The effects of climate change on human health are increasingly well studied and quantified. They can be grouped into direct effects (for example due to heat waves, extreme weather events) or indirect effects. The latter take place through changes in the biosphere for example due to changes in water and air quality, food security and displacement. Social dynamics such as age, gender or socioeconomic status influence to what extent these effects become wide-spread risks to human health.: 1867 Health risks are unevenly distributed across the world.Disadvantaged populations are especially vulnerable to climate change impacts.: 15 For example, young children and older people are the most vulnerable to extreme heat.
Studies have found that communication on climate change is more likely to lead to engagement by the public if it is framed as a health concern, rather than just as an environmental matter.
Effects of climate change
Climate change affects the physical environment, ecosystems and human societies. Changes in the climate system include an overall warming trend, more extreme weather and rising sea levels. These in turn impact nature and wildlife, as well as human settlements and societies. The effects of human-caused climate change are broad and far-reaching, especially if significant climate action is not taken. The projected and observed negative impacts of climate change are sometimes referred to as the climate crisis.
Recent warming has strongly affected natural biological systems. It has degraded land by raising temperatures, drying soils and increasing wildfire risk.: 9 Species worldwide are migrating poleward to colder areas. On land, many species move to higher ground, whereas marine species seek colder water at greater depths. At 2 °C (3.6 °F) of warming, around 10% of species on land would become critically endangered.: 259
A 2021 report published in The Lancet found that climate change does not affect people's health in an equal way. The greatest impact tends to fall on the most vulnerable such as the poor, women, children, the elderly, people with pre-existing health concerns, other minorities and outdoor workers.: 13
There are certain predictors of health patterns that determine the social vulnerability of the individuals. These can be grouped into "demographic, socioeconomic, housing, health (such as pre-existing health conditions), neighbourhood, and geographical factors".
Overview of health impacts
Types of pathways affecting health
The effects of climate change on human health can be grouped into direct and indirect effects.: 1867 Both types of effects interact with social dynamics. The combination of effects and social dynamics determines the eventual health outcomes. Mechanisms and social dynamics are explained further below:
Direct mechanisms or risks: changes in extreme weather and resultant increased storms, floods, droughts, heat waves and wildfires
These health risks vary across the world and between different groups of people. For example, differences in health service provision or economic development will result in different health risks for people in different regions, with less developed countries facing greater health risks. In many places, the combination of lower socioeconomic status and cultural gender roles result in increased health risks to women and girls as a result of climate change, compared to those faced by men and boys (although the converse may apply in other instances).
Health and health care provision can also be impacted by the collapse of health systems and damage to infrastructure due to climate-induced events such as flooding. Therefore, building health systems that are climate resilient is a priority.: 15
Impacts on mental health
The effects of climate change on mental health and well-being are documented, especially for vulnerable populations and those with pre-existing serious mental illness. There are three broad pathways by which these effects can take place: directly, indirectly or via awareness. The direct pathway includes stress related conditions being caused by exposure to extreme weather events, such as post-traumatic stress disorder (PTSD). Scientific studies have linked mental health outcomes to several climate-related exposures—heat, humidity, rainfall, drought, wildfires and floods. The indirect pathway can be via disruption to economic and social activities, such as when an area of farmland is less able to produce food. The third pathway can be of mere awareness of the climate change threat, even by individuals who are not otherwise affected by it.
Assessing the exact mental health effects of climate change is difficult; increases in heat extremes pose risks to mental health which can manifest themselves in increased mental health-related hospital admissions and suicidality.: 9
Impacts of higher global temperatures will have ramifications for the following aspects: vulnerability to extremes of heat, exposure of vulnerable populations to heatwaves, heat and physical activity, change in labor capacity, heat and sentiment (mental health), heat-related mortality.
The global average and combined land and ocean surface temperature show a warming of 1.09 °C (range: 0.95 to 1.20 °C) from 1850–1900 to 2011–2020, based on multiple independently produced datasets. The trend is faster since the 1970s than in any other 50-year period over at least the last 2000 years.
A 2023 study estimated that climate change since 1960–1990 has put over 600 million people (9% of the global population) outside the "temperature niche" - the average temperature range at which humans flourish.
Vulnerable people with regard to heat illnesses include people with low incomes, minority groups, women (in particular pregnant women), children, older adults (over 65 years old), people with chronic diseases, disabilities and co-morbidities.: 13 Other people at risk include those in urban environments (due to the urban heat island effect), outdoor workers and people who take certain prescription drugs. Exposure to extreme heat poses an acute health hazard for many of the people deemed as vulnerable.
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.
A wet-bulb temperature that is too high means that human bodies would no longer be able to adequately cool the skin. A wet bulb temperature of 35 °C is regarded as the limit for humans (called the "physiological threshold for human adaptability" to heat and humidity).: 1498 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. Global warming above 1.5 degrees risks making parts of the tropics uninhabitable because the threshold for the wet bulb temperature may be passed.
