Douglas Kenrick, rangy and grizzled, squints through the shimmering heat of a late-summer afternoon in the Sonora desert. “You live here long enough,” he says, crossing to the south side of an empty street for the five-minute walk across the campus of Arizona State University, “and you become like a desert animal, searching out shade.” Having grown up on Long Island, and coming from the frequently snowbound campus of Montana State University, he relished the heat when he moved to Phoenix in 1980, but by the end of his first full summer, it had become oppressive. “I came from New York with the attitude that it can’t ever be too hot for me,” says Kenrick, “but I was wrong.” It seems likely that most people who move to Phoenix, where the temperature reached 118 degrees one day last June, make the same discovery, but as an evolutionary psychologist, Kenrick wanted to do more than complain about the climate. So he did an experiment.

His method had the elegance of all great science: He recruited a volunteer to stop her car at a green light and he counted the seconds until the driver behind honked the horn. He did this once a week from April to August, on days when the high temperature ranged from 84 degrees to 108, and he found that the thermometer accurately predicted how soon, and how many times, thwarted drivers would protest before the light changed. “When the weather was comfortably cool, the typical driver just politely tapped on the horn for a second,” Kenrick wrote. “When it got up near 100, though, they started blaring their horns, yelling out the window, and making hand signals they probably did not learn in driver’s education.”

The link between heat and anger—people are “fired up” or “steamed up,” or they “keep their cool”—is so deeply embedded in folk wisdom that it has gone mostly unquestioned. But it is increasingly a subject for psychologists and other social scientists concerned about the implications of a world in which 108 degrees may no longer be exceptional. Under one scenario studied by the Centers for Disease Control and Prevention, by the end of the century, today’s North Carolina summers would become the norm for New Hampshire, while Louisiana’s climate would migrate up to Illinois. In Phoenix itself, “temperatures could regularly hit the 130s...by the second half of this century,” University of Arizona climatologist Jonathan Overpeck has predicted.

The various environmental effects of greenhouse gases are potentially devastating, as we have often heard. The latest Intergovernmental Panel on Climate Change report, made public in March, underscored the danger of widespread hunger, even starvation, resulting from crop failures. Other health threats have been enumerated by Robert Repetto, a United Nations Foundation economist, who says climate change will intensify smog, leading to “increased outbreaks of asthma and allergies,” and “exacerbate vector-borne diseases such as hantavirus, West Nile virus, Lyme disease and dengue fever.” Repetto also worries about the “extreme weather events” that some researchers say climate change will engender. “Biological systems and engineering systems are all designed for a range of climatic conditions,” he says. “Within those limits, we’re OK, ...but outside those limits, the damage increases rapidly and becomes catastrophic, and we’re going outside those limits.” Heat waves themselves pose a health risk, especially for young children and the elderly—and world-class athletes. Temperatures at the Australian Open in January reached 104 degrees for four consecutive days, a condition that one tennis player called “inhumane” after competitors collapsed on the court.

The weather is always changing, to be sure, and any given event might have happened independent of global warming, but some trends are clear. Melting glaciers and disappearing sea ice, combined with the thermal expansion of the oceans, will almost certainly lead to increased coastal flooding of low-lying areas around the world, including parts of the United States. Like the iconic polar bear stranded on a shrinking ice floe, we are all facing an uncertain and perilous ecological future.

There may be hordes of climate refugees, fleeing homes on islands and coasts made uninhabitable by climate change—anywhere from 25 million to 1 billion people by 2050, according to the International Organization for Migration. Even people who don’t have to move will experience a bewildering sense of dislocation as the environment changes around them—as Northern winters start to be measured in weeks rather than months. Glenn Albrecht, an Australian philosopher, coined the term “solastalgia” for this emotion, a kind of homesickness you can experience without leaving home.

“We will see the emergence of novel climates, environments we’ve not seen before in human times, and the extinction of others, around the Arctic and in high Alpine regions,” says Laurence C. Smith, a professor of geography at UCLA and author of The World in 2050. Smith says cities, industry and agriculture may benefit in places such as Canada and Scandinavia, though at some cost in psychological and cultural disruption. “Very bitterly cold winters will be less common in some places,” he says, “but instead of a nice blanket of white snow, they will have slush.” And people who move north for the weather, or for jobs that may open up as the Arctic melts, will discover that climate change doesn’t make the winter nights any shorter.

