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An illustration showing displays with high temperatures, and images of people walking in a desert environment. Christina Animashaun/Vox

100 degrees for days: the looming Phoenix heat wave that could harm thousands

How climate change could bring the Katrina of extreme heat to Arizona.

To live in the 21st century is to live with the threat of weather growing more and more wicked.

Droughts, heat waves, and wildfires are growing more intense and dangerous from global warming and rising greenhouse gas emissions. Meanwhile, we’re not reckoning with scientists’ predictions that worst-case weather scenarios will be more likely — and common — if we don’t change course. Only 41 percent of the American public believes climate change will affect them personally, a 2018 survey by Yale and George Mason University found.

Phoenix, Arizona, is susceptible to a heat wave that could peak at a staggering 122 degrees Fahrenheit. Southern California could face a wildfire that burns 1.5 million acres of land. Tampa, Florida, could see 26 feet of storm surge flooding from a hurricane, just below the record-breaking 28-foot storm surge of Hurricane Katrina.

Heat waves rise up from the ground while a woman crosses the railroad tracks as temperatures climb past 112-degrees in Phoenix, Arizona, on July 5, 2018.
Ross D. Franklin/AP

In every case, these “Big Ones” could be huge disasters not just because of geography and proximity to threats, but also because of decisions to build homes and offices in certain places, ignoring nature. Many other communities in the same regions have similar vulnerabilities.

For too long, we’ve been complacent about climate change and the really scary possibilities of 2 degrees Celsius (3.6 degrees Fahrenheit) or more of average warming. Two degrees is the amount of warming we are likely to experience by midcentury, and it’s double the warming we’ve experienced to date. As David Wallace-Wells, author of The Uninhabitable Earth, put it in a Vox interview, “being scared about what is possible in the future can be motivating.”

Californians have long been taught to fear and prepare for the next big earthquake — and the state now has stronger infrastructure and wide engagement in earthquake readiness and planning. If more communities around the country feared climate “Big Ones,” they and their leaders would be more engaged in both stopping fossil fuel use and readying for disaster.

The scenarios in Phoenix, Southern California, and Tampa we describe in this three-part series are hypothetical. But they’re based on models scientists use to project what’s possible today, or tomorrow. There’s always uncertainty in these models. Things can change. These aren’t premonitions, but tastes of what’s possible.

We face more and more expensive disasters, and we are still building right in the paths of tempests. But our ability today to anticipate dangerous future weather creates an opportunity to reconcile with where we let people build, how we manage vegetation that could burn, and whether we replace more trees and soil with heat-magnifying concrete.

For a look at what’s in store, let’s begin in Arizona.


One day in the future, a massive wave of high-pressure air will park over Phoenix.

As the sun rises amid an already scorching summer, the pressure will hold the accumulated heat in place and the triple-digit temperatures will tick up higher and higher. 119 degrees Fahrenheit. 120. 121. 122. Health officials will warn citizens to stay inside, but some will venture out and emergency room visits will spike. At night, the temperature will drop only to 100.

It’s an alarming prospect for a rising population. Phoenix is home to 1.6 million people and is the second-fastest-growing city in the United States. The metro area is home to 4 million and is projected to grow to 6.6 million people by 2050. By then, more than 20 percent of the population will be older than 65.

A chart showing the probability for higher outdoor air temperatures in Phoenix, up to 104 degrees Fahrenheit, will increase by 2050. Christina Animashaun/Vox

The Centers for Disease Control and Prevention reports that heat is already the deadliest weather phenomenon in the US, killing hundreds of people a year, more than floods, fires, earthquakes, lightning strikes, tornadoes, or hurricanes. With climate change, the threat is only getting worse, particularly for the elderly and the impoverished.

Over the next 24 hours of this heat wave, electricity use will surge higher as millions of air conditioners blast at full force, and the power grid will sputter as power lines strain. Power plants will run dangerously low on cooling water as the rivers that feed the region slow to a trickle and heat up. Generators will become less efficient.

