drinking water, causing cholera outbreaks.[75] As flooding increases there is also an increase in cholera epidemics as the flood waters that are carrying the bacteria are infiltrating the drinking water supply.[76] El Nino has also been linked with cholera outbreaks because this weather pattern warms the shoreline waters, causing the cholera bacteria to multiply rapidly.[75][76]

Warmer freshwater due to global warming is increasing the presence of the amoeba Naegleria fowleri, in freshwater and the parasite Cryptosporidium in pools, and both can cause a severe disease.[77]

Climate change can facilitate outbreaks of zoonoses, i.e. diseases that pass from animals to humans.[78] One example of such outbreaks is the COVID-19 pandemic.

With regards to zoonotic diseases (e.g., diseases that pass from animals to humans), deforestation, climate change, and livestock agriculture are among the main causes of increased disease risk.[79] As of 2016, outbreaks have cost lives and financial losses amounting to billions of dollars; future pandemics could cost trillions of dollars.[79]

It is projected that interspecies viral sharing, that can lead to novel viral spillovers, will increase due to ongoing climate change-caused geographic range-shifts of mammals (most importantly bats). Risk hotspots would mainly be located at "high elevations, in biodiversity hotspots, and in areas of high human population density in Asia and Africa".[80]

Ticks are also thriving in the warmer temperatures allowing them to feed and grow at a faster rate.[81] The black legged tick, a carrier of Lyme disease, when not feeding, spends its time burrowed in soil absorbing moisture.[52][82] Ticks die when the climate either becomes too cold or when the climate becomes too dry, causing the ticks to dry out.[52][82] The natural environmental controls that used to keep the tick populations in check are disappearing, and warmer and wetter climates are allowing the ticks to breed and grow at an alarming rate, resulting in an increase in Lyme disease, both in existing areas and in areas where it has not been seen before.[52][81]

Other diseases on the rise due to extreme weather include hantavirus,[83] schistosomiasis,[72][73] onchocerciasis (river blindness),[73] and tuberculosis.[68] It also causes the rise in hay fever, as when the weather gets warmer there is a rise in pollen levels in the air.[84][85]

Climate change has impacts on terrestrial and marine food security and undernutrition. Food insecurity is increasing (some of the underlying causes are related to climate change, for example an increase in extreme weather events, pests and pathogens)[86] and has affected 2 billion people in 2019.[4]^: 13

Over 500,000 more adult deaths are projected yearly by 2050 due to reductions in food availability and quality due to climate change.[87][88]

Changes in atmospheric carbon dioxide may reduce the nutritional quality of some crops.[89] Elevated CO₂ may reduce the nutritional values of crops, with for instance wheat having less protein and less of some minerals.[90] Studies have shown that increases in CO₂ lead to decreased concentrations of micronutrients in crop and non-crop plants with negative consequences for human nutrition,[91] including decreased B vitamins in rice.[92][93]

Climate change induced drought stress in Africa will likely lead to a reduction in the nutritional quality of the common bean.[94] Studies have also shown that higher CO₂ levels lead to reduced plant uptake of nitrogen (and a smaller number showing the same for trace elements such as zinc) resulting in crops with lower nutritional value.[95][96] This would primarily impact on populations in poorer countries less able to compensate by eating more food, more varied diets, or possibly taking supplements.

A headline finding in 2021 regarding marine food security stated that: "In 2018–20, nearly 70% of countries showed increases in average sea surface temperature in their territorial waters compared with in 2003–05, reflecting an increasing threat to their marine food productivity and marine food security".[4]^: 14

With seafood being a major protein source for so much of the population, there are inherent health risks associated with climate change. Increased agricultural runoff and warmer water temperature allows for eutrophication of ocean waters. This increased growth of algae and phytoplankton in turn can have dire consequences. These algal blooms can emit toxic substances that can be harmful to humans if consumed. Organisms, such as shellfish, marine crustaceans and even fish, feed on or near these infected blooms, ingest the toxins and can be consumed unknowingly by humans.

As the climate warms, it changes the nature of global rainfall, evaporation, snow, stream flow and other factors that affect water supply and quality. Rising sea levels cause saltwater to enter into fresh underground water and freshwater streams. This reduces the amount of freshwater available for drinking and farming.