But climate is about more than ecology: It’s also a force in human behavior, a fact often overlooked in global-warming scenarios. And new research suggests that a hotter world may, for one thing, be more dangerous, and not just because of road rage. Craig A. Anderson, of Iowa State University, pioneered research on climate and aggression, and derived the formula that each additional degree of warming increases the rate of violent crime (homicides and assaults) by 4.19 cases per 100,000 people. Solomon Hsiang, a public policy specialist at UC Berkeley, has found that climate change historically leads to social disruption, up to and including war. Property crime, personal violence, domestic violence, police violence—everything you want less of, climate change seems to bring more of, either directly by making individuals more violence-prone, or indirectly by promoting conflict related to diminishing resources or deteriorating economic conditions.

For reasons Hsiang is still studying, hotter temperatures depress economic activity. In a study of 28 Caribbean economies, he found that “short-term increases in surface temperature are associated with large reductions in economic output. I was stunned by how large the effect was. I don’t want to be alarmist, but I think the evidence is extremely concerning, and it hasn’t been seriously considered by policy makers.”

Violence, disease, social chaos—these are irresistible themes for science fiction, at least since the classic “Twilight Zone” episode “The Midnight Sun” in 1961, in which a cosmic accident sends the Earth out of its orbit and spiraling toward the Sun. Since then, of course, we’ve come to realize that humanity has supplied the mechanism for calamity all by itself, through greenhouse gas emissions. Global warming does pose some special challenges for fiction, as the editor Gordon Van Gelder points out: “It’s hard to write a story where the characters are grappling with climate change. You can’t just pull out a laser gun and shoot at it.” Still, Van Gelder managed to recruit 16 contributors for his 2011 collection of stories, Welcome to the Greenhouse. Families driven out of their homes struggle to reach the Arctic, where temperatures are bearable; monster tornadoes level whole towns; the military battles six-inch-long honeybees. And, in a story in Van Gelder’s magazine Fantasy & Science Fiction, tribunals in the future pass judgment on “tippers,” the wastrels whose giant carbon footprints led the world over the edge to disaster.

Science fiction is one way to get a feel for what daily life might be like in a hotter world. Another way is to go to Phoenix during a late-September heat wave when temperatures hover around 105, where the first thing you learn about the future is that it will apparently be lived indoors.

***

It is, as they say, a dry heat. On the East Coast, summer heat envelops you, like a hot, wet blanket, but step outside in Phoenix and it swats you, like a rolled-up newspaper. “When I worked in Atlanta it was hot and humid, but there was never a day I couldn’t go outside and hit a tennis ball,” says Royal Norman, a meteorologist for station KTVK. “But there are days here where I’m never outside except to get in and out of my car.” An advertisement for air conditioners in Phoenix uses the slogan, “Some of the best moments in life happen indoors,” which could well be true, unless your passion is, say, golf or gardening. Newcomers have to learn the hard way what happens to a soda can left inside a car parked in the sun, or to dogs whose owners take them out on sidewalks without protective booties.

Outside my hotel, in the heart of downtown, the streets illustrate why the noun “desert” is cognate with “deserted.” At mid-morning on an ordinary weekday, I can walk around the block twice without encountering another human being on foot. In late afternoon, I meet a radio reporter named Jude Joffe-Block. She arrives a few minutes late, apologetically; she says she was once two minutes late to meet a friend at a bar, which happened to be closed that day; he was gone, unable to bear 120 seconds on the sidewalk. Phoenicians defend their city with variations of the claim that “everyone has air conditioning,” but during a heat wave last June, whose average high temperature was 107, Joffe-Block interviewed people who were doing without it, usually because they couldn’t afford monthly electric bills of $400 or more. Sharon Harlan, a sociologist at Arizona State University, who has been studying how communities are affected by extreme heat, says that in some poor neighborhoods a third of the population says the high cost of electricity keeps them from using their air conditioning. Joffe-Block herself was living in a rented apartment with a device called a “swamp cooler,” a machine that blows air over a water-saturated pad, lowering the temperature by evaporation. On a recent 105-degree day, the swamp cooler chilled Joffe-Block’s apartment all the way down to 95. The machines are common in the small stucco and cinder-block houses that line the streets of south-central Phoenix, a low-income neighborhood a 15-minute walk from the skyscrapers of downtown, if anyone was around to walk it.

And by the iron law of real estate values, people too poor for air conditioning tend to live in the hottest parts of town, flat and shadeless under the relentless desert sun, far from the soothing balm of golf courses and parks. Wealthy neighborhoods receive the “microclimate ecosystem services” of trees and shrubs. Over the course of a summer, Harlan measured temperatures in the yards of houses in various neighborhoods and found differences up to 14 degrees. Plants provide shade, intercept sunlight and cool the surrounding environment as water evaporates from their leaves, whereas the built environment absorbs energy from the sun and radiates it back as heat. Driving by a golf course at night during summer, with the windows down, the change in air temperature can be “startling,” says Chris Martin, a professor of horticulture at Arizona State.

