How shade trees and cool roofs can slow the billion-dollar drain of urban heat in Phoenix

There's a lot of shade at Gilbert Riparian Preserve at Water Ranch.
There's a lot of shade at Gilbert Riparian Preserve at Water Ranch.

The Phoenix area’s rising urban heat will cost hundreds more lives and billions of dollars in lost economic production each year by midcentury if the region doesn’t adopt widespread tree planting and cool roof installation, a new study predicts.

The cost of additional lost lives alone could reach $1.5 billion on average by 2050, on top of an average figure of $1.3 billion from recent years, according to the study led by the Nature Conservancy and the consulting firm AECOM. The report's costs are in 2021 dollars.

Hotter days and nights, expected to be 3 to 5 degrees warmer on average, are also projected to drain nearly half a percent of the region’s economic output by 2050 and cost electric ratepayers an additional $10 per degree each month from May through October.

“Heat is a really significant issue,” said Anna Bettis, who directs TNC’s Healthy Cities Program in Arizona. “It’s the leading weather-related cause of death, and the highest rates are in Arizona.”

In 2020, according to Maricopa County, 323 people died of heat-related causes. The county has confirmed 252 heat deaths this year and is investigating another 86.

But, Bettis said, “We do know that bringing nature into the city can help.” That means lots of desert-adapted trees.

Phoenix this year opened one of the nation’s first city heat mitigation offices, with a key goal of bumping the city’s tree canopy coverage from about 13% now to 25% within a decade. Some neighborhoods in the region have tree coverage in the low single digits.

Another goal is to encourage the installation of reflective roofs and pavements. Studies have shown these methods cool surface temperatures to an extent that can influence human health, and this new study offers a cost-benefit analysis for widespread tree planting and conversion to “cool roofs.”

Achieving tree shade over 25% of the metro area would cost $4 billion, according to the study, but over a 40-year period would save more than $15 billion in costs from deaths, hospital visits, road and other repairs and lost labor productivity, according to the study. Converting every roof to reflective materials would cost an estimated $1.5 billion but save $7.9 billion.

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A steering committee of experts from Arizona State University, the city of Phoenix, the utilities Arizona Public Service and Salt River Project, the Greater Phoenix Economic Council, Vitalyst Health Foundation and the Maricopa County Public Health Department provided data and guidance for the study.

The study used a baseline climate period of 1986-2005, a period during which the region averaged 12 days a year with high temperatures at or above 110 degrees. Depending on global greenhouse gas emissions scenarios, the study projects those extreme heat days will rise to a range of 36-42 by 2030 and 48-63 by 2050.

The annual average cost of local inaction by 2050 is projected at between $1.9 billion and $2.3 billion, depending on global emissions, according to the study. The biggest costs are in deaths and labor productivity, with smaller but substantial costs in energy consumption, hospital visits and road maintenance.

As big as the dollar figures sound, they’re based on incremental changes in average temperatures, not outlandish predictions for new highs, said Mac McCullough, a health economist at ASU who worked on the study.

“It’s basically: What if we have similar stuff, just a little bit more of it?” he said. For instance, each day the heat season expands in length brings new risks.

The sun sets over downtown Phoenix as it glows red from smoke from the Telegraph Fire burning east of the city on June 15, 2021. The National Weather Service in Phoenix issued an excessive heat warning for portions of south-central Arizona through the week.
The sun sets over downtown Phoenix as it glows red from smoke from the Telegraph Fire burning east of the city on June 15, 2021. The National Weather Service in Phoenix issued an excessive heat warning for portions of south-central Arizona through the week.

The study’s mortality section relies on a current U.S. Department of Transportation value of $11.9 million per life, McCullough said. That figure takes into account a range of lost economic opportunities and figures such as what workers in high-risk fields are paid. While it doesn’t capture a person’s intrinsic value, he said, it is intended to help agencies make cost-benefit decisions about investments such as highway safety upgrades. The existing figure of $1.3 billion a year is based on an average of 112 deaths a year from 1986-2005, though recent years have seen substantially higher death tolls.

The study additionally projects that heat-related emergency medical visits, now at $2 million a year, will escalate by $700,000 in 2030 and $1.2 million in 2050; and hospitalizations, now $5.3 million, will rise by $2.6 million in 2030 and $4.7 million in 2050.

McCullough said the findings that trees and reflective roofs can more than pay for themselves gives him hope, though he understands it will be important for cities, companies and residents to understand that their investments may help others in future years.

“It’s easy to see the costs of that because the person who plants the tree has to buy the tree and water it,” he said. “But it’s hard to see the benefits decades in the future.”

APS, the electric utility, helped frame the study and supports efforts at cooling streets and neighborhoods as part of its corporate promise to help shape "a long-term sustainable future for Arizona," said Eric Massey, the company's director of sustainability. The company also has an interest in keeping down nighttime temperatures so Arizonans won't need to use so much air conditioning during hours when solar power is not available, he said.

The cost of a cool roof can sound prohibitive, but the study assumed home and business owners will only incur them when they need to re-roof anyway, said Anne deBoer, project manager with AECOM. The premium paid to create a cool roof ranges widely, up to $5 or more per square foot for white glaze applied to clay tiles, she said. On average, though, the added cost per square foot for the area’s most common roof types ranges from 5 cents to $3.

The study's projected return on climate resiliency investments demonstrates the importance of taking action, said Kristen Stephenson, senior vice president for research and analytics at the Greater Phoenix Economic Council, which seeks to attract business to the region.

"We have an opportunity here in greater Phoenix to be a leader," she said.

The Nature Conservancy does not intend to use the study to tell people, companies or governments how they should respond to rising temperatures, Bettis said. Instead, the goal is to show there is an economic case for urban cooling investments.

Warmer cities: Outdoor workers could be exposed to even more days of extreme heat

Brandon Loomis covers environmental and climate issues for The Arizona Republic and Reach him at

Environmental coverage on and in The Arizona Republic is supported by a grant from the Nina Mason Pulliam Charitable Trust. Follow The Republic environmental reporting team at and @azcenvironment on Facebook, Twitter and Instagram.

This article originally appeared on Arizona Republic: Without more trees and cool roofs, Phoenix may lose billions