Why climate change is still the greatest threat to human health

Polluted air and steadily rising temperatures are linked to health effects ranging from increased heart attacks and strokes to the spread of infectious diseases and psychological trauma.

Workers fumigate for mosquitoes on a city street in New Delhi, India, as a preventive measure against the spread of dengue, malaria, and chikungunya. The impact of vector-borne diseases will increase as global temperatures rise. An editorial co-published in hundreds of medical journals called for urgent action to reduce emissions.
Photograph by Raj K Raj, Hindustan Times via Getty Images

People around the world are witnessing firsthand how climate change can wreak havoc on the planet. Steadily rising average temperatures fuel increasingly intense wildfires, hurricanes, and other disasters that are now impossible to ignore. And while the world has been plunged into a deadly pandemic, scientists are sounding the alarm once more that climate change is still the greatest threat to human health in recorded history.

As recently as August—when wildfires raged in the United States, Europe, and Siberia—World Health Organization Director-General Tedros Adhanom Ghebreyesus said in a statement that “the risks posed by climate change could dwarf those of any single disease.”

On September 5, more than 200 medical journals released an unprecedented joint editorial that urged world leaders to act. “The science is unequivocal,” they write. “A global increase of 1.5°C above the pre-industrial average and the continued loss of biodiversity risk catastrophic harm to health that will be impossible to reverse.”

Despite the acute dangers posed by COVID-19, the authors of the joint op-ed write that world governments “cannot wait for the pandemic to pass to rapidly reduce emissions.” Instead, they argue, everyone must treat climate change with the same urgency as they have COVID-19.

Here’s a look at the ways that climate change can affect your health—including some less obvious but still insidious effects—and why scientists say it’s not too late to avert catastrophe.

Air pollution

Climate change is caused by an increase of carbon dioxide and other greenhouse gases in Earth’s atmosphere, mostly from fossil fuel emissions. But burning fossil fuels can also have direct consequences for human health. That’s because the polluted air contains small particles that can induce stroke and heart attacks by penetrating the lungs and heart and even traveling into the bloodstream. Those particles might harm the organs directly or provoke an inflammatory response from the immune system as it tries to fight them off. Estimates suggest that air pollution causes anywhere between 3.6 million and nine million premature deaths a year.

“The numbers do vary,” says Andy Haines, professor of environmental change and public health at the London School of Hygiene and Tropical Medicine and author of the recently published book Planetary Health. “But they all agree that it’s a big public health burden.”

People over the age of 65 are most susceptible to the harmful effects of air pollution, but many others are at risk too, says Kari Nadeau, director of the Sean N. Parker Center for Allergy and Asthma Research at Stanford University. People who smoke or vape are at increased risk, as are children with asthma.

Air pollution also has consequences for those with allergies. Carbon dioxide increases the acidity of the air, which then pulls more pollen out from plants. For some people, this might just mean that they face annoyingly long bouts of seasonal allergies. But for others, it could be life-threatening.

“For people who already have respiratory disease, boy is that a problem,” Nadeau says. When pollen gets into the respiratory pathway, the body creates mucus to get rid of it, which can then fill up and suffocate the lungs.

Even healthy people can have similar outcomes if pollen levels are especially intense. In 2016, in the Australian state of Victoria, a severe thunderstorm combined with high levels of pollen to induce what The Lancet has described as “the world’s largest and most catastrophic epidemic of thunderstorm asthma.” So many residents suffered asthma attacks that emergency rooms were overwhelmed—and at least 10 people died as a result.

Climate change is also causing wildfires to get worse, and wildfire smoke is especially toxic. As one recent study showed, fires can account for 25 percent of dangerous air pollution in the U.S. Nadeau explains that the smoke contains particles of everything that the fire has consumed along its path—from rubber tires to harmful chemicals. These particles are tiny and can penetrate even deeper into a person’s lungs and organs. (Here’s how breathing wildfire smoke affects the body.)

Extreme heat

Heat waves are deadly, but researchers at first didn’t see direct links between climate change and the harmful impacts of heat waves and other extreme weather events. Haines says the evidence base has been growing. “We have now got a number of studies which has shown that we can with high confidence attribute health outcomes to climate change,” he says.

Most recently, Haines points to a study published earlier this year in Nature Climate Change that attributes more than a third of heat-related deaths to climate change. As National Geographic reported at the time, the study found that the human toll was even higher in some countries with less access to air conditioning or other factors that render people more vulnerable to heat. (How climate change is making heat waves even deadlier.)

That’s because the human body was not designed to cope with temperatures above 98.6°F, Nadeau says. Heat can break down muscles. The body does have some ways to deal with the heat—such as sweating. “But when it’s hot outside all the time, you cannot cope with that, and your heart muscles and cells start to literally die and degrade,” she says.

If you’re exposed to extreme heat for too long and are unable to adequately release that heat, the stress can cause a cascade of problems throughout the body. The heart has to work harder to pump blood to the rest of the organs, while sweat leeches the body of necessary minerals such as sodium and potassium. The combination can result in heart attacks and strokes.

Dehydration from heat exposure can also cause serious damage to the kidneys, which rely on water to function properly. For people whose kidneys are already beginning to fail—particularly older adults—Nadeau says that extreme heat can be a death sentence. “This is happening more and more,” she says.