The grid will succumb to brownouts and blackouts. Air conditioners will wheeze out, leaving many in homes that will grow dangerously hot. Water pumps will shut off, threatening people with dehydration. Freezers will thaw and food will spoil. Lines will form at gas stations as stalled pumps force drivers to refuel by hand.

Anxiety will grow about the region’s water supply. The Phoenix metropolitan region will already be in a drought and what little water is left will start becoming too hot to use. The nearby water reservoirs will be at record lows. Golf courses turn yellow as water restrictions go into effect.

By Day 5, city officials will declare a public health emergency. Officials will ask people to congregate in malls, libraries, and rec centers to minimize the cooling load.

In 2017, PetSmart sold new elastic booties, used as protective coverings to protect dogs’ paws from the hot pavement, as temperatures in Phoenix were forecasted to hit 120 degrees.
Angie Wang/AP

Gasoline will be in short supply as people start to leave the city. Traffic will gridlock as asphalt bubbles and roads close. Flights will be grounded as the heat makes the air too thin to generate enough lift for aircraft to safely take off and climb. Cacti will wilt. Air pollution will reach record levels as dust and ozone build up, leading to another spike in emergency room visits.

This surging heat with temperatures peaking around the 120s will linger for two weeks, as rising average temperatures increase the length, severity, and frequency of extreme heat. The city’s economy will grind to a halt. Lights will switch off. By the end of this cascade, many may die.

Phoenix is already hot, but the heat will only get worse

Many of these effects have already happened before. Scientists, engineers, and city planners are worried they could happen again with more devastation. In 2018, researchers mapped out how extreme heat could lead to a rippling collapse in infrastructure, warning that Phoenix might face “a [Hurricane] Katrina of extreme heat.”

Such a scenario could happen in Phoenix in the next few decades as the climate changes. The city’s past brushes with heat extremes are illustrative.

The hottest temperature recorded in Phoenix was 122 degrees in 1990. And a searing late-June heat wave in 2017 lasted more than a week and melted mailboxes. Letters slid off street signs. Asphalt bubbled. Airplanes couldn’t take off. Power consumption soared to record highs. There was no measurable rain during the month.

A pedestrian uses an umbrella to get some relief from the sun as she walks past a sign displaying the temperature on June 20, 2017 in Phoenix, Arizona.
Record temperatures were hit on June 20, 2017, in Phoenix.
Ralph Freso/Getty Images

High temperatures killed a record 172 people in the Phoenix metropolitan area that summer, up from 150 heat deaths in 2016 and 85 in 2015.

But as the climate changes, heat waves will get worse. Not only is Phoenix one of the hottest cities in the United States, it’s one of the fastest warming. By 2060, the city will have 132 days above 100 degrees, according to Climate Central, a consortium of researchers and reporters.

With the city still growing, the number of people suffering from the heat will rise as well.

How a heat wave forms over Phoenix

A heat wave is simply a prolonged period of extreme heat. It’s a relative measurement, meaning that what counts as a heat wave is different depending on the local climate. It’s often defined as temperatures above the 95th percentile of the historical distribution for a region.

In Phoenix, a heat wave builds on Arizona’s already hot, dry climate. Some of the forces behind the climate begin as cold water currents near Alaska’s Aleutian Islands. That cold water is a key reason why the Southwest is a desert, since it doesn’t evaporate as easily, explained Nancy Selover, the Arizona state climatologist.

The little moisture that does evaporate into the air currents gets wrung out over the mountains in California to the west. By the time the air reaches Phoenix, it is extremely dry, bringing scarcely any clouds. Those clear skies mean the sun gets an unobstructed line of sight to the sweeping stretches of concrete below. The air then sinks over the city, compressing and heating up.

Then, during a heat wave, a high atmospheric pressure system builds, trapping the ordinary heat in place. As that pressure system gains strength and heat, it pushes the jet stream — a high-altitude, fast-moving air-current — further north. It acts as a barrier, forcing any potentially cooling precipitation away from the city.

