The Heat Will Kill You First

1. A CAUTIONARY TALE

WHEN THE BABYSITTER arrived to take care of Miju at around 11 a.m. on Monday, August 16, 2021, she was surprised to find the house empty. Miju was the one-year-old daughter of Jonathan Gerrish and Ellen Chung, who had recently fled the Bay Area to start a new life in California’s Sierra Nevada foothills, not far from the old Gold Rush town of Mariposa. Their modern three-bedroom house sat on ten acres of lightly forested land. It had wood floors and a big stone chimney and tall rectangular windows that looked over a rugged treeless canyon called Devil’s Gulch. From the second-floor bedroom, you could just see the top of El Capitan, the iconic granite formation in Yosemite Valley, about thirty-five miles to the east. The house was their refuge from the hustle of Silicon Valley, where Gerrish worked as a software engineer at Snapchat, the instant messaging app company.

The babysitter, who had a key, let herself in and called out their names. No response. It had been a hot weekend, but the inside of the house was cool, thanks to the air conditioner, which was going strong. Oddly, Chung and Gerrish had left behind their wallets. Even more confusing, the diaper bag that the couple always took with them was there.

The babysitter had last seen them the previous Friday when she had finished straightening up the house. That evening, Chung had happily texted her a video of Miju starting to walk. She had made no mention of plans to be away that Monday. Gerrish and Chung, who doted on Miju and seemed deliriously happy with their new life in the Sierra foothills, were not the kind of people who would disappear for a last-minute road trip to Las Vegas.

Worried, the babysitter called the construction manager who was working with Gerrish on another home he owned, and whom she knew was in frequent contact with him. The construction manager was not initially concerned because Chung and Gerrish were a “very active family,” an investigator later wrote in the police report. Nevertheless, the babysitter and the construction manager started making calls and sending texts to friends, asking if anyone had seen the couple. Steve Jeffe, a friend who lived in Mariposa, posted on Facebook: “Hi, please has anyone been in contact with Jonny Gerrish or Ellen Chung in the last two days… Please.” By 5 p.m. that day, several friends began driving around looking for the family. At 11 p.m., they called the Mariposa County sheriff.

A few hours later, a deputy sheriff found Gerrish’s truck at the Hites Cove trailhead a few miles from their home. By 4 a.m., a search and rescue team was on the scene. They headed down the trail in an ATV, flashlights cutting through the darkness. They radioed back that they had found tracks on the trail. But when they followed the tracks down to the Merced River, the tracks disappeared. By that time, the sun was rising. A helicopter was called in. More search and rescue teams arrived. One team headed down a steep trail with numerous switchbacks toward the river. They were a mile and a half down the trail when, at about 9:30 a.m., they discovered the bodies of Gerrish, Miju, and their dog, Oski. Gerrish was in a seated position, with Miju and Oski beside him.

At first, the search and rescue team saw no sign of Chung. About a half hour later, a deputy walking back up the trail from where Gerrish was found noticed “some disturbed dirt on the uphill side of the trail that appeared that something or someone had tried to go up the hill.” He spotted a shoe, then Chung’s body. Investigators would later conclude that the family had been hiking up the trail when they died. The location of Chung’s body indicated that she had abandoned the trail and was climbing straight up the mountainside—a sign, investigators believed, of the urgency of their situation and her desperate attempt to reach their truck.

But even if Chung had made it to the truck, she might not have been able to get in. During a search of the area, investigators discovered a Ford key fob on the trail about a hundred feet below Gerrish’s body. Had it accidentally fallen out of his pocket? Or did he have it in his hand and drop it and not realize it—perhaps a sign that he had been panicked or disoriented?

Rescuers found no signs of foul play. No marks on the bodies, no obvious signs of struggle. Because of the remote location and the difficulty of the terrain, the bodies could not be removed immediately. Instead, two deputies spent the night at the scene, guarding the bodies from bears and coyotes. The next morning, a California Highway Patrol helicopter airlifted the family off the trail.

