Based on Wikipedia: Effects of climate change on human health

In the summer of 2003, Europe experienced something unprecedented. Temperatures soared. Air conditioners, relatively uncommon in European homes at the time, couldn't keep up. And over the course of a few scorching weeks, more than 70,000 people died.

That single heat wave killed more Europeans than any natural disaster in the continent's recorded history.

This wasn't a volcanic eruption or an earthquake. It was hot weather—the kind of hot weather that climate scientists had been warning would become more common as the planet warmed. The World Health Organization now calls climate change the biggest global health threat of the 21st century, and understanding why requires looking at how rising temperatures affect human bodies in ways both obvious and surprisingly subtle.

The Human Climate Niche

For roughly six thousand years, humans have thrived within a particular range of average temperatures. Researchers call this the "human climate niche"—the thermal sweet spot where our civilizations have flourished, our crops have grown reliably, and our bodies have functioned comfortably.

Between 1960 and 1990, climate change pushed an estimated 600 million people—about nine percent of the world's population—outside this niche. They didn't move; the climate around them shifted into territory that human beings had historically avoided settling.

The projections are starker. Without significant reductions in greenhouse gas emissions, regions inhabited by a third of humanity could become as hot as the hottest parts of the Sahara Desert within fifty years. The projected annual average temperature for these areas—above 29 degrees Celsius, or about 84 degrees Fahrenheit—would fall outside what scientists consider the biologically suitable temperature range for human beings.

This isn't about discomfort. It's about survival.

How Heat Kills

The human body has a remarkably simple cooling system: sweat evaporates from the skin, carrying heat away with it. This works beautifully in dry conditions but fails in humid ones. When the air is already saturated with moisture, sweat can't evaporate. The body overheats.

Scientists measure this using something called "wet-bulb temperature"—essentially what a thermometer would read if you wrapped it in a wet cloth. A wet-bulb temperature of 35 degrees Celsius (95 degrees Fahrenheit) represents the absolute limit of human tolerance. At that point, even a young, healthy person sitting still in the shade, with access to unlimited water, cannot cool their body enough to survive.

As of 2020, only two weather stations on Earth had ever recorded wet-bulb temperatures of 35 degrees, and only for brief periods. But the frequency of such events is expected to rise.

More concerning is that dangerous conditions begin well below this theoretical maximum. A wet-bulb temperature of 31 degrees is already considered dangerous even for young, healthy people. If global temperatures rise by 3 degrees Celsius—which remains the most likely scenario if fossil fuel use continues at current rates—large swaths of Pakistan, India, China, sub-Saharan Africa, the United States, Australia, and South America will regularly exceed this threshold.

The Vulnerable

Heat doesn't affect everyone equally. The elderly are especially susceptible—heat-related deaths in people over 65 reached an estimated 345,000 in 2019 alone, a record high. Their bodies regulate temperature less efficiently, and many take medications that interfere with sweating or blood flow to the skin.

Children face unique risks. Their bodies heat up faster than adults' bodies do, and they depend on caregivers to recognize when they're in danger.

Pregnant women experience complications from high temperatures, including preterm birth and low birth weight babies. The developing fetus, it turns out, is remarkably sensitive to thermal stress.

People with chronic conditions face compounded risks. Those with diabetes, cardiovascular disease, or respiratory conditions like chronic obstructive pulmonary disease (a progressive lung condition that makes breathing difficult) find their symptoms worsen dramatically in heat. Studies suggest that the risk of dying from chronic lung disease during a heat wave runs 1.8 to 8.2 percent higher than during average summer temperatures. Hospitalization rates for people with chronic obstructive pulmonary disease increase by 8 percent for every single degree Celsius the temperature rises above 29 degrees.

Mental health conditions create hidden vulnerabilities. People with depression, dementia, or Parkinson's disease experience more severe effects from heat, partly because these conditions affect the brain's temperature regulation and partly because they may impair judgment about when to seek relief. Research shows that extreme heat increases both mental health-related hospitalizations and suicide rates.

