Phoenix’s Bigger Problem Isn’t Water. It’s the Heat.

I want to say something that might be unpopular: the water crisis in Phoenix is serious, but it’s not our biggest problem. The heat is. And unlike water, we don’t have a good plan for it.

Before you stop reading — I know. The Colorado River is shrinking. Lake Mead has hit historic lows. Arizona’s junior water rights put us in a legally precarious position. I’ve read the reports, I follow the news, and I take the water issue seriously. Water scarcity is real, and we should be working on it.

But here’s what I keep coming back to: water is a problem with tools. You can conserve it, recycle it, renegotiate it, and engineer around it. Rain will always fall somewhere. Snowpack will vary. Aquifer recharge is slow, but it happens. The water crisis is a solvable problem. It’s a difficult, expensive, politically messy problem to solve, but fundamentally tractable.

The urban heat island effect is something different. And I think we’re not talking about it nearly enough.

01What the Numbers Actually Show

Let’s start with what’s happened to Phoenix’s temperature over the past few decades, because the data is alarming. Since 1983, Phoenix’s average daily summer temperature has increased by 3.6°F, its daily high has gone up 3.2°F, and — most alarming — its nighttime low has risen by 4.4°F, according to NOAA. That last number matters most, and I’ll explain why shortly.

Arizona State University research has found that over the last 50 years, Phoenix has seen roughly a 7-degree Celsius increase in average temperatures, with the city’s explosive growth rate identified as the primary reason. In 2024, some Phoenix neighborhoods were recorded as much as 10°F warmer than nearby rural areas. That’s not weather. That’s us. That’s the concrete, the asphalt, the rooftops, the artificial turf, the loss of vegetation. We built a heat machine, and now we live inside it.

The number of days above 110°F in Phoenix has jumped 30% over the last ten years, according to NOAA. In 2024 alone, Phoenix endured 34 days above 110°F, compared to just 17 such days a decade earlier. We’re not talking about incremental change. We’re talking about a fundamental shift in what Phoenix summer means.

02Plants Don’t Add Heat. They Remove It.

Here’s something that gets lost in the rock-and-gravel aesthetic that’s taken over Phoenix landscaping: plants are not neutral in this equation. They actively cool the environment around them, in two distinct ways.

First, shade. A tree blocks solar radiation before it ever hits the ground, a wall, or a roof. Research across multiple cities has found that shaded surfaces can be 20 to 45°F cooler than surfaces in direct sun. A well-placed tree on the south or west side of a home can reduce interior temperatures by as much as 10°F and cut summer air conditioning costs by 15 to 25%. A single mature tree has been found to provide the cooling equivalent of several room-sized air conditioners operating 20 hours a day… for free.

Second, transpiration. Trees pull water up through their roots and release it as vapor through tiny pores in their leaves. This process — essentially the plant equivalent of sweating — absorbs heat from the surrounding air as the moisture evaporates. A Berkeley Lab study found that a single tree can transpire 100 gallons of water per day, with a cooling effect equivalent to five standard air conditioning units. In a hot, dry climate like Phoenix, where the humidity gradient between tree leaves and the surrounding air is steep, this effect is powerful. Research published in 2025 found that trees provide the largest air conditioning energy reductions — up to 17% — in the hot dry city-climates of Riyadh and Phoenix, precisely because of this enhanced evapotranspirative cooling.

Asphalt absorbs up to 95% of solar radiation and stores it underground, releasing it slowly at night when we need relief. Plants do the opposite. They intercept that radiation, convert it through photosynthesis, and release moisture that cools the air. One of these surfaces is working against us. The other is working for us.

The idea that removing plants is a low-maintenance, water-smart choice misses the bigger picture. Yes, a rock yard uses less irrigation water. But it also runs your air conditioner harder, contributes to the heat that’s killing people in Maricopa County at record rates, and destroys the very microclimate that made any plant life possible in the first place.

03Why the Nights Matter More Than the Days

Phoenix has always been hot during the day. Desert plants evolved over thousands of years to handle brutal daytime heat. The strategy many of them use is called CAM photosynthesis — Crassulacean Acid Metabolism. In CAM plants like saguaros and many other succulents, the stomata open at night to absorb carbon dioxide when temperatures are cooler and humidity is relatively higher, minimizing water loss. The absorbed CO2 is stored overnight as organic acids, which the plant then uses during daylight hours to perform photosynthesis with its stomata closed.