Gerrish and Chung had moved to Mariposa about a year and a half earlier, just before Miju, their first child, was born. They had been living in San Francisco, where Chung taught yoga while finishing her degree in counseling psychology, and Gerrish wrangled computer code at Snapchat. But then Miju came along and the Covid-19 pandemic erupted and they needed a change. They decided they wanted to get out of the city and raise Miju closer to nature. Mariposa, which is just an hour outside the entrance to Yosemite National Park, was an ideal mix of wilderness and charm. “They fell in love with the Mariposa area,” one friend of the family said.

Gerrish was born in Grimsby, an old fishing port in the northeast of England, where his father was an elementary school teacher and his mother a receptionist in a doctor’s office. His brother Richard, who was two years younger, recalled their mother and father dragging them on long walks when they were kids. “My brother and I built dams across mountain streams and played manhunt (a more exciting version of hide and seek) in the woods,” Richard wrote in an essay about his childhood. “But usually we ended up complaining about the distance we walked and crying because I was tired, hungry and my feet were sore.” After high school, Gerrish attended Newcastle University, majoring in computer science. He worked for a few software companies in England, then got a job with Google in London. When the company offered him the chance to move to California, he jumped at it. “Jonny was somewhat awkward in his own skin growing up,” Richard told me. “When he moved to San Francisco, he found his people. He loved it there.”

Gerrish was six foot four and a bit shaggy, with a beard and longish hair that looked like a comb had never run through it. He wore Vans and supported Greenpeace and listened to techno and deep house music. Burning Man, the clothing-optional psychedelic super-rave that is held in the Nevada desert during the late summer, was his most holy holiday. His friends (of whom there were many) called him Jonny, a name that captured his boyish enthusiasm and charm. “You don’t see many men happier than Jonny,” a friend told me.

Chung grew up in Orange, California, and graduated from University of California, Berkeley in 2012. Her parents had immigrated from South Korea in the 1970s and eventually opened a successful restaurant in Orange. After graduation, she worked for a few years in marketing at a tech company but decided she wanted a change. She enrolled in classes at the California Institute of Integral Studies, a private university in San Francisco with roots in Eastern culture and philosophy, where she discovered she was drawn to—and good at—helping people talk about their problems. She wore stylish straw hats and loved the way the light filtered through the redwoods on the California coast and the long, open vistas at Zion National Park in Utah.

Gerrish and Chung both doted on Miju. You can see it in every photo of them together, the big beaming happy-father smile on Gerrish’s face, the joy and new-mother exhaustion on Chung’s. And Miju, waking up to the world, her eyes wide. She was just beginning to walk, to track birds flying across the sky, to take in all the wonders around her. Gerrish and Chung were protective of their young daughter, and careful about her surroundings. At one point, they asked a local contractor to make their daughter’s bedroom cooler because it was “too stuffy.”

The day before their hike, Gerrish mapped out the route using the AllTrails app on his Google Pixel 4. The app helps users find local trails, giving maps and elevations, as well as a place for other hikers to leave comments. Gerrish had logged sixteen hikes in 2021, most of them three or four miles long, all of them in the mountains and canyons near his house.

The hike he planned for his family was not a remote wilderness adventure. It started at a trailhead a few minutes’ drive from their house and ended at the top of Devil’s Gulch, which was practically in their front yard. The trail went along a ridge, then down fairly steeply to the south fork of the Merced River, which flowed out of Yosemite and through the canyons toward Mariposa. The trail wandered along the flat ground on the banks of the river for about three miles. From there, Gerrish plotted a right turn, which would take them on a steep 2,300-foot climb through Devil’s Gulch and back to their truck. All in all, it was an eight-mile loop.

Gerrish loved nature, but he was not a serious outdoorsman. His brother Richard, who now lives in Scotland with his wife and four children, had spent years as an Outward Bound leader, guiding teenagers on adventures into the wilderness. Richard had also rappelled into some of the deepest caves in the world (including one in Austria called Fit for Insane Worms and Geckos). Gerrish, in contrast, was more of a weekend adventurer. The construction manager who worked with him renovating one of his houses called Gerrish and Chung “city folk,” pointing out that Gerrish would go to the store and get firewood rather than chopping his own.

As it happened, Gerrish called Richard the day before the hike for some parenting advice. Gerrish mentioned to his brother that he had been out exploring the property that day and that it had been unusually hot. Gerrish also mentioned that he was planning a family hike the next day to scout out possible swimming holes on the Merced River. Richard, who was well aware of the dangers of hiking in the heat, cautioned his brother, telling him to carry plenty of water and get an early start. Gerrish agreed, and promised they would be off the trail before it got too hot.

