The New Thermal Divide - Part 2: From Savanna to City-Humanity's Failed Adaptation

The New Thermal Divide - Part 2: From Savanna to City—Humanity’s Failed Adaptation

When extreme heat arrives, it operates as an invisible force that works upon the body in ways people cannot anticipate or control. Humanity has adapted to survive extreme conditions over millennia. Evolution equipped humans with sophisticated cooling mechanisms honed for life in our planetary Goldilocks Zone. This zone is the specific temperature range where life thrives. However, the modern world is changing rapidly, pushing global systems outside their functional parameters. As fossil fuel consumption unleashes heat, humanity finds its biological limits challenged.

Heat is no longer an incremental change but an active, destructive force. Understanding how heat shapes us, and how we fail to adapt, is crucial for survival in this rapidly warming world. The greatest consequence of this shift is the widening thermal divide. This invisible but undeniable line separates the cool from the suffering, and the lucky from the damned.

Evolutionary Triumph: Built for the Heat

The history of human success on Earth is fundamentally linked to heat management. Early human ancestors, like Lucy, began standing upright 3.2 million years ago in Ethiopia’s Awash River valley. Bipedalism allowed Lucy to dissipate body heat more easily. It also lifted her body off the ground, which is always significantly warmer than the air just a few feet above it.

As our ancestors moved out of the trees onto the open savanna, they faced constant exposure to heat. The evolution of the eccrine sweat gland became a key innovation in human survival. This specialized gland is essentially an internal sprinkler system, squirting water onto the skin to cool the body through evaporation. Losing most body hair, coupled with millions of eccrine glands, created a system that allows humans to cool down while in motion.

This superior heat management system turned early humans into endurance athletes and excellent hot-weather hunters. Unlike panting predators like hyenas or lions, humans could sweat as they ran. They could chase prey, like the fast kudu antelope, for hours until the animal collapsed from heat exhaustion. By managing heat, humans expanded their hunting range and traveled far from water sources. However, these brilliant adaptations were optimized for the climate existing over the last ten thousand years. The modern world is changing far too fast for evolutionary processes to keep pace.

A Cautionary Tale: Heatstroke on the Trail

The failure to recognize and respect extreme heat can turn a simple outdoor adventure into a fatal event. The tragedy of the Gerrish-Chung family in California’s Sierra Nevada foothills illustrates how quickly modern heat can overwhelm biological defenses. Jonathan Gerrish, Ellen Chung, their one-year-old daughter Miju, and their dog Oski died from hyperthermia and probable dehydration during a hike in August 2021.

The family was hiking on a steep, exposed trail near Yosemite. Gerrish and Chung were happy, active “city folk” who were protective of their daughter. They began their hike early, around 7:30 a.m., when the temperature was in the mid-seventies. They planned to finish the 8-mile loop by 1 p.m., before the sun blazed strongest.

By 10:29 a.m., they began their ascent back toward the truck. The temperature had risen to nearly 100 degrees Fahrenheit. By 11:56 a.m., records show the air temperature reached 107 degrees. Gerrish attempted to send a text: “[name redacted] can you help us. No water or ver [over] heating with baby”. The family’s single 85-ounce hydration pack was empty.

The body operates as a heat machine and works hard to maintain an internal temperature of about 98 degrees. When external heat combines with the heat generated by muscle activity, the core temperature rises fast. The body responds by dilating blood vessels to push overheated blood toward the skin for cooling via sweat evaporation. When humidity is high, or when the body is dehydrated, sweat cannot evaporate effectively, blocking the cooling mechanism.

The family was caught in a lethal feedback loop. Their hearts pumped madly to cool the body, which increased their metabolism, which generated even more heat. As the blood was shunted away from vital organs like the liver, kidneys, and brain, they became starved of oxygen. Hyperthermia, or abnormally high body temperature, quickly causes dizziness and light-headedness. The typo in Gerrish’s final text suggests the heat was already causing cognitive difficulty.

The dog, Oski, was likely in trouble first, as dogs cannot sweat and rely only on panting to release heat. Miju, in a carrier on her father’s back, was trapping heat like an extra layer of clothing. Infants cannot fully release heat because their sweat glands are underdeveloped and their smaller blood volume means less reserve for cooling internal organs. Even young, fit adults cannot survive heatstroke once the core temperature reaches 107 degrees Fahrenheit. At this stage, cellular proteins essential for life begin to unfold and cell membranes melt. The body unravels, leading to organ collapse, clotting cascades, and massive internal hemorrhaging.