Now recall that the global average temperature has risen by about 1 degree Celsius since the dawn of the industrial revolution. Temperatures over land have risen about 1.5 degrees Celsius. Those small changes in averages lead to big changes in extremes as heat-trapping gases increase the thermal energy in the atmosphere, leading to more intense, frequent, and longer heat waves.

So Phoenix’s record high of 122 degrees Fahrenheit may recur. “I anticipate at some point in the next number of years it will happen again,” Selover said. “All of a sudden we’ll get a whole series of weeks of high pressure just [sitting] there baking us just at the right time of year, and potentially we could go over that.”

A Salvation Army hydration station in Phoenix, on June 19, 2017.
Ross D. Franklin/AP

Phoenix’s infrastructure contributes to the heat and suffers from it

Beyond emissions of greenhouse gases, decisions about planning roads, building houses, erecting utility poles, and paving parking lots also contribute to Phoenix’s capacity to absorb heat. These surfaces take in an immense amount of heat, and when the evenings stay warm, the roads, sidewalks, and buildings can’t shed that energy.

At 517 square miles, larger than Los Angeles by area, Phoenix is one of the most sprawling cities in the US. The ever-expanding reaches of concrete, asphalt, glass, and steel soak up the heat and radiate it slowly across the city, even block by block.

So day after day, the heat builds, and builds, and builds. This is a phenomenon known as the heat island effect, and the warming it causes can be just as significant as warming due to emissions of greenhouse gases, albeit at different times of day.

“Warming due to the expanding built environment is similar to warming resulting from greenhouse gases during nighttime hours, while warming during daytime hours is dominated by greenhouse gas-induced climate change,” explained Matei Georgescu, an associate professor of geophysical sciences and urban planning at Arizona State University.

The heat island effect is found in major cities throughout the world, but Phoenix stands out for just how high temperatures can get. The heat in turn threatens the stability of this urban environment. Mikhail Chester, an associate professor of civil engineering at Arizona State University, explained that heat can be just as big a threat to health and infrastructure as a hurricane or earthquake, but because it builds up slowly, it’s easy to overlook.

In the built environment, the impact of heat mounts over time and doesn’t always manifest as a sudden, catastrophic failure. Instead, rising temperatures weaken roadways, cause metal fatigue in bridges, and make the metal pipes that move water expand, crack, and leak.

But the biggest threat from a heat wave may be a power failure. In addition to losing efficiency in power plants, power lines have a diminished capacity when heated to high temperatures, and equipment like transformers and inverters see a higher failure rate.

How cities can prepare for extreme heat Amanda Northrup/Vox

Chester, who coauthored the study warning that Phoenix could face the Katrina of extreme heat, explained that losing power jeopardizes not just air conditioning, but traffic lights, commuter rail, water sanitation systems, even fuel pumps for gasoline. So a blackout or brownout during a time when the city needs energy the most stands to create a propagating series of failure and disruption, halting the economy and potentially taking lives.

“Whether it is a hurricane in New Orleans or an [extreme heat event] in the Southwest, critical infrastructure systems are at risk for cascading failure in ways that are unpredictable and surprising due to their complex interdependencies and fragility to extreme conditions,” the authors wrote.

“For each of these, you can find precedent,” Chester said. “We’re not just making up doomsday scenarios. In the Southwest, we’ve had these scenarios unfold.”

Heat waves will be increasingly deadly in the warmer future

Higher temperatures make it harder for the body to shed excess heat. When outside temperatures exceed the body’s temperature, more heat flows into the body than out of it. Higher temperatures make physiological processes harder. It can cause proteins to malform. Oxygen is moved with less efficiency.

When the body temperature rises up too high, it creates conditions like hyperthermia, which can then lead to heat stroke and death.

One upside to Phoenix’s heat is that it comes with little humidity. High levels of humidity impede the body’s ability to cool off by sweating, though once temperatures get high enough, heat risks will rise regardless. “We don’t tend to factor in humidity very much, because we almost never have enough humidity to make a huge difference in terms of heat stress,” Selover said. “The temperature itself is what gets us.”