Fifty percent of the world's fresh water consumption is dependent glacial runoff.[97] Earth's glaciers are expected to melt within the next forty years, greatly decreasing fresh water flow in the hotter times of the year, causing everyone to depend on rainwater, resulting in large shortages and fluctuations in fresh water availability which largely effects agriculture, power supply, and human health and well-being.[44] Many power sources and a large portion of agriculture rely on glacial runoff in the late summer. "In many parts of the world, disappearing mountain glaciers and droughts will make fresh, clean water for drinking, bathing, and other necessary human (and livestock) uses scarce" and a valuable commodity.[44]

Climate change poses a threat to our current systems for water management, which in turn poses a threat to the availability of drinking water. Traditional water management systems use rely heavily on the assumption of stationarity. Stationarity in water management is "the idea that natural systems fluctuate within an unchanging envelope of variability."[98] The significant anthropogenically induced change in Earth's climate has altered hydrologic stationarity, changing the means and extremes of precipitation, evapotranspiration, and river discharge rates.[99][98][100][101] Traditionally, water plans are based on historic data, such as streamflow data and historic precipitation rates.[102] With the variability of climate change, many of these models are rendered ineffective. For example, in the Colorado basin, there has been an observed increase in high and low stream flow by 24% when compared to historic data.[101] This increase variability in the water cycle has the potential to render traditional methods of water planning and management ineffective, leading to shortage of drinking water supply.[103]

Water quality can be affected by climate change in several ways. In some regions, climate change will drive an increase in precipitation. The increasing volume of water has the potential to overwhelm sewer systems and water treatment plants, resulting in contaminated water entering municipal water supplies.[104][105] Moreover, heavy downpours can increase runoff into surface water bodies. Runoff contaminants may include sediments, nutrients, pollutants, animal excrement, and other harmful materials. The increase in runoff into surface waters can result in a degradation of water quality.[104][106] In addition, freshwater resources along the coastline are at risk of saltwater contamination. As the sea level rises with climate change, saltwater will move into freshwater areas, contaminating drinking water supplies.[105][104] Moreover, the increase in water consumption in regions of drought can cause salt waters to infiltrate further upstream as freshwater is drained from rivers and reservoirs upstream. The increase in droughts can lead to saltwater contamination in once-reliable freshwater sources.[104][106]

In areas of increased flooding and precipitation, water treatment plants will not be able to keep up with the increased water volume, leading to contamination.[104] On the other end of the spectrum, the increase in droughts and temperatures can result in lower streamflow, therefore treatment will have to be increased to meet minimum flow requirements in some regions.[104] On top of this, rising sea levels from climate change can damage infrastructure and reduce treatment efficiency.[104] As water treatment becomes less effective, water-borne diseases will become more prevalent.

Climate change increases the risk of illness via "increasing temperature, more frequent heavy rains and runoff, and the effects of storms".[107]

The warming oceans and lakes are leading to more frequent harmful algal blooms.[51][73][75] Also, during droughts, surface waters are even more susceptible to harmful algal blooms and microorganisms.[115] Algal blooms increase water turbidity, suffocating aquatic plants, and can deplete oxygen, killing fish. Some kinds of blue-green algae (cyanobacteria) create neurotoxins, hepatoxins, cytotoxins or endotoxins that can cause serious and sometimes fatal neurological, liver and digestive diseases in humans. Cyanobacteria grow best in warmer temperatures (especially above 25 degrees Celsius), and so areas of the world that are experiencing general warming as a result of climate change are also experiencing harmful algal blooms more frequently and for longer periods of time.[116]

One of these toxin producing algae is Pseudo-nitzschia fraudulenta. This species produces a substance called domoic acid which is responsible for amnesic shellfish poisoning.[117][118] The toxicity of this species has been shown to increase with greater CO₂ concentrations associated with ocean acidification.[117] Some of the more common illnesses reported from harmful algal blooms include; Ciguatera fish poisoning, paralytic shellfish poisoning, azaspiracid shellfish poisoning, diarrhetic shellfish poisoning, neurotoxic shellfish poisoning and the above-mentioned amnesic shellfish poisoning.[117]

Ozone pollution in urban areas is especially concerning with increasing temperatures, raising heat-related mortality during heat waves.[119] During heat waves in urban areas, ground level ozone pollution can be 20% higher than usual.[120] 

One study concluded that from 1860 to 2000, the global population-weighted fine particle concentrations increased by 5% and near-surface ozone concentrations by 2% due to climate change.[121]

Ozone pollution has many well-known health effects:[122][123]

1. Ozone can cause adverse effects to the respiratory system such as shortness of breath and inflammation of the airways in the general population
2. Long term exposure to ozone is likely one of the many causes of asthma development
3. Ozone exposure is a potential cause of premature deaths

Higher levels of indoor and outdoor CO₂ levels may impair human cognition.[124][125][126]

Researchers found that there is a strong correlation between higher winter temperatures and drowning accidents in large lakes, because the ice gets thinner and weaker.[127]