In a hot environment, even being well-hydrated cannot prevent exertional heatstroke. Drinking water keeps sweat flowing, but it does not cool the inner-core body temperature itself. The Gerrish family’s death was an official result of hyperthermia and probable dehydration due to environmental exposure. It was a tragedy shaped by a failure to reckon with the risks of living in a rapidly warming world.

The Urban Heat Trap: Heat Islands

In many modern cities, the threat of heat is exacerbated by the built environment itself. Urban areas become “Heat Islands” where concrete and buildings absorb and radiate heat back. Asphalt can shimmer, and metal bus stops become convection ovens. City heat feels more cruel and intimate than heat found in nature.

This heat disproportionately impacts the vulnerable and exposes deep fissures of inequity and injustice. A measurement in Portland, Oregon, during the 2021 heat wave dramatically illustrated this thermal divide. In Lents, one of Portland’s poorest neighborhoods with few trees and plentiful concrete, the air temperature hit 124 degrees. Meanwhile, in Willamette Heights, a wealthy, tree-lined suburb, the temperature measured 99 degrees. Wealth in a heat wave can afford twenty-five degrees of coolness.

Poverty equates directly to vulnerability. Those with money can turn up air-conditioning, install backup generators for blackouts, and move to cooler locations. Those who are poor swelter in uninsulated apartments, cannot afford to run old air-conditioning units, or are afraid to lose their jobs by moving away. As one former mayor testified, “Some of us are sitting on aircraft carriers while others are just bobbing along on a floatie” in the storm.

Global Vulnerability: Concrete and Collapse

In cities worldwide, the lack of financial resources turns the heat into a predatory event that “culls out the most vulnerable people”. In Phoenix, Arizona, a major blackout triggered by a wildfire, substation failure, or cyberattack could lead to cascading failures of urban infrastructure. Modern, tightly sealed, efficient buildings become dangerous heat traps when the power fails. Traffic signals would fail, highways would gridlock, and hospitals would overflow. A blackout lasting more than thirty-six hours in Phoenix could easily result in “Katrina-like numbers” of deaths—thousands—requiring the National Guard to maintain order.

The death of Stephanie Pullman, 72, in Sun City West, Arizona, exemplifies this vulnerability. Pullman lived on a fixed income of less than $1,000 a month. Her utility company, Arizona Public Service (APS), cut off her power over an unpaid bill of just $51.84. She died in her bed on a 107-degree day from heat exposure. APS data showed the company cut off power over 110,000 times in 2018, including more than 39,000 cutoffs during the scorching months of May through September. Her death forced Arizona regulators to ban summer power shutoffs.

In the Global South, urbanization has created massive heat challenges. Chennai, India, is one of the world’s wettest major cities, but poor water storage led to a severe water crisis during a 2019 heat wave. Anjalai, a 39-year-old resident, lives in a small hut and relies on basic cooling strategies, such as wetting down the thatched roof and dirt base of her home. Her daily routine, including commuting on a bike to clean houses, is defined and driven by heat. The wealthy clients she works for have air-conditioning, providing her temporary relief, while her husband, who has a heart condition, must work outdoors in construction.

In Perumbakkam, Chennai, families relocated after flooding now live in concrete high-rises that lack air-conditioning, trees, and greenery. Mercy Muthu, 41, noted that her children get heat rashes if they play outside. The lack of reliable running water forces her to collect water from a courtyard well. A particularly spooky risk for the elderly involves elevators that stop during power outages, trapping people for hours with no cooling. Muthu bluntly noted, “No one wants to get cooked in an elevator”.

The Sweat Economy: Labor and Fatal Inequity

Heat is a daily workplace hazard for the 15 million people in the U.S. who work outside, plus those in poorly ventilated warehouses and factories. The world’s “sweat economy” is supported by essential workers—the people who build houses, fix roads, and grow food—who have no air-conditioned respite.

