How Your Body Adapts to Extreme Cold

Scientists are finding a dynamic story in human physiology linked to frigid temperatures—a story that climate change may rewrite.

A bitter winter storm is sweeping across the north-east of North America this weekend, and is expected to bring significant snow to New York City for the first time in two years. Low temperatures around freezing are expected to last into next week.

If this is making you miserable, it’s because you, like most people, overwhelmingly prefer hot places. That group does not include Cara Ocobock, a biological anthropologist at University of Notre Dame who is one of the scientists trying to understand how the human body adjusts to extreme cold. “I just handle cold much better than I can handle heat,” says Ocobock.

Researchers like Ocobock have recently uncovered a variety of physiological adaptations linked to cold. Those range from anatomical to metabolic changes, and can stem from generations of natural selection or simply the short-term effects of acclimatization. These discoveries help people make practical decisions today, and most important to Ocobock, they hint at what we should expect in an increasingly capricious climate where winter cyclones freeze people in what are normally hot places, and heat waves make people swelter in what are normally icy ones.

Climate change is driving up ocean temperatures, fueling powerful winter storms in the northeast US seemingly every year. Strong polar winds are bringing harsher, earlier, cold fronts. This October, temperatures in Houston, Texas, dropped a record 43 degrees Fahrenheit within 24 hours. Denver, colourado, tied its earliest freeze in history on September 8, 2020, just hours after reaching 93 degrees in a record-setting heat wave. A deep freeze engulfed Texas for nine days in February 2021. It was the state’s coldest storm in 132 years. Scientists have debated whether an Atlantic Ocean current will collapse, triggering a massive drop in temperatures in Europe, but many disagree. Meanwhile, Earth’s summers have been brutal, even in the most frigid places, such as Siberia, which endured record heat in 2021 and 2023.

Ocobock wonders what we can learn about human bodies in a changing climate. “There are people who have been living in these climates for generations upon generation upon generation, and we are now seeing unprecedented rapid change in weather as well as climate,” she says. “So how are our bodies responding to it? Is there a limit to how they can respond to it?”

Climate change also creates and amplifies refugee crises, which send people to entirely new climates. “People are migrating into environments they've never been in before,” Ocobock says, noting the presence of Sudanese refugees in Finland. New insights help us understand how to cope with extreme cold—and how to prepare for losing it.

Ocobock’s wintery work began a decade ago, when she was a graduate student traveling to Wyoming to collect data from students of the nonprofit National Outdoor Leadership School. During six-week-long winter expeditions in Wyoming’s backcountry, she would hike out in parallel to the students and pitch a tent a couple of kilometers away. Each day, she’d pop in to measure their weights and collect urine samples, activity monitors, and travel logs. She hoped these measurements could help her predict the poorly understood energy demands that cold environments place on human bodies. “People would go out on these courses, and they would come back having dropped a lot of weight. And in some cases, that's OK. But in other cases, people were losing a lot of muscle mass and coming back feeling horrible,” she says.

Her dissertation revealed that the winter backpackers expend surprisingly few calories to stay warm. At first, this seems counterintuitive: “They don't have external heating sources. They're living in tents, and they just have their clothes with them. They're very exposed to the elements every single day,” Ocobock says. These conditions should make muscles shiver as the body works overtime. But Ocobock noticed that exercise lifted that burden. Simply moving around a snowy environment on cross-country skis and snowshoes changed the bodily calculus. “The wonderful part about muscle is that it's inefficient,” she says. Only 20 to 30 percent of the calories your muscles burn go to actually doing things, and much of the rest is “wasted” as heat. In the cold, though, this heat is no waste—it lowers the cost of thermal regulation.

Scientists had never shown what Ocobock did in a natural setting. It fueled her curiosity about cold climates—“populations who have been there for millennia, rather than American students who are just hopping into the Rocky Mountains for the winter,” she says. So after Wyoming, Ocobock began a project with reindeer herders in Finland that include the Sámi, an Indigenous group. Ocobock spent three years building relationships and trust before collecting data. Worth the wait, she felt, since scientists still knew little about how bodies respond to extreme cold. “There hadn't been any recent work on cold physiology,” Ocobock says. “A lot of things were left over from the 1930s, and even older.”