It’s a brilliant adaptation. But it depends on one thing: the nights have to cool down.

Research from the Arizona-Sonora Desert Museum confirms the problem directly. The CAM photosynthetic pathway works well under high temperatures but begins to break down when evening temperatures exceed roughly 95°F. This has been the case for several consecutive weeks in Phoenix during recent summers, when temperatures have rarely dropped below 100°F at night.

And what happens when those plants can’t photosynthesize efficiently? According to researchers, with sustained high overnight temperatures, plants may be losing more carbon and water than they’re able to take in. The leaves yellow and drop. Plants essentially starve to death.

We saw this play out in 2023. The Desert Botanical Garden reported saguaro cacti collapsing during the peak of July’s heatwave. The Garden has also reported that nearly 20% of their saguaros have died in recent years, attributing the losses to intensified heat and drought stress. A 2023 study in the Journal of Arid Environments found that when nighttime temperatures consistently remain above 85°F, saguaros experience reduced recovery from daily heat stress, preventing them from replenishing water stores overnight and leading to progressive internal damage.

The saguaro can live over 200 years. We’re watching them collapse in Phoenix because our nights no longer cool down enough for them to recover.

Why don’t the nights cool down? According to NASA’s Jet Propulsion Laboratory, dark asphalt and concrete have a high heat capacity, so most of the heat they absorb during the day goes into storage below the ground. That heat gets released slowly at night, making air temperatures much warmer in dense urban areas. NASA documented Phoenix’s built surfaces hovering around 100°F for hours after sunset during the 2023 heatwave, growing progressively hotter day over day because they never fully cooled overnight before the next day began.

04Even Native Plants Are Struggling — and Need More Help Than They Used To

Here’s an important point: the plants that evolved specifically to survive in this desert are having trouble surviving in the city that now occupies it. That’s how far we’ve pushed conditions outside the natural range.

Native plants in urban Phoenix aren’t just facing natural desert heat. They’re facing temperatures elevated by 7 to 10 degrees above surrounding rural areas, with nights that don’t cool down the way the desert floor does. Researchers at Cronkite News have reported that after thousands of years of evolution, native plants are now struggling under a new wave of environmental conditions that have no real precedent in their evolutionary history.

What that means practically is that a native plant installed in a Phoenix urban landscape today may need supplemental water during establishment and heat stress periods — not because it’s a water hog by nature, but because we’ve built a city that imposes thermal stress far beyond what it evolved to handle. The plants aren’t wrong. The environment is wrong.

Researchers have noted that as a short-term measure, some native plants in distress may benefit from supplemental water or even temporary shade structures to get through extreme heat events. We’ve pushed conditions so far that the desert’s own tools aren’t enough anymore without some help from us.

05People Are Already Dying From This

The ecological consequences are serious. The human consequences are already catastrophic.

In 2023, Maricopa County recorded 645 heat-related deaths — shattering the previous record set just the year before by more than 50%. For eight straight years, Maricopa County had set a new annual record for heat-related deaths. That is not a blip. That is a trend line pointing straight up.

Arizona’s heat-related death rate has increased roughly tenfold over the past two decades, while the national rate roughly doubled over the same period. Texas broke its own record in 2023 with over 300 heat-related deaths, yet Maricopa County had twice as many — 645 — despite having less than one-sixth of Texas’s population.

2024 saw 602 confirmed heat-related deaths, a slight improvement, and worth acknowledging. Targeted interventions like expanded cooling centers and overnight respite sites made a real difference. But 602 people still died. The water crisis has not killed hundreds of people per year. Heat has. And this is not a future scenario. It is already happening.

06The Reversibility Problem

Here’s the core of my argument, and the part I think gets overlooked most.

Water problems are, at their root, management problems. We can change behavior, policy, and infrastructure to address them. Phoenix already uses less water today than it did two decades ago, despite significant population growth, largely through more efficient outdoor irrigation. City officials have stated that Phoenix is not running out of water, having built a diversified water portfolio over decades that includes the Salt and Verde rivers, the Colorado River, and groundwater banking.

The long-term picture on the Colorado River is genuinely uncertain, and I don’t want to understate that. But people are actively working the problem. Tools exist. Progress is measurable.