On Sunday morning, Gerrish and Chung were up at dawn. They skipped breakfast and gathered up their gear: hiking poles, baby carrier, diapers and sippy cup for Miju, and a leash for eight-year-old Oski, a big strong dog that was part Akita. For drinking water, Chung carried an Osprey hydration pack, which held 85 ounces (about two and a half quarts) of water. Gerrish wore dark shorts, a yellow T-shirt, and tennis shoes. Chung wore hiking boots, spandex shorts, and a yellow tank top. They woke up Miju and dressed her in a short-sleeved onesie and pink shoes. Then they loaded everything into their 2020 gray Ford Raptor, an off-road version of the F-150 pickup, and headed out for the five-minute drive to the trailhead.

At about 7:30 a.m., a woman walking her dogs along Hites Cove Road, which was not much more than a narrow dirt track, saw their truck drive past and park at the trailhead. Gerrish took their first selfie of the family on the trail at 7:44 a.m. The temperature was in the midseventies, not humid, a warm but lovely morning. Under normal conditions, Gerrish might well have calculated that the eight-mile loop might take four or five hours to complete. If all went well, they would be off the trail by 1 p.m., just as the afternoon sun began to blaze.

The Sierra foothills are still marked by the California Gold Rush of the 1850s and 1860s. You see piles of old tailings along the rivers, abandoned mining shacks and sluices. In the Mariposa area, quartz veins—the geological homeland of gold—run twelve feet thick through the mountains. Hites Cove, where Gerrish and Chung were hiking, was once a mining camp with over a hundred people. Gold Rush fever is long gone, but you still occasionally run into prospectors with metal detectors wandering through the area. These days, most hikers come for the spring wildflowers, especially the spectacular fields of orange California poppies, which thrive in hot, dry, rocky soil. While hiking, you might see a bear, bobcat, or coyote (or, more likely, their scat). At the bottom of the canyon, big rainbow trout rest in the deep pools and eddies of the Merced.

In recent years, climate-driven heat and drought have turned the area into a tinderbox. The area was badly burned in the 2018 Ferguson Fire, which consumed almost a hundred thousand acres and forced Yosemite National Park to close for the first time in decades. Two firefighters died. The fire, caused by a spark from the catalytic converter of a vehicle, turned the dry grass and bark-beetle-infested trees into an inferno. In the three years since the fires, wildflowers had returned and a few saplings were rising out of the rocky soil, but most of the trees were charred sticks poking up at the sky and offering little shade for hikers or wildlife on a hot afternoon.

Even before the 2018 fire, the trail that Gerrish had selected for the hike was a risky choice. The steep ascent out of the canyon was along a southeastern-facing slope, which meant it was exposed to the full brutality of the sun. “It’s a horrible trail,” one local wrote on social media. “With the poison oak, rattlesnakes, and potential for broken ankles, it just isn’t worth it.” Another local, who hiked the trail on a mild spring day, praised the wildflowers blooming on the mountainside, but noted that it was dangerously exposed: “I wouldn’t want to do this [hike] on a hot day.”

For Gerrish and his family, the hike started off easy. The first two miles were mostly downhill. The morning sun would have felt good, the light slanting across the mountains. It only took them a little over an hour to get to the river, where they stopped to take another family selfie at 9:05 a.m. They rambled along the river for the next hour and a half, where they may well have stopped for a drink from their hydration pack and even doused their hands and faces in the cool river water.

At 10:29 a.m., they took a final family selfie along the river, then began the climb. It had been three hours since they left the truck. The temperature had risen to nearly 100 degrees and it was getting hotter with each passing minute. The recently burned trees along the steep trail were black and leafless. The tall grass was sunburned to a golden, crispy brown, like straw.

If there is one idea in this book that might save your life, it is this: The human body, like all living things, is a heat machine. Just being alive generates heat. But if your body gets too hot too fast—it doesn’t matter if that heat comes from the outside on a hot day or the inside from a raging fever—you are in big trouble.