Unfortunately, the cooling effects of plants come at a cost, namely water, which is becoming increasingly a precious commodity in the Southwest as the climate warms and population increases. With the advent of air conditioning and high-insulation building materials, people felt less need to surround their houses with shade trees. Improvements in artificial turf have made it an acceptable alternative to grass in small patches, even in wealthy neighborhoods. Such a yard “can be 15 or 20 degrees warmer at night than the same yard if it were irrigated,” Martin says. “You can see very nice homes in a yard without a single living thing in it. It’s one hot place, but most people are inside so they don’t care.”

***

Phoenix, like most big cities, is what meteorologists call a “heat island,” hotter than the surrounding countryside, or than the land would be without the burden of civilization: of asphalt parking lots and tinted-glass skyscrapers, of the air conditioners, automobile engines, appliances and light bulbs of 1.5 million people. (Or, for that matter, the people themselves: The population of the Phoenix metropolitan area, over four million, generates as much energy in the form of body heat as a medium-size power plant.) The heat island effect creates a phenomenon that meteorologists and ordinary citizens find even more disturbing than the occasional 115-degree afternoon: the trend toward higher nighttime temperatures.

Citing National Weather Service data, Norman, the meteorologist, said the last record low in Phoenix was in December 1990. “Since then we have set 144 record [daytime] highs and 230 record-high [nighttime] lows. Back in the 1980s, even in the hottest part of the year, there were cool mornings, but this year there were nights it never got out of the 90s. I wonder if eventually we will never get below freezing, and that worries me because when it happens, the next summer we get hammered by the bugs—spiders, roaches, ants—even mice.”

Fifteen to 20 times a year, Ken Waters of the National Weather Service issues a heat warning for the region, based on predicted highs and, equally important, nighttime lows. “No question it has a major impact on people,” he says. “When it stays above 90 all night, it makes it very difficult to recover from daytime heating.” If you don’t have a home to go to, you are at the mercy of the elements, Harlan says, no less than someone sleeping on a subway grate in Manhattan to stay warm in December. In a study that looked at heat-related death by occupation, men in the category “unknown”—which usually means homeless—had a rate ten times that of men in known occupations.

For the rest of us—well, we will just have to get used to sweating more, and put up with what Anderson, of Iowa State, describes as the “crankiness” factor. “Being uncomfortable colors the way people see things,” he says. “Minor insults may be perceived as major ones, inviting, even demanding, retaliation.”

That was just what Richard Larrick of Duke University’s Fuqua School of Business, along with his co-authors, found when they examined the box scores of some 57,000 Major League Baseball games played since 1952—about 4.5 million plate appearances in all. They were looking into whether hot weather made pitchers more likely to throw at batters, and based on records of game-time temperatures, they found that it did, but in a specific and telling way. In theory, hot weather might increase the incidence of wild pitches by affecting pitchers’ control (distracting them, or making their palms sweaty), but that’s not what the study focused on. Instead, it found that after one or more batters were hit, intentionally or not, hot weather made it more likely that the opposing pitcher would retaliate later in the game. “What’s interesting is that the same act—your teammate being hit by a pitch—seems to mean something different in a hot temperature than a low one,” Larrick says. “An ambiguous act now seems more provocative when your own mind is in turmoil because of the heat.”

Of course, very cold weather makes people uncomfortable also, and in laboratory experiments cold has in fact been shown to increase aggression. But that doesn’t appear to translate into more crime during cold spells. There is some evidence from brain imaging that the perception and regulation of heat involves some of the same regions that process anger—the proverbial “hothead”—although the significance of those findings is unclear. Anderson speculates that in evolutionary history, extreme cold has generally posed a more immediate threat to personal survival than heat, and people are driven to escape it, with clothing, fire and shelter. “If I’m cold, I have to deal with that right away,” he muses. “I don’t have time to be irritable.”

And if you suffer from the heat, like Kenrick, the Arizona researcher, and you work on an academic schedule, you can head north for relief. “I go to Vancouver for a couple of weeks a year,” he says, “and I enjoy being able to go out for coffee without having to stop each time and think, is it worth it.” He should enjoy it while he can, because Vancouver recorded its two hottest days ever in 2009, and the city is considered at risk of flooding owing to climate change in the coming decades.

That honking sound you hear? It may be the climate apocalypse, right behind you.