Phoenix’s urban heat island effect is increasingly deadly

In July 2005, Phoenix suffered through what was then a record-breaking heat wave.  For