Every organism manages heat in a different way (more about that in an upcoming chapter). We humans work hard to maintain an internal temperature of about 98 degrees, no matter what the temperature is outside. If it is cold outside, we pull blood into our vital organs to keep them warm. If it is hot outside, we push blood toward our skin so it can be cooled by sweat. That’s why dry heat is often less dangerous than wet heat—the more humid the air is, the more difficult it is for our sweat to evaporate and dissipate heat. And like all living things, our bodies have thermal limits. Those limits vary depending on age, general health, and a number of other factors. But there is general consensus among researchers that a wet bulb temperature of 95 degrees—which basically means both outdoor air temperature and humidity levels are high (see glossary for a more precise definition of wet globe temperature)—is the upper end of human adaptability to humid heat. Beyond that, our body generates heat faster than it can dissipate it.

And that’s where trouble begins. Hyperthermia, or abnormally high body temperature, causes a range of physiological responses that might start with dizziness and heat cramps and lead all the way to heatstroke—a condition that can be, and often is, fatal.

Broadly speaking, there are two kinds of heatstroke: classic and exertional. Classic heatstroke hits the very young, the elderly, the overweight, and people suffering from chronic conditions like diabetes, hypertension, and cardiovascular disease. Alcohol and certain medications (diuretics, tricyclic antidepressants, antipsychotics) can increase one’s susceptibility. Classic heatstroke is often what happens to babies who are left in cars, or older people trapped in upper-story apartments with no air-conditioning on summer days.

Exertional heatstroke, on the other hand, often hits the young and fit. Exercise drastically accelerates temperature rise. Anytime you flex a muscle, it generates heat. In fact, when you move a muscle, only about 20 percent of the energy you expend actually goes to muscle contraction; the other 80 percent is released as heat. That is why marathon runners, cyclists, and other athletes sometimes push into what is called exercise-induced hyperthermia, where internal body temperatures typically hit 100 to 104 degrees. Usually, there’s no lasting damage. But as your body temperature climbs higher, it can trigger a cascading disaster of events as your metabolism, like a runaway nuclear reactor, races so fast that your body can’t cool itself down.

It doesn’t take long. And being young or in great shape won’t save you. In fact, being young and strong allows you to fight off the warning signs of heat exhaustion until it is too late. A few years ago, eighteen-year-old Kelly Watt, a track star and aspiring journalist in Virginia, parked his car on a hilly road where he often trained and went out for a fifty-minute run on a hot summer afternoon. A few hours later, Watt’s father found his body in some bushes not far from his car. Handprints on the car suggested Watt had made it back to the car after his run, but had become disoriented from heat, couldn’t open the door, and then wandered off into the bushes, where he collapsed and died.¹ In 2021, Philip Kreycik, a thirty-seven-year-old ultramarathoner and father of two young children, drove up into the hills near Pleasanton, California, on a July morning to go for a run. He parked his Prius on a dirt road, left his water bottle in the center console, and headed out. By noon, the temperature was 105 degrees. After Kreycik’s wife reported him missing a few hours later, hundreds of people rallied for a search and rescue effort that the Alameda County Sheriff’s Office called one of the largest ever on the West Coast. Twelve thousand people joined a Facebook group to help with the search and a fundraiser for Kreycik’s family has raised more than $150,000. His body was finally found in a remote area twenty-four days later. Cause of death: hyperthermia.

Both Watt and Kreycik were excellent athletes. Both knew it was going to be hot during their run. Neither carried water. But would it have mattered? In 2016, thirty-four-year-old Michael Popov, one of the world’s top ultrarunners, who routinely ran hundreds of miles in the rugged Sierra Nevada mountains, set out on a six-mile jog in Death Valley on a hot August day. He was carrying four bottles of water and ice. Two hours later, he was found collapsed on the side of a road. He died later that day.

There is a lot of confusion about the relationship between water and heat exhaustion and heatstroke. Water is necessary to keep sweat flowing. If you get dehydrated, you can’t sweat. And if you can’t sweat, you can’t cool off. But drinking water does not in itself cool off inner-core body temperatures. Put another way, dehydration can exacerbate heat exhaustion and heatstroke, but you can still die of heatstroke and be well hydrated. In one study in Montana, a wildfire fighter working in extreme heat for a seven-hour period who continuously drank a huge amount of water—more than twice as much as the firefighters around him—still had a core body temperature of 105 degrees, which is well into heatstroke-land.