People with cognitive health issues (e.g. depression, dementia, Parkinson's disease) are more at risk when faced with high temperatures and ought to be extra careful as cognitive performance has been shown to be differentially affected by heat. People with diabetes and those who are overweight, have sleep deprivation, or have cardiovascular/cerebrovascular conditions should avoid too much heat exposure.
The risk of dying from chronic lung disease during a heat wave has been estimated at 1.8-8.2% higher compared to average summer temperatures. An 8% increase in hospitalization rate for people with Chronic Obstructive Pulmunary Disease has been estimated for every 1 °C increase in temperatures above 29 °C.
The effects of heatwaves tend to be more pronounced in urban areas because they are typically warmer than surrounding rural areas due to the urban heat island effect.: 2926 This is caused from the way many cities are built. For example, they often have extensive areas of asphalt, reduced greenery along with many large heat-retaining buildings that physically block cooling breezes and ventilation. Lack of water features are another cause.: 2926
Extreme heat exposure in cities with a wet bulb globe temperature above 30 °C tripled between 1983 and 2016. It increased by about 50% when the population growth in these cities is not taken into account.
Cities are often on the front-line of climate impacts due to their densely concentrated populations, the urban heat island effect, their frequent proximity to coasts and waterways, and reliance on ageing physical infrastructure networks.
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".: 9
More than 70,000 Europeans died as a result of the 2003 European heat wave. Also more than 2,000 people died in Karachi, Pakistan in June 2015 due to a severe heat wave with temperatures as high as 49 °C (120 °F).
Mortality due to heat waves could be reduced if buildings were better designed to modify the internal climate, or if the occupants were better educated about the issues, so they can take action on time. Heatwave early warning and response systems are important elements of heat action plans.
Reduced labour capacity
Heat exposure can affect people's ability to work.: 8 The annual Countdown Report by The Lancet investigated change in labour capacity as an indicator. It found that during 2021, high temperature reduced global potential labour hours by 470 billion - a 37% increase compared to the average annual loss that occurred during the 1990s. Occupational heat exposure especially affects laborers in the agricultural sector of developing countries. In those countries, the vast majority of these labour hour losses (87%) were in the agricultural sector.: 1625
Working in extreme heat can lead to labor force productivity decreases as well as participation because employees' health may be weaker due to heat related health problems, such as dehydration, fatigue, dizziness, and confusion.: 1073–1074
Sports and outdoor exercise
With regards to sporting activities it has been observed that "hot weather reduces the likelihood of engaging in exercise".: 1625 Furthermore, participating in sports during excessive heat can lead to injury or even death.: 1073–1074 It is also well established that regular physical activity is beneficial for human health, including mental health.: 1625 Therefore, an increase in hot days due to climate change could indirectly affect health due to people exercising less.
Impacts caused by weather and climate events other than heat
Climate change is increasing the periodicity and intensity of some extreme weather events. Confidence in the attribution of extreme weather to anthropogenic climate change is highest in changes in frequency or magnitude of extreme heat and cold events with some confidence in increases in heavy precipitation and increases in the intensity of droughts.
Extreme weather events, such as floods, hurricanes, droughts and wildfires can result in injuries, death and the spread of infectious diseases. For example, local epidemics can occur due to loss of infrastructure, such as hospitals and sanitation services, but also because of changes in local ecology and environment.
Due to an increase in heavy rainfall events, floods are expected to become more severe in future when they do occur.: 1155 However, the interactions between rainfall and flooding are complex. In some regions, flooding is expected to become rarer. This depends on several factors, such as changes in rain and snowmelt, but also soil moisture.: 1156 Floods have short and long-term negative implications to people's health and well-being. Short term implications include mortalities, injuries and diseases, while long term implications include non-communicable diseases and psychosocial health aspects.
For example, the 2022 Pakistan Floods (which were likely more severe because of climate change) affected people's health directly and indirectly. There were outbreaks of diseases like malaria, dengue, and other skin diseases. Flood runoff can wash soil contaminants like fertilisers and toxins into estuaries, lakes and seas, posing a threat to human health by reducing water quality.
Stronger hurricanes create more opportunities for vectors to breed and infectious diseases to flourish.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.
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 result in 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.
Climate change affects multiple factors associated with droughts, such as how much rain falls and how fast the rain evaporates again. Warming over land increases the severity and frequency of droughts around much of the world.: 1057 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.
Climate change increases wildfire potential and activity. 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.
