Two months later, the deaths were found to be the work of a much more quotidian source: heat. The young parents, Jonathan Gerrish and Ellen Chung, were healthy and fit — but the day the family died, temperatures near the ground on the trail were 109 degrees. They had carried water but were in the middle of a steep climb back to their truck. There was little shade.

If there is a lesson from their deaths, journalist Jeff Goodell writes in his new book, “The Heat Will Kill You First: Life and Death on a Scorched Planet,” it is that extreme heat can kill anyone. You just have to be in the wrong place at the wrong time.

For years, journalists and writers have struggled to describe the toll of global-warming-induced extreme heat. The changes in our climate so far — on average, our planet has warmed about 1.1 degrees Celsius (or 2 degrees Fahrenheit) since humans began using fossil fuels widely for energy — seem small, the difference between relative comfort and ever so mild discomfort. But the true toll of climate change is not the average temperature but the extremes: 125-degree days in Phoenix. A sweeping heat wave in Paris. The week in Jacobabad, Pakistan — considered one of the hottest cities in the world — where temperatures were over 126 degrees, with humidity, every day.

In 14 whirlwind chapters, Goodell, a longtime climate journalist and contributing editor for Rolling Stone, earns his book’s grim title. The chapters travel from the Arctic Circle to the tropics and back again, tracing the effects of heat on melting ice and suffering corals, but also on enthused mosquitoes, whose ranges are stretching wider as temperatures warm.

Goodell is at his most effective when he describes how heat affects the human body. Most of us don’t think much about how sweat — our “internal sprinkler system,” as Goodell calls it — allows us to survive in temperatures and situations that would otherwise lead to a swift death. Humans evolved to sweat to gain a hunting advantage; releasing up to three gallons of water a day through pores enabled early humans to chase prey for hours and hours, until the antelope or deer collapsed because of heat exhaustion.

How money corrupts our politics and destroys our climate

Even with this critical tool, however, humans are fairly weak creatures, at least where temperature is concerned. Without protective clothing and heating, we die quickly in the cold; without air conditioning and water, we die just as quickly in the heat. When the body gets too hot, blood rushes to the skin in an attempt to cool off, abandoning the internal organs. At 102 or 103 degrees Fahrenheit, Goodell writes, one might feel wobbly or pass out (the brain’s attempt to get more blood to fulfill its basic functions). At 106 degrees, the body convulses into seizures.

Above 107 degrees, things start to break down at the cellular level. “As the heat rises, the proteins unfold and the bonds that keep the structures together break,” Goodell writes. “At the most fundamental level, your body unravels. … Your insides melt and disintegrate — you are hemorrhaging everywhere.”

Some are more vulnerable than others. To the Gerrish-Chung family, Goodell adds the story of Sebastian Perez, a recent immigrant from Guatemala who died working at a nursery in Oregon’s Willamette Valley. “He was lying in the field, among the variegated boxwood, barely breathing,” Goodell writes. Perez is an emblem of thousands of untold stories — of farmworkers, construction workers and UPS drivers, who toil all day in extreme heat and risk their lives to do so.

But they aren’t the only ones at risk. We have, as a society, tried to insulate ourselves from heat. The greatest example of this is air conditioning, which has enabled a vast population of Americans to settle in the Sun Belt, comfortably living in areas that stretch the boundaries of habitable. But in certain moments, that triumph shows itself to be razor-thin. A/C units already account for about one-fifth of all energy used from buildings — in a heat wave, with millions of A/Cs churning at once, the electricity grid can be in danger of collapse. What it leaves behind is a society that has forgotten the architectural methods of dealing with heat: airflow, white roofs, thick walls. “Air-conditioning is not just a technology of personal comfort; it is also a technology of forgetting,” Goodell writes.

An environmental writer asks: What world will my daughter inherit?

Then there are the places that never adapted to heat in the first place. In 2021, more than 600 people died in a heat wave in normally cool British Columbia, as temperatures spiked 36 degrees above normal. Goodell brings up an earlier example — the 2003 heat wave that killed 15,000 people in Paris alone. Historically mild temperatures created a city with no “climate culture,” as Goodell calls it: Parisians aren’t used to dealing with any sort of extremes. They have built beautiful zinc-roofed apartments that are ill-suited to rising temperatures. And affection for the city’s historical architecture makes those buildings difficult to transform.

Yet people in places like Paris, one of Goodell’s sources notes, have only three choices: “Roast, flee, or act.” In one grisly anecdote, a young woman returned home to her apartment to find it filled with blood. Her elderly upstairs neighbor had died during the heat wave; fluids from her decomposing body had leaked through the floor into the downstairs unit.

Shortly after I finished “The Heat Will Kill You First,” I went for a two-hour drive through central Oregon in a car with a broken air conditioner. It wasn’t brutally hot, only in the mid-90s, but the car’s mini-greenhouse effect trapped heat as I drove down Interstate 5. I opened a window and hung my arm out of it, feeling the blood in that arm cool and circulate through the rest of my body. I was reminded of Goodell’s graphic descriptions of heat stroke and heat exhaustion, and also of a recent conversation with emergency room doctors in Phoenix, who had explained how quickly someone can get heat stroke in a car with no climate control. The barrier between safety and heat felt thinner than ever.

The scariest thing about the heat-infused future, Goodell notes, is that we don’t treat it with the respect and concern it deserves. When the heat rises, plants, animals and people die. But the coronavirus pandemic showed just how much death and destruction a society can accept. Suffering and death “will become part of what it means to live in the twenty-first century,” Goodell writes. “Something we accept.”

The Heat Will Kill You First

Life and Death on a Scorched Planet

Little, Brown. 383 pp. $29