Sam Cheuvront, a heat and hydration expert who spent more than twenty years at the US Army Research Institute of Environmental Medicine in Massachusetts, put it to me this way: “Both heat exhaustion and heatstroke can occur in the absence of dehydration. We can speculate that proper hydration can, however, delay heat exhaustion because dehydration exacerbates heat exhaustion. But proper hydration cannot prevent heatstroke.”

Drinking water when it’s hot is certainly important. A common recommendation is about one half quart (16 ounces) of water per person per hour of moderate activity in moderate temperatures. But in extreme conditions, even that isn’t enough. A well-hydrated human can sweat up to three quarts per hour, but no matter how much water you drink, your body can only replace about two quarts of water per hour—so if you are in a hot place for a long time, dehydration is a concern.

Even at a sweating rate of two quarts per hour, which is what a firefighter might do while working in a hot environment and wearing protective clothing, it takes an hour to exceed 2 percent dehydration, which is really where dehydration starts contributing to heart strain, mostly due to reduced blood volume. It also exacerbates the competition for blood flow between your muscle, your skin, your brain, and your organs.

The only effective treatment for heatstroke is to get a person’s core body temperature down, fast. A cold shower or bath, or tubs (or, as I mentioned in the prologue, body bags) of ice, is one way to do this. Another is to quickly cool places on the body where, because of the structure of our veins, a lot of blood circulates close to the surface: the bottoms of the feet, the palms of the hands, the upper part of the face. Taking Tylenol or aspirin doesn’t help. In fact, both can interfere with kidney function, and make it harder for your body to deal with rising temperatures. Only after core body temperature is lowered do the damages from heatstroke stop and hope of repair and recovery begin.

About an hour and a half after Gerrish, Chung, Miju, and Oski left the banks of the Merced, they were in trouble. They had climbed two miles up the trail, but still had another mile and a half of steep switchbacks to go before they got back to their truck.

At 11:56 a.m., Gerrish pulled his phone out of his pocket and attempted to send a text: “[name redacted] can you help us. On savage lundy trail heading back to Hites cove trail. No water or ver [over] heating with baby.” Records would later show that the air temperature at that time was 107 degrees. But on the trail, with full sun and no shade and rocks absorbing and amplifying the heat, the actual temperature that Gerrish and his family were experiencing was certainly much higher.

Gerrish and Chung surely had a moment when they stopped to consider whether it was better to stop climbing and turn around and seek refuge by the river. There wasn’t a lot of shade down there either, but there was some. And they could have found some relief by wading out into the cool water. But at the same time, if they retreated to the river, they would still have to hike out eventually, and the afternoon was just going to get hotter. Waiting until the temperature dropped and the sun lost its edge meant waiting until late afternoon or early evening. While that might have been the safe decision, it had its own risks: they were out of water, and signs along the river warned hikers not to drink from it due to toxic algae. The actual risk of getting sick from toxic algae was extremely low compared to the danger of heatstroke, but Gerrish and Chung may not have known that.

There was also the issue of food for Miju. They had not brought enough diapers or baby formula with them for an entire day. Perhaps, out of love for Miju, they decided that it was better to suffer in the heat themselves, push through the climb back to the truck, blast the AC, and feel the relief of having escaped a heat-driven nightmare.

The typo in Gerrish’s text message (“or ver”) may be nothing more than a sign that he was in a hurry to get the message out. But it also could be a sign that the heat was already causing him some cognitive difficulty, which is common during extreme heat exhaustion. If that was the case, it would make clear-headed decision-making about whether to keep climbing in the heat or seek refuge near the river all the more difficult.

Whatever he was thinking at that moment, Gerrish was clearly aware that their situation was growing more dire. Over the next twenty-seven minutes, he attempted five phone calls, but because of the lack of service in the area, none went through. He did not call 911. If he had, there’s a chance he might have gotten a response, given that in remote areas, 911 calls are routed differently to cell towers and so are sometimes picked up in areas where other calls are not. Gerrish may not have been aware of this, or he simply might have been too disoriented to consider it. In any case, he made one last attempt to call someone for help at 12:36 p.m. By then, two hours had passed since they left the shaded banks of the Merced.