Infectious diseases that are sensitive to climate can be grouped into: vector-borne diseases (transmitted via mosquitos, ticks etc.), water-borne diseases (transmitted via viruses or bacteria, e.g.: E. Coli) and food-borne diseases (e.g.: Salmonella).: 1107 Climate change is affecting the distribution of these diseases due to the expanding geographic range and seasonality of these diseases and their vectors.: 9
Climate change may also lead to new infectious diseases due to changes in microbial and vector geographic range. Microbes that are harmful to humans can adapt to higher temperatures, which will allow them to build better tolerance against human endothermy defences.
Mosquito-borne diseases that are sensitive to climate include malaria, elephantiasis, Rift Valley fever, yellow fever, dengue fever, Zika virus, and chikungunya. Scientists found in 2022 that rising temperatures are increasing the areas where dengue fever, malaria and other mosquito-carried diseases are able to spread.: 1062 Warmer temperatures are also advancing to higher elevations, allowing mosquitoes to survive in places that were previously inhospitable to them.: 1045 This risks malaria making a return to areas where it was previously eradicated.
Diseases from ticks
Ticks are changing their geographic range because of rising temperatures, and this puts new populations at risk. Ticks can spread lyme disease and tick-borne encephalitis. It is expected that climate change will increase the incidence of these diseases in the Northern Hemisphere.: 1094 For example, a review of the literature found that "In the USA, a 2°C warming could increase the number of Lyme disease cases by over 20% over the coming decades and lead to an earlier onset and longer length of the annual Lyme disease season".: 1094
Scientist expect that disease outbreaks caused by vibrio (in particular the bacterium that causes cholera, called vibrio cholerae) are increasing in occurrence and intensity.: 1107 One reason is that the area of coastline with suitable conditions for vibrio bacteria has increased due to changes in sea surface temperature and sea surface salinity caused by climate change.: 12 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.: 12 Furthermore, the increasing occurrence of higher temperature days, heavy rainfall events and flooding due to climate change could lead to an increase in cholera risks.: 1045
Climate change affects many aspects of food security through "multiple and interconnected pathways".: 1619 Many of these are related to the effects of climate change on agriculture, for example failed crops due to more extreme weather events. This comes on top of other coexisting crises that reduce food security in many regions. Less food security means more undernutrition with all its associated health problems. Food insecurity is increasing at the global level (some of the underlying causes are related to climate change, others are not) and about 720–811 million people suffered from hunger in 2020.: 1629
The number of deaths resulting from climate change-induced changes to food availability are difficult to estimate. The 2022 IPCC Sixth Assessment Report does not quantify this number in its chapter on food security. A modelling study from 2016 found "a climate change–associated net increase of 529,000 adult deaths worldwide [...] from expected reductions in food availability (particularly fruit and vegetables) by 2050, as compared with a reference scenario without climate change."
Reduced nutritional value of crops
Changes in atmospheric carbon dioxide may reduce the nutritional quality of some crops, with for instance wheat having less protein and less of some minerals.: 439  The nutritional quality of C3 plants (e.g. wheat, oats, rice) is especially at risks: lower levels of protein as well as minerals (for example zinc and iron) are expected.: 1379 Food crops could see a reduction of protein, iron and zinc content in common food crops of 3 to 17%. This is the projected result of food grown under the expected atmospheric carbon-dioxide levels of 2050. Using data from the UN Food and Agriculture Organization as well as other public sources, the authors analyzed 225 different staple foods, such as wheat, rice, maize, vegetables, roots and fruits.
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 within 2003–05, reflecting an increasing threat to their marine food productivity and marine food security".: 14
Water resources can be affected by climate change in various ways. The total amount of freshwater available can change, for instance due to dry spells or droughts. Heavy rainfall and flooding can have an impact on water quality: pollutants can be transported into water bodies by the increased surface runoff. In coastal regions, more salt may find its way into water resources due to higher sea levels and more intense storms. Higher temperatures also directly degrade water quality: warm water contains less oxygen. Changes in the water cycle threaten existing and future water infrastructure. It will be harder to plan investments for water infrastructure as there are significant uncertainties about future variability of the water cycle.
Climate change may increase the risk of violent conflict, which can lead to injuries, such as battle injuries, and death. Conflict can result from the increased propensity towards violence after people become more irritable due to excessive heat. There can also be follow-on effects on health from resource scarcity or human migrations that climate change can cause or aggravate in already conflict prone areas.
However, the observed contribution of climate change to conflict risk is small in comparison with cultural, socioeconomic, and political causes. There is some evidence that rural-to-urban migration within countries worsens the conflict risk in violence prone regions. But there is no evidence that migration between countries would increase the risk of violence.: 1008, 1128
The relationship between surface ozone (also called ground-level ozone) and ambient temperature is complex. Changes in air temperature and water content affect the air's chemistry and the rates of chemical reactions that create and remove ozone. Many chemical reaction rates increase with temperature and lead to increased ozone production. Climate change projections show that rising temperatures and water vapour in the atmosphere will likely increase surface ozone in polluted areas like the eastern United States.