Sebastian Perez, a 38-year-old undocumented migrant from Guatemala, died working in an Oregon field during the 2021 Pacific Northwest heat wave. Perez was earning money to build his family a house. He worked alone, dragging 30-pound irrigation pipes as the temperature hit 106 degrees Fahrenheit. He kept working, driven by his $12,000 debt to the coyote who guided him across the border. After he collapsed in the field among the boxwood, his coworkers rushed him into the thin shade of a fir tree. By the time emergency services arrived, delayed because workers struggled to communicate their location, Perez had stopped breathing.

Historically, heat was weaponized to justify horrific working conditions. During the era of slavery, doctors and racial theorists promoted the idea that African Americans were better suited to manual labor in tropical climates than white people. This scientific racism argued that the African body was an “animal machine” constitutionally protected from the sun by a thick skull and woolly hair. They used this falsehood to ignore the lethal consequences of forcing slaves to work in fields where the sun burned their heads and backs. Even after slavery, these stereotypes persisted, arguing that Mexican workers were “hot-weather plants” contented to work in the desert.

Today, the systemic exploitation continues, particularly for farmworkers who are excluded from national laws regarding overtime pay or collective bargaining. At the time of Perez’s death, only Washington and California had rules for outdoor workers. Farmworker advocates noted that growers resisted heat rules, claiming labor cost changes would make them uncompetitive. Farm owners frequently use independent labor contractors to distance themselves from responsibility for worker treatment. After Perez’s death, Oregon finally announced emergency rules requiring shade and water when temperatures rise above 80 degrees. The death of Sebastian Perez, a victim of the hard math of the climate crisis, was entirely preventable with only shade, cool water, and rest.

Limits of Adaptation and Climate Flight

The idea that eight billion people can thrive on a hotter planet merely by increasing air-conditioning use or seeking shade is a profound misunderstanding of the future. Adaptation has its limits, even for the wealthy. Technology offers some hope: new high-tech cooling fabrics and planting shade trees are underway in cities like Paris and Los Angeles. Plant geneticists are also developing new crops, like Kernza, that tolerate higher temperatures and drought.

But adaptation is expensive and slow. Rebuilding infrastructure to handle 120-degree heat—installing rail lines that do not melt, or retrofitting houses that are designed like ovens—would take decades and immense resources. In many places, heat is rising faster than our ability to adapt.

When adaptation fails, people flee. As the world heats up, species, animals, and humans are on the move, seeking a more habitable climate niche. During the 1930s Dust Bowl, farmers largely chose to migrate to California rather than adapt their farming techniques. Today, a vast remapping of the world’s populations is likely as up to three billion people are projected to be left outside the stable climate conditions that gave rise to civilization.

The decision to migrate is complex, often driven by personal reasons like jobs or family. However, a lack of food and water, both aggravated by extreme heat, is high on the list of driving factors. Migration itself is becoming deadly. On the US/Mexican border, the Devil’s Highway in the Sonoran Desert acts as a “migration barrier,” killing anything that attempts to cross its merciless heat. Alarmingly, the US Border Patrol has been accused of using heat as a weapon, funneling migrants through the hottest and most dangerous regions of the border to deter crossings.

Despite the clear dangers, people continue to move toward high-risk areas due to cheaper living costs and job markets. Williamson County, Texas, an area with high heat risk, saw the highest net migration rate among fifty analyzed US counties between 2016 and 2020. Living in these booming, hot cities—like Austin, Texas—becomes tenuous. Comfort depends entirely on the fragile technology of air-conditioning and the complex economic logic maintaining the power grid.

Beyond the Goldilocks Zone

The extreme heat we are experiencing is driving entropy and disorder. Heat is the engine of planetary chaos, amplifying wildfires, drought, and sea-level rise. For all living things, rising temperature above their Goldilocks Zone means death.

Humanity stands at a crossroads, equipped with biological systems optimized for a world that no longer exists. The first and most striking consequence of our trip beyond the Goldilocks Zone will be the further widening of the thermal divide. Until we address the root cause—the fossil fuels that unleash this deliberate, premeditated heat—millions will continue to suffer and die on the wrong side of that invisible line. The immediate choice is stark: We roast, we flee, or we act.

Coming up next: Part 3 details the global collapse of critical systems, including ecosystems, food supplies, and the rise of vector-borne diseases, all amplified by planetary heat.