The Urban Heat Trap

Cities create their own microclimates, and these tend to be hotter than surrounding areas—sometimes dramatically so. This phenomenon, called the urban heat island effect, results from a perfect storm of design choices.

Asphalt and concrete absorb heat during the day and radiate it back at night, preventing the natural cooling that occurs in rural areas after sunset. Tall buildings block cooling breezes and create canyons that trap warm air. The absence of trees means no shade and no evaporative cooling from leaves. Air conditioning units pump heat from building interiors into already-hot streets.

The result: city dwellers face far more extreme heat exposure than their suburban or rural counterparts. Between 1983 and 2016, exposure to extreme heat in cities—defined as wet-bulb globe temperatures above 30 degrees Celsius—tripled. Even accounting for population growth in these cities, exposure increased by about 50 percent.

Cities are often located near coasts and waterways, which makes them additionally vulnerable to the flooding that accompanies more severe storms. Their aging infrastructure—water mains, electrical grids, hospitals—wasn't designed for the climate we're entering. And their densely concentrated populations mean that when systems fail, millions of people are affected simultaneously.

Beyond Direct Heat

The health effects of climate change extend far beyond heat stroke. Rising temperatures alter the behavior of disease-carrying insects in ways that expand their range and extend their active seasons.

Mosquitoes that carry dengue fever and malaria are moving into regions previously too cold for them. Ticks that spread Lyme disease are surviving winters they couldn't have survived a few decades ago. These aren't theoretical concerns—they're already happening, and they're accelerating.

Waterborne diseases become more common as flooding overwhelms sewage systems and droughts concentrate pathogens in shrinking water supplies. Diarrheal diseases, which already kill hundreds of thousands of children annually, are expected to increase.

Air quality degrades on multiple fronts. Hotter temperatures accelerate the formation of ground-level ozone, the main component of smog. Longer, more severe wildfire seasons fill the air with particulate matter that penetrates deep into lungs. Both trigger respiratory crises, particularly in people with asthma or other lung conditions.

Food security—and with it, nutrition—faces mounting pressure. Climate change reduces yields for staple crops in many regions, raises food prices, and forces populations to rely on less nutritious alternatives. The resulting undernutrition affects cognitive development in children, weakens immune systems, and reduces the body's ability to cope with other stressors, including heat.

The Kidney Disease Mystery

In certain farming communities, particularly in Central America and South Asia, an epidemic of chronic kidney disease has emerged over the past few decades. Unlike typical kidney disease, which usually results from diabetes or high blood pressure, this variant strikes young, healthy agricultural workers in their twenties and thirties.

Researchers have linked it to the combination of extreme heat, physical exertion, and inadequate hydration—conditions that are increasingly common as temperatures rise. Workers laboring in sugarcane fields and other agricultural settings experience repeated episodes of heat stress and dehydration, and over years, their kidneys sustain cumulative damage.

This represents a new kind of occupational disease, one created by the intersection of climate change and the economic necessity of outdoor labor. It's a preview of health impacts that may become more widespread as working conditions in many parts of the world become more dangerous.

The Economics of Heat

In 2021, high temperatures reduced global potential labor hours by 470 billion—a 37 percent increase compared to the average annual loss during the 1990s. Most of this loss, about 87 percent, occurred in the agricultural sector of developing countries, where workers have the least ability to escape the heat and the most urgent economic need to continue working.

The impacts cascade. Workers suffering from heat exhaustion make mistakes. Dehydration causes fatigue, dizziness, and confusion. Chronic heat exposure reduces the hours people can safely work, cutting into already-thin earnings and making poverty harder to escape.

Even exercise—one of the most effective interventions for preventing chronic disease—becomes more difficult. Research confirms what anyone who has tried to run in summer heat already knows: hot weather reduces the likelihood of exercising. As the number of dangerously hot days increases, opportunities for safe outdoor physical activity shrink.

This creates a bitter irony. Climate change is caused in part by the combustion-powered transportation that replaced walking and cycling. The warming it produces makes walking and cycling less safe, pushing more people toward air-conditioned cars, which continue the cycle.

Floods, Droughts, and Cascading Failures

Climate change doesn't just raise average temperatures—it amplifies weather extremes. Droughts become more severe because warmer air evaporates water from soil more quickly. When rain does fall, it often comes in more intense bursts, overwhelming drainage systems and causing flash floods.

The health impacts of floods extend well beyond immediate drowning risks. In the short term, floodwaters spread waterborne diseases and create breeding grounds for mosquitoes. In the medium term, they damage or destroy hospitals, disrupt emergency services, and contaminate drinking water supplies. Over the long term, flood survivors experience elevated rates of mental health problems, including post-traumatic stress disorder, depression, and anxiety.

Perhaps most concerning is how these impacts interact. A flood that destroys a hospital affects not just the immediate victims but also everyone in the region who might need medical care in the following weeks and months. Food insecurity weakens immune systems, making infectious diseases more deadly. Mental health impacts from one disaster reduce people's capacity to prepare for or respond to the next.

The Inequality Multiplier

Climate change doesn't create new inequalities so much as it amplifies existing ones. People with money can buy air conditioning, relocate to cooler regions, and access healthcare when they fall ill. People without these resources cannot.

This plays out at every scale. Wealthy nations have more resources to adapt than poor ones. Within nations, affluent neighborhoods have more tree cover and fewer heat-trapping surfaces than low-income areas. Within households, the people least able to control their environment—children, the elderly, those with disabilities—face the greatest risks.

Women and girls often face disproportionate impacts, though the specific mechanisms vary by context. In many societies, they bear primary responsibility for collecting water and fuel, tasks that become more time-consuming and dangerous as droughts intensify. Pregnancy creates biological vulnerabilities to heat. And in disasters, pre-existing gender inequalities often determine who can evacuate, who receives aid, and who recovers.

Adaptation and Its Limits

The most obvious solution to heat—air conditioning—illustrates the contradictions of climate adaptation. Air conditioning works by moving heat from inside buildings to outside, which means it makes outdoor spaces hotter. It consumes enormous amounts of electricity, much of which is still generated by burning fossil fuels. And it creates peak demand on electrical grids at exactly the moment those grids are most likely to fail.

During heat waves, air conditioning-related electricity demand spikes. Power plants struggle to meet this demand, transmission lines sag in the heat and become less efficient, and transformers can fail. When the grid goes down, the air conditioning goes with it, and people die.

Better building design offers a more sustainable path. Passive cooling strategies—white roofs that reflect sunlight, strategic window placement for cross-ventilation, thick walls that resist heat transfer—can dramatically reduce indoor temperatures without electricity. But these features must be built in from the start or require expensive retrofits.

Early warning systems help. Cities that implement heat action plans—public messaging campaigns, opening cooling centers, checking on vulnerable residents—see lower mortality during heat waves. But these systems require coordination, funding, and public trust, all of which can be in short supply.

The Trajectory

Every decade since the 1970s has been warmer than the one before it—a rate of warming faster than any 50-year period in at least the last 2,000 years. Compared to the second half of the 19th century, global temperatures have already risen by about 1.09 degrees Celsius.

That single degree has produced the effects described here: record-breaking heat waves, expanding disease ranges, agricultural disruption, and hundreds of thousands of excess deaths. The Paris Agreement set a goal of limiting warming to 1.5 degrees, but current policies put the world on track for roughly 3 degrees of warming by the end of this century.

At 3 degrees, the impacts don't simply double. They interact and amplify. Heat waves become more deadly in populations weakened by food insecurity. Floods cause more damage when health systems are already overwhelmed. Migration increases as regions become uninhabitable, straining resources in destination areas.

The window for limiting these impacts is narrowing. But it remains open. The same scientific enterprise that identified these risks has also identified solutions—technologies and policies that could reduce emissions quickly enough to avoid the worst scenarios. Whether humanity deploys them in time remains the central question of our era.

What's not in question is that climate change is already affecting human health, in ways both dramatic and subtle, in every region of the world. The heat wave that killed 70,000 Europeans in 2003 was once considered exceptional. Now it's a preview.