On the other hand, ozone concentrations could decrease in a warming climate if anthropogenic ozone-precursor emissions (e.g., nitrogen oxides) continue to decrease through implementation of policies and practices. Therefore, future surface ozone concentrations depend on the climate change mitigation steps taken (more or less methane emissions) as well as air pollution control steps taken.: 884
High surface ozone concentrations often occur during heat waves in the United States. Throughout much of the eastern United States, ozone concentrations during heat waves are at least 20% higher than the summer average. Broadly speaking, surface ozone levels are higher in cities with high levels of air pollution.: 876 Ozone pollution in urban areas affects denser populations, and is worsened by high populations of vehicles, which emit pollutants NO2 and VOCs, the main contributors to problematic ozone levels.
There is a great deal of evidence to show that surface ozone can harm lung function and irritate the respiratory system. Exposure to ozone (and the pollutants that produce it) is linked to premature death, asthma, bronchitis, heart attack, and other cardiopulmonary problems. High ozone concentrations irritate the lungs and thus affect respiratory function, especially among people with asthma. People who are most at risk from breathing in ozone air pollution are those with respiratory issues, children, older adults and those who typically spend long periods of time outside such as construction workers.
The warming oceans and lakes are leading to more frequent harmful algal blooms. Also, during droughts, surface waters are even more susceptible to harmful algal blooms and microorganisms. 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.
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.
Available evidence on the effect of climate change on the epidemiology of snakebite is limited but it is expected that there will be a geographic shift in risk of snakebite: northwards in North America and southwards in South America and in Mozambique, and increase in incidence of bite in Sri Lanka.
The health benefits (also called "co-benefits") from climate change mitigation measures are significant: potential measures can not only mitigate future health impacts from climate change but also improve health directly. Climate change mitigation is interconnected with various co-benefits (such as reduced air pollution and associated health benefits) 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).
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.: 26 Access to urban green spaces provides benefits to mental health as well.: 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.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.
It is possible that a potential health benefit from global warming could result from fewer cold days in winter:: 1099 This could lead to some mental health benefits. However, the evidence on this correlation is regarded as inconsistent in 2022.: 1099
Estimating deaths (mortality) or DALYs (morbidity) from the effects of climate change at the global level is very difficult. A 2014 study by the World Health Organization estimated the effect of climate change on human health, but not all of the effects of climate change were included. For example, the effects of more frequent and extreme storms were excluded. The study assessed deaths from heat exposure in elderly people, increases in diarrhea, malaria, dengue, coastal flooding, and childhood undernutrition. The authors estimated that climate change was projected to cause an additional 250,000 deaths per year between 2030 and 2050 but also stated that "these numbers do not represent a prediction of the overall impacts of climate change on health, since we could not quantify several important causal pathways".
Climate change was responsible for 3% of diarrhoea, 3% of malaria, and 3.8% of dengue fever deaths worldwide in 2004. Total attributable mortality was about 0.2% of deaths in 2004; of these, 85% were child deaths. The effects of more frequent and extreme storms were excluded from this study.
Much of the health burden associated with climate change falls on vulnerable people (e.g. indigenous peoples and economically disadvantaged communities). As a result, people of disadvantaged sociodemographic groups experience unequal risks. Often these people will have made a disproportionately low contribution toward man-made global warming, thus leading to concerns over climate justice.
Climate change has diverse impacts on migration activities, and can lead to decreases or increases in the number of people who migrate.: 1079 Migration activities can have impacts on health and well-being, in particular for mental health. Migration in the context of climate change can be grouped into four types: adaptive migration (see also climate change adaptation), involuntary migration, organised relocation of populations, and immobility (which is when people are unable or unwilling to move even though it is recommended).: 1079
^ abcdefghMarina Romanello, Claudia Di Napoli, Paul Drummond, Carole Green, Harry Kennard, Pete Lampard, Daniel Scamman, Nigel Arnell, Sonja Ayeb-Karlsson, Lea Berrang Ford, Kristine Belesova, Kathryn Bowen, Wenjia Cai, Max Callaghan, Diarmid Campbell-Lendrum, Jonathan Chambers, Kim R van Daalen, Carole Dalin, Niheer Dasandi, Shouro Dasgupta, Michael Davies, Paula Dominguez-Salas, Robert Dubrow, Kristie L Ebi, Matthew Eckelman, Paul Ekins, Luis E Escobar, Lucien Georgeson, Hilary Graham, Samuel H Gunther, Ian Hamilton, Yun Hang, Risto Hänninen, Stella Hartinger, Kehan He, Jeremy J Hess, Shih-Che Hsu, Slava Jankin, Louis Jamart et al. (2022) The 2022 report of the Lancet Countdown on health and climate change: health at the mercy of fossil fuels, The Lancet, Vol 400 November 5, DOI: 10.1016/ S0140-6736(22)01540-9
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