Historical research had provided cold physiology with three guiding principles relevant to many warm-blooded animals: Bergmann’s rule, Allen’s rule, and Thomson’s rule. German anatomist Carl Bergmann theorized in 1847 that animals of similar species tend to be larger in cold climates. For example, polar bears have a couple feet of height and a few hundred pounds on the average grizzly. Thirty years after Bergmann, American ornithologist Joel Asaph Allen tacked “shorter appendages” onto the theory of larger bodies. Polar bears have stockier limbs and smaller ears than black bears. In the 1920s, British anthropologist Arthur Thomson argued that people in cold places have longer, narrower noses.

Bergmann’s and Allen’s theories were all about the importance of bodies retaining heat in their cores; Thomson’s supposed that nasal cavities condition ambient air before it reaches the lungs. Cold dry air can irritate the airways and lungs, and may weaken our sense of smell. But in a narrower cavity, cold dry air mingles longer with warm blood and moisture.

Thomson’s rule simplifies what’s actually a complex part of our evolution likely influenced by other factors, like sexual attraction. But researchers in the past decade have found supporting evidence based on studying people with roots in Northern Europe, West Africa, South Asia, and East Asia showing that wider nostrils appear more frequently among people in colder places, suggesting an environmental adaptation.

Bergmann’s and Allen’s rules also held up when comparing old data on body sizes in warm climates to those of Sámi, Inuit, and Yuit populations. But a 2013 study found that Bergmann’s rule only applies when groups are 50 degrees of latitude apart, or live in places with 30 degree difference in temperature. When the distances and temperatures aren’t so different, body sizes aren’t meaningfully different, either.

Today, physiologists and anthropologists like Ocobock are focused more on distinguishing what happens within bodies that are accustomed to cold. Our bodies make their own vitamin D out of a precursor chemical, 7-dehydrocholesterol, that absorbs UV-B rays from sunlight. Near the equator, there’s enough strong sun for people to get their vitamin D supply. In fact, the risk is too much cancer-causing sunlight, so people have more melanin, a skin pigment that absorbs UV. But as frigid climates get less sun, melanin competes with 7-dehydrocholesterol for weaker sunlight, so the body risks underproducing vitamin D. Experts believe this prompted ancient humans who lived in northern latitudes to develop lighter skin tones, which synthesize vitamin D faster, an adaptation to life far from the equator.

Other adaptations keep the body warm. Blood vessels constrict when it’s cold to limit blood flow to extremities like hands and feet. It’s uncomfortable and limits dexterity, but it also minimizes heat loss. When skin temperature drops enough, however, the body briefly lets warm blood reenter the fingers, toes, ears, and nose. This blood vessel dilation explains why your ears get red and painful in the cold. Populations in cold parts of the world have reportedly faster cycles between vasoconstriction and vasodilation, which provides a more balanced temperature regulation in extreme conditions. And this is nothing new: DNA from 4,000-year-old hair preserved in Greenland showed signs of vasoconstriction.

That same hair sample also showed genetic signs of high body-mass index, which is another adaptation to cold. Fat and muscle insulate the body, and populations that live in cold places also maintain more of both, on average. That fat has a job, particularly a type of fat called brown adipose tissue. Scientists had once believed that bodies just shiver to generate heat—a belief that was upended once they realized that brown adipose tissue allows rodents produce heat without shivering.

Then, about 20 years ago, scientists discovered brown fat in adult humans. No one has a lot of it. Human limits max out around 100 grams, distributed mostly around the neck, back, shoulders, heart, and kidneys. Anthropologists traced it predominantly to people who live in cold environments. Brown fat became quasi-synonymous with cold adaptation and the purported health benefits of cold exposure: It burns calories to produce heat when you feel cold, and studies have suggested it helps regulate obesity and blood sugar. Last year, Ocobock reported that brown fat speeds up the metabolisms of Finnish reindeer herders by about 9 percent when they feel cold.

But the amount of brown fat varies from person to person. Early life exposure to cold matters a lot, says Stephanie Levy, a biological anthropologist at Hunter College who studies the “plasticity” of brown fat throughout people’s lives. Levy’s central question: Do people who spend more time in the cold as kids have more brown fat as adults? In a small study published in 2021, she found that adults had more brown fat when they had more cold exposure roughly between ages 2 and 5. It may be possible to tack on more brown fat with cold exposures later in life, but early childhood seems to be particularly plastic.