A minor further effect are increases of pollen season lengths and concentrations in some regions of the world which can affect people with pollen allergies.[128][129][130]

Climate change may have influence on the risk of violent conflict, including organized armed conflict. Evidence shows links between armed conflict and variations in temperature: conflict incidence substantially increases during warmer periods.[131] Climate change is predicted to diminish natural resource availability.[132] Water scarcity, food shortages, and decreased livelihoods may lead to an increase in desperate populations, enhancing the risk of intra and interstate conflicts.[132] Climate hazards are a driving force of involuntary migration with a growing impact and a potential important contributor to violent conflicts even though the importance is small compared to other factors such as culture, politics, and economy. Climate hazards are associated with increased violence against vulnerable groups and the mitigation could potentially exacerbate violent conflicts. Future increases in violent-conflict-related deaths induced by climate change have been estimated at conflict-prone region.

The health benefits (also called "co-benefits") from climate change mitigation measures are predicted to be signification: potential measures can not only mitigate future health impacts from climate change but also improve health directly.[133] Climate change mitigation is interconnected with various co-benefits (such as reduced air pollution and associated health benefits)[134] and how it is carried out (in terms of e.g. policymaking) could also determine its impacts on living standards (whether and how inequality and poverty are reduced).[135]

There are many health co-benefits associated with climate action. These include those of cleaner air, healthier diets (e.g. less red meat), more active lifestyles, and increased exposure to green urban spaces.[4]^: 26 Access to urban green spaces provides benefits to mental health as well.[4]^: 18

Compared with the current pathways scenario (with regards to greenhouse gas emissions and mitigation efforts), the "sustainable pathways scenario" will likely result in an annual reduction of 1.18 million air pollution-related deaths, 5.86 million diet-related deaths, and 1.15 million deaths due to physical inactivity, across the nine countries, by 2040. These benefits were attributable to the mitigation of direct greenhouse gas emissions and the commensurate actions that reduce exposure to harmful pollutants, as well as improved diets and safe physical activity.[136] Air pollution generated by fossil fuel combustion is both a major driver of global warming and the cause of a large number of annual deaths with some estimates as high as 8.7 million excess deaths during 2018.[137][138]

Placing health as a key focus of the Nationally Determined Contributions could present an opportunity to increase ambition and realize health co-benefits.[136] Some politicians, such as Arnold Schwarzenegger with his slogan "terminate pollution", say that activists should generate optimism by focusing on the health co-benefits of climate action.[139]

It has been projected that climate change will bring some benefits in temperate areas, such as fewer deaths from cold exposure, and some mixed effects such as changes in range and transmission potential of malaria in Africa.[140]^: 48 Benefits are projected to be outweighed by negative health effects of rising temperatures, especially in developing countries.

The World Bank suggested that climate-sensitive health system resilience should be increased through investment in two areas:[156]

• Health system strengthening to improve resilience and build capacity to prepare for the varied environmental impacts and health impacts caused by climate change (for example early warning systems, disaster preparedness systems); and
• Programmatic (e.g., disease-specific, nutrition-focused) responses to address the changing burden of disease related to climate change.

Much of the health burden associated with climate change falls on vulnerable people (e.g. coastline inhabitants, indigenous peoples, economically disadvantaged communities). As a result, people of disadvantaged sociodemographic groups experience unequal risks.[157] Often these people will have made a disproportionately low contribution toward man-made global warming, thus leading to concerns over climate justice.[158][85][148] Climate change can worsen the existing, often enormous, health problems, especially in the poorer parts of the world.

Notably, in the United States, housing policy and zoning laws have historically limited the opportunity for minority populations to seek places of living. The common options are often areas with a disproportionately higher risk of exposure to environmental hazards. These environmental hazards are compounded by the growing effects of climate change and air pollution, resulting in a higher chance of adverse impacts on minority populations. These communities have the highest estimated temperature-related deaths, heat-related illnesses, pesticide exposure, and childhood asthma diagnosis. Furthermore, this leads to inequitable health outcomes regarding climate change and air pollution. There is not only an equity issue with the systems and infrastructure that restrict minority populations to more high-risk living areas but also with the emergency response. More affluent and white communities are afforded a more prompt emergency response following a climate or environmental crisis. Too often, communities with minority populations are not afforded this rapid emergency response and even have to wait until the issue stockpiles into an even larger tragedy.

Studies have found that when communicating climate change with the public, it can help encourage engagement if it is framed as a health concern, rather than as an environmental issue. This is especially the case when comparing a health related framing to one that emphasised environmental doom, as was common in the media at least up until 2017.[159][160]