The heat island is structurally different. You can’t easily undo decades of paving. Urban development and expansion has been identified in peer-reviewed research as the primary contributor to Phoenix’s urban heat island effect. Every new road, parking lot, rooftop, and artificial turf installation makes the problem worse. And unlike a policy change or a water recycling plant, you can’t reverse it quickly.

And the biology makes it even harder. The very thing that would most help cool our cities — living plants — can’t survive in the conditions we’ve already created in many parts of the metro. As temperatures rise and nights stay too hot, native vegetation dies. As vegetation dies, there’s less transpirational cooling. Less cooling means higher temperatures. Higher temperatures kill more plants. We’re already inside this feedback loop.

07Stop Demonizing Plants — All of Them

This is where I want to push back on something that’s become a lazy default in desert landscaping conversations.

Plants are not the problem. They were here before humans arrived, and in some form, they’ll be here long after we’re gone. What we’ve done is make the environment so hostile that even the hardiest ones are struggling. And our response has been to remove them rather than support them.

There’s also an important nuance that often gets glossed over in the native-plant-only conversation: the idea of “native” isn’t as fixed as people think. Desert ecosystems have never been static. The Sonoran Desert today is not what it was 2,000 years ago, and it will not be what it is today in another 2,000 years. Climate shifts, species migrate, plant communities change. What we call “native” is a snapshot of one point in time, not a permanent ecological truth.

That doesn’t mean all non-native plants are equivalent or that invasive species aren’t a real problem — they absolutely are. Buffelgrass is a destructive invasive that crowds out native species, fuels fire, and simplifies ecosystems into monocultures. That’s not what I’m talking about.

What I am talking about is a drought-tolerant, non-invasive tree from another part of the world that can grow in Phoenix, survive our heat, provide genuine shade, and not require large amounts of water. Research published through the American Geophysical Union studied 14 tree species — both native and non-native — planted in Phoenix, and found that Pinus eldarica (Afghan pine), Arizona cypress, desert willow, Bauhinia, and Arizona ash were among the best performers for providing intermediate to full shade across seasons relative to their water use under arid conditions. Several of these are not Sonoran natives. All of them are doing real cooling work.

The City of Phoenix itself recognizes this. Their tree grant program offers a list of 17 trees — half native, half non-native but drought-tolerant and desert-adapted. Even local experts working on the heat crisis acknowledge that a mature non-native fruit tree providing shade and food shouldn’t be ripped out in favor of a new native planting, because the shade and habitat value it provides is real and immediate.

The native-only conversation, taken to its extreme, can end up discouraging people from planting anything at all — because the choices feel too narrow, too complicated, or too restricted. And an empty landscape of rock and decomposed granite, however ecologically “pure” in concept, is a heat island in practice.

08A Note on the Water Issue

To be clear: I’m not dismissing the water crisis. Anyone paying attention to Colorado River allocations, post-2026 federal guidelines, and the state of Lake Mead knows that Arizona faces real challenges that will require expensive solutions and difficult negotiations. This is serious.

What I’m arguing is that water gets the attention and heat doesn’t — and that imbalance needs to correct. Water is visible. A shrinking reservoir is a striking image. Heat is invisible until it kills someone or collapses a saguaro that’s been standing for 150 years.

There’s also a dimension of this that rarely gets discussed: more plants in the urban landscape can actually help the water situation over time. Trees that shade homes reduce air conditioning loads, which reduces energy demand, which reduces waste heat pumped into the atmosphere by HVAC systems. Trees that shade asphalt and concrete keep those surfaces from absorbing as much heat, which means cooler ambient temperatures, which means people run their AC less. The cooling effect of trees isn’t in competition with water conservation — in Phoenix’s climate, they’re part of the same solution.

09What This Means for How We Landscape

As someone who works in Phoenix landscapes every day, I see the individual decisions that add up. Rock and decomposed granite instead of living ground cover. Artificial turf instead of drought-adapted plants. Wide concrete driveways. Dark hardscape that soaks up afternoon sun and radiates it back at 2 a.m.

None of these choices are unreasonable in isolation. People are making reasonable decisions with the information they have. But the cumulative effect across a metro area of five million people is a city that can’t cool down at night, that’s progressively inhospitable to the plant life that evolved here, and that’s getting measurably hotter in ways that can’t be quickly undone.

Phoenix can’t fully reverse the heat island. But it can stop accelerating it. And right now, a lot of the metro is still accelerating it, one landscaping decision at a time.

That’s what I think we need to be talking about more.