Studies have also drawn links between higher temperatures and preterm birth and other pregnancy complications. It’s unclear why, but Haines says that one hypothesis is that extreme heat reduces blood flow to the fetus.

Food insecurity

One of the less direct—but no less harmful—ways that climate change can affect health is by disrupting the world’s supply of food.

Climate change both reduces the amount of food that’s available and makes it less nutritious.  According to an Intergovernmental Panel on Climate Change (IPCC) special report, crop yields have already begun to decline as a result of rising temperatures, changing precipitation patterns, and extreme weather events. Meanwhile, studies have shown that increased carbon dioxide in the atmosphere can leech plants of zinc, iron, and protein—nutrients that humans need to survive.

Malnutrition is linked to a variety of illnesses, including heart disease, cancer, and diabetes. It can also increase the risk of stunting, or impaired growth, in children, which can harm cognitive function.

Climate change also imperils what we eat from the sea. Rising ocean temperatures have led many fish species to migrate toward Earth’s poles in search of cooler waters. Haines says that the resulting decline of fish stocks in subtropic regions “has big implications for nutrition,” because many of those coastal communities depend on fish for a substantial amount of the protein in their diets.

This effect is likely to be particularly harmful for Indigenous communities, says Tiff-Annie Kenny, a professor in the faculty of medicine at Laval University in Quebec who studies climate change and food security in the Canadian Arctic. It’s much more difficult for these communities to find alternative sources of protein, she says, either because it’s not there or because it’s too expensive. “So what are people going to eat instead?” she asks.

Infectious diseases 

As the planet gets hotter, the geographic region where ticks and mosquitoes like to live is getting wider. These animals are well-known vectors of diseases such as the Zika virus, dengue fever, and malaria. As they cross the tropics of Cancer and Capricorn, Nadeau says, mosquitoes and ticks bring more opportunities for these diseases to infect greater swaths of the world.

“It used to be that they stayed in those little sectors near the Equator, but now unfortunately because of the warming of northern Europe and Canada, you can find Zika in places you wouldn’t have expected,” Nadeau says.

In addition, climate conditions such as temperature and humidity can impact the life cycle of mosquitoes. Haines says there’s particularly good evidence showing that, in some regions, climate change has altered these conditions in ways that increase the risk of mosquitos transmitting dengue.

There are also several ways in which climate change is increasing the risk of diseases that can be transmitted through water, such as cholera, typhoid fever, and parasites. Sometimes that’s fairly direct, such as when people interact with dirty floodwaters. But Haines says that drought can have indirect impacts when people, say, can’t wash their hands or are forced to drink from dodgier sources of freshwater.

Mental health

A common result of any climate-linked disaster is the toll on mental health. The distress caused by drastic environmental change is so significant that it has been given its own name—solastalgia.

Nadeau says that the effects on mental health have been apparent in her studies of emergency room visits arising from wildfires in the western U.S. People lose their homes, their jobs, and sometimes their loved ones, and that takes an immediate toll. “What’s the fastest acute issue that develops? It’s psychological,” she says. Extreme weather events such as wildfires and hurricanes cause so much stress and anxiety that they can lead to post-traumatic stress disorder and even suicide in the long run.

Another common factor is that climate change causes disproportionate harm to the world’s most vulnerable people. On September 2, the Environmental Protection Agency (EPA) released an analysis showing that racial and ethnic minority communities are particularly at risk. According to the report, if temperatures rise by 2°C (3.6°F), Black people are 40 percent more likely to live in areas with the highest projected increases in related deaths. Another 34 percent are more likely to live in areas with a rise in childhood asthma.

Further, the effects of climate change don’t occur in isolation. At any given time, a community might face air pollution, food insecurity, disease, and extreme heat all at once. Kenny says that’s particularly devastating in communities where the prevalence of food insecurity and poverty are already high. This situation hasn’t been adequately studied, she says, because “it’s difficult to capture these shocks that climate can bring.”

Why there’s reason for hope

In recent years, scientists and environmental activists have begun to push for more research into the myriad health effects of climate change. “One of the striking things is there’s been a real dearth of funding for climate change and health,” Haines says. “For that reason, some of the evidence we have is still fragmentary.”

Still, hope is not lost. In the Paris Agreement, countries around the world have pledged to limit global warming to below 2°C (3.6°F)—and preferably to 1.5°C (2.7°F)—by cutting their emissions. “When you reduce those emissions, you benefit health as well as the planet,” Haines says.

Meanwhile, scientists and environmental activists have put forward solutions that can help people adapt to the health effects of climate change. These include early heat warnings and dedicated cooling centers, more resilient supply chains, and freeing healthcare facilities from dependence on the electric grid.

Nadeau argues that the COVID-19 pandemic also presents an opportunity for world leaders to think bigger and more strategically. For example, the pandemic has laid bare problems with efficiency and equity that have many countries restructuring their healthcare facilities. In the process, she says, they can look for new ways to reduce waste and emissions, such as getting more hospitals using renewable energy.

“This is in our hands to do,” Nadeau says. “If we don’t do anything, that would be cataclysmic.”

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