Maricopa County, which encompasses Phoenix, has reported more 150 heat-related deaths in each of the past few years. “[Heat] is a huge public health hazard,” Chester said.

A graphic showing how to distinguish heat stroke from exhaustion. Heat stroke is more severe, and symptoms include nausea, no sweating, a rapid pulse, and red, hot, dry skin. Amanda Northrop/Vox

High temperatures are lethal to people with existing problems like heart disease and emphysema. It accelerates the formation of ozone, a major pollutant. That can also contribute to breathing difficulties. The indirect harms from heat may in fact be far larger than maladies caused directly from heat, but they are much more difficult to track.

Time matters when it comes to risk from heat. If the temperature is hot enough, it can start to harm people in just a few minutes. But it’s not just the peaks that are dangerous; exposure to lower heat for a longer period of time can be harmful as well.

When high temperatures endure, the body gets little time to cool off. And in cities like Phoenix, such relief becoming increasingly scarce as nighttime temperatures continue to rise as well. In the past 20 years, nighttime temperatures in Phoenix have shot up 9 degrees Fahrenheit on average. This is a major health risk factor.

David Sailor, director of the Urban Climate Research Center at Arizona State, examined what would happen under the combined effect of heat on health and infrastructure. In a study published in May, he reported that the risk of power outages under extreme heat is only growing. Sailor and his colleagues found that even under the current level of warming, a blackout would be devastating for Phoenix.

“In the end, using a combination of census data and results from a city-level survey we conducted of elderly residents, we estimate that about 8,000 residents of Maricopa County would be particularly vulnerable to a major heat disaster (power outage coincident with summer heat),” he said in an email.

How Phoenix is trying to cool off

To combat warming, city officials are taking aim at the hotspots within Phoenix’s heat island. “Two neighborhoods two miles apart can have a 13-degree [Fahrenheit] temperature difference,” said Mark Hartman, chief sustainability officer for the City of Phoenix. That means cooling tactics have to be targeted at the most vulnerable areas, which will require careful planning and measurement.

But he’s hopeful that many of the worst health effects can be avoided. “We can cool our city to more than offset our increasing temperatures,” said Hartman.

The biggest need, according to Hartman, is to rethink urban surfaces. Painting roads and roofs light, reflective colors can go a long way toward mitigating heat. Vegetation like trees and even cacti can transport moisture through the air that can cool an area. Shade for walkable areas, particularly in neighborhoods where residents have to walk to transit stops, can shield people from the heat.

A man runs through section of South Mountain Park at sunrise to avoid the excessive heat in Phoenix.
The 2017 heat wave hit Arizona, Nevada and California, bringing 120-degree temperatures to Phoenix.
Matt York/AP

Implementing these measures requires a comprehensive view of the warming situation and aggressive action on the part of the city. Eventually Phoenix could limit its absorption of heat from the desert sun. The world, however, is still warming, and temperatures after the sun sets will likely continue to rise. “Even for such low emissions pathways, these strategies barely offset any nighttime warming,” ASU’s Georgescu said.

Climate change is undoubtedly hitting Phoenix hard, and even in a city famous for its heat, it will profoundly change its way of life.

With few tolerable outdoor hours in the day, many jobs will increasingly become nocturnal. Road construction crews in Phoenix already work in the middle of the night on highways, since it’s safer for workers and because it’s easier to work with materials like concrete at lower temperatures. Recreation activities like hiking are beginning to become more popular before the sun comes up.

That means millions of people will have to live hermetically sealed off from the world outside for a long and growing stretch of the year. In turn, the city’s appetite for energy and water will increase. More residents may soon be a broken AC or a missed bill payment away from death.

Though the heat is already here, the worst is yet to come.


Correction: An earlier headline said 122 degrees Fahrenheit could last for days when instead it could be a peak during a heat wave. Clarifications have been added. The text has also been corrected to show that cold coastal water, not cold air, is the progenitor of desert conditions.

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