Effects of climate change on human health

• 1. Introduction

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  The effects of climate change on human health reach every age, from infancy through old age. In 2019 alone, heat-related deaths among people older than 65 reached an estimated 345,000 worldwide. The World Health Organisation has called climate change the biggest global health threat of the 21st century. That designation raises urgent questions: who bears the heaviest burden, how do rising temperatures translate into illness and death, and what happens when multiple threats arrive at once? The answers involve heat waves, floods, wildfires, infectious disease, and the food on people's plates. By 2050, a modelling study projected a climate change-associated net increase of 529,000 adult deaths worldwide from reductions in food availability alone.

• 2. The Heat Burden

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  Since the 1970s, the surface of the Earth has grown warmer each decade. Compared to the second half of the 19th century, temperatures in the 21st century show a warming of 1.09 degrees Celsius. That shift is faster than any other 50-year period over at least the last 2,000 years.

  More than 70,000 Europeans died as a result of the 2003 European heat wave. In June 2015, over 2,000 people died in Karachi, Pakistan, when temperatures climbed as high as 49 degrees Celsius. From the decade 2003-12 to 2013-22, heat mortality in Europe increased by 17 deaths per 100,000 people, with women more vulnerable than men.

  Between 1960 and 1990, climate change pushed over 600 million people, roughly 9% of the global population, outside the human climate niche. That niche is the average temperature range in which people have been able to thrive across the last 6,000 years. Without reductions in greenhouse gas emissions, regions home to a third of humanity could become as hot as the hottest parts of the Sahara within 50 years, with projected annual average temperatures above 29 degrees Celsius.

  The human body depends on sweat evaporation to prevent overheating. A wet-bulb temperature of 35 degrees Celsius is the recognised physiological threshold for human adaptability. As of 2020, only two weather stations had ever recorded that figure, and only very briefly. A wet-bulb temperature of 31 degrees is already considered dangerous even for young, healthy people. If global temperatures rise by 3 degrees, large areas of Pakistan, India, China, sub-Saharan Africa, the United States, Australia, and South America would regularly exceed that limit.

• 3. Who Bears the Worst

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  A 2021 report in The Lancet found that the greatest impact of climate change on health falls on the poor, women, children, the elderly, people with pre-existing health conditions, and outdoor workers. Vulnerability is not random; it is shaped by housing, neighbourhood, socioeconomic position, and geography.

  People with cognitive conditions such as depression, dementia, and Parkinson's disease face elevated risk in high temperatures because cognitive performance is differentially affected by heat. People with diabetes, those who are overweight, and those with cardiovascular or cerebrovascular conditions are also at heightened risk.

  The risk of dying from chronic lung disease during a heat wave has been estimated at 1.8-8.2% higher than during average summer temperatures. For every 1 degree Celsius increase above 29 degrees Celsius, hospitalisation rates for people with chronic obstructive pulmonary disease rise by an estimated 8%.

  Urban residents face compounded exposure. Cities are typically warmer than surrounding rural areas because of asphalt, reduced greenery, and large heat-retaining buildings that block cooling breezes. Extreme heat exposure in cities with a wet-bulb globe temperature above 30 degrees Celsius tripled between 1983 and 2016, even before accounting for population growth.

• 4. Floods, Storms, and Wildfire Smoke

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  Floods carry short and long-term health consequences. In the immediate period, they cause mortalities, injuries, and infectious disease outbreaks. Over the medium term, they can sever access to hospitals even for facilities far from the affected zone. The 2022 Pakistan floods, which research suggests were made more severe by climate change, triggered outbreaks of malaria, dengue, and skin diseases.

  Storms are becoming wetter under climate change. Maximum and mean rainfall rates are rising for both tropical cyclones and extratropical cyclones. Scientists expect fewer tropical cyclones overall, but predict that their strength will increase and that their tracks are shifting toward the poles, exposing regions that historically have not faced such events.

  Wildfires produce particulate matter in wood smoke that harms the respiratory system. Wildfire smoke exposure has been linked to the development and worsening of asthma and chronic obstructive pulmonary disorder, increased risk of lung cancer, mesothelioma, and tuberculosis, as well as changes in inflammatory mediators and coagulation factors. Climate change extends fire season by promoting earlier snowmelt, drier vegetation, and longer summer droughts.

• 5. Infectious Disease and Water

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  Vector-borne diseases spread by mosquitoes and ticks, including dengue fever and malaria, are projected to expand into new regions as temperatures shift. In 2004, climate change was responsible for 3% of diarrhea deaths, 3% of malaria deaths, and 3.8% of dengue fever deaths worldwide; in that same year, 85% of total attributable mortality from climate change fell on children.

  Warming oceans and lakes are producing more frequent harmful algal blooms. During droughts, surface waters grow even more susceptible to these blooms and to other microorganisms. Some blue-green algae, also called cyanobacteria, produce neurotoxins, hepatotoxins, cytotoxins, and endotoxins that can cause neurological, liver, and digestive diseases in humans. Cyanobacteria grow best above 25 degrees Celsius, so climate change extends both their frequency and duration.

  One algal species, Pseudo-nitzschia fraudulenta, produces domoic acid, which causes amnesic shellfish poisoning. Its toxicity has been shown to increase as CO2 concentrations rise through ocean acidification. A 2021 headline finding stated that between 2018 and 2020, nearly 70% of countries recorded increases in average sea surface temperature in their territorial waters compared with 2003-05, signalling a growing threat to marine food security.

• 6. Food, Air, and Hidden Costs

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  Around 720-811 million people suffered from hunger in 2020. Climate change contributes to that figure through failed crops, reduced yields, and higher food prices, all of which worsen undernutrition and its cascade of health consequences.

  Surface ozone concentrations during heat waves across much of the eastern United States run at least 20% above the summer average. Ozone exposure is linked to premature death, asthma, bronchitis, and heart attack. Air pollution from fossil fuel combustion has been estimated to cause as many as 8.7 million excess deaths in 2018 alone; a separate estimate puts the annual death toll from air pollution at over 13 million.

  The annual Countdown Report by The Lancet tracked labour capacity as a health indicator. During 2021, high temperatures reduced global potential labour hours by 470 billion, a 37% increase compared to the average annual loss during the 1990s. In developing countries, 87% of those lost hours came from the agricultural sector. Mental health is also affected: extreme heat raises the risk of mental health-related hospitalisations and suicidal outcomes, and migration driven by climate pressures carries its own mental health consequences, particularly for people who are forced to relocate involuntarily.

• 7. A Path Through Mitigation

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  Climate change mitigation has been described as the greatest global health opportunity of the 21st century. Cleaner air, healthier diets, more active lifestyles, and access to urban green spaces are among the co-benefits that accompany action on emissions.

  Future sustainable pathway scenarios project an annual reduction of 1.18 million air pollution-related deaths, 5.86 million diet-related deaths, and 1.15 million deaths from physical inactivity across nine countries by 2040. Globally, the cost of limiting warming to 2 degrees Celsius is less than the value of the extra years of life gained from cleaner air; in India and China, that margin is even wider.

  Research on communications has found that framing climate change as a health concern, rather than an environmental or doomsday issue, more effectively encourages public engagement. In 2019, the Australian Medical Association formally declared climate change a health emergency. The United States Environmental Protection Agency had issued a 100-page report on global warming and human health as far back as 1989. A 2014 World Health Organization study projected that climate change would cause an additional 250,000 deaths per year between 2030 and 2050, while also noting that the estimate excluded several important causal pathways. If warming reaches or exceeds 2 degrees Celsius this century, a review found that roughly 1 billion premature deaths could result from anthropogenic global warming.