This plasticity may explain our ability to adapt to our environments within our lifetime, rather than through generations of natural selection. In a separate study, Levy compared brown fat between people from Illinois and Indigenous Yakut people from northeastern Siberia. Adults in Yakutia had more brown fat than Illinoisans, who live in a comparatively milder climate. “There's mounting evidence that this could be a pan-human adaptation—we all have some level,” says Ocobock. In September, one of her former students reported finding brown fat in Samoan people, a tropical population.

“There’s also kind of a mystery,” Levy adds. Although people with more brown fat burn more calories when they’re exposed to cold than people who don’t, brown fat alone doesn’t explain the difference. That means brown fat may also play an indirect role for heating, such as releasing fuel or signaling molecules to other organs.

Ocobock’s strangest discovery while observing Finnish and Sámi herders involved the calories they burn at rest. Resting metabolism is typically higher in the cold, and it scales with body size, so men have higher rates. Ocobock figured that the cold climate would give every inhabitant a higher resting metabolism, compared to people living in warm climates. But in this group, only the female herders had high rates. In fact, the females’ resting metabolic rates were higher than that of males. “This has never been seen before,” she says.

Ocobock suspects this metabolic oddity results from another player in wintery physiology: thyroid hormone. This hormone helps set our metabolic rates, and increases when we feel cold. “Thyroid hormone is critically important for setting your own body's internal thermostat,” she says. “However, it's also critically important for maintaining a pregnancy, particularly in the first eight weeks.” People in cold climates may need higher baselines for regular life and pregnancy, so Ocobock’s hunch is that while men’s levels might drop easily, women’s levels may face a sort of “physiological resistance” that safeguards reproductive success.

The dynamic between resting metabolism and thyroid hormone is tricky and unsettled, though, Levy notes. Her Siberian studies show Yakut thyroid hormone levels decreasing from summer to winter. The body seems to produce more thyroid hormone in the cold, but consume more too. Ocobock still works with reindeer herders and hopes to confirm the thyroid theory soon.

Metabolic details matter to predict health in the modern world, Ocobock says. The same genetic programming that arose to protect someone in the Arctic—like high BMI and faster metabolism—could become liabilities. Many of Ocobock’s study subjects have been overweight and obese with normal cholesterol and blood sugar. Being “fat but fit,” which has been beneficial in extreme cold, “could now also be falling apart because of climate change, and could be leading to worsening health,” she says. If people’s diets and activity levels remain the same, but their metabolic rates drop as the climate warms, their obesity risk will rise. “The lowered resting metabolic rates among males might be an embodiment of climate change,” she says.

In February, Ocobock traveled to Inari, Finland, which sits 165 miles north of the Arctic circle. February is usually the coldest month of the year, with highs around 15 degrees Fahrenheit. This year, several days topped 40 degrees. “So literally in February, there were days I didn't bother wearing a coat in the Arctic Circle. That's deeply messed up.”

But experts caution that biological adaptations alone don’t determine whether someone is cut out for the cold. For one thing, humans only migrated to colder climates less than 100,000 years ago—a blink in evolutionary timescales. “Some of these adaptations are actually not as dramatic as we think,” says François Haman, who studies thermal physiology at the University of Ottawa, Canada. Haman notes that traits like the size and shapes of bodies, hands, feet, and ears vary a lot within any population, as does a person’s amount of brown fat.

“When a trait is highly variable like what we see for cold, what we realize is that behavior was actually more important to survive than genetics,” Haman says. What’s most important is that the individual learns to adapt to the risks of cold places, like the risk of falling through thin ice on a lake, or the risk of not dressing appropriately. “What [cold-dwelling populations] have that we don't have is thousands of years of practice of living in cold conditions. Their behavior and their decisionmaking is much, much better than ours,” Haman continues. (For example, caribou-skin clothing made by Inuit populations is warmer than standard-issue Canadian army winter uniforms.)

That said, there is one X factor that seems neither genetic nor learned: whether you like being cold. Levy and Ocobock are both from Michigan, but Levy hates the cold. Ocobock hears conflicting perceptions from herders and Finns too. “It runs the gamut, just like you expect anywhere else,” she says. “Even native Finns that have been there their entire lives, and their families too, there are some who cannot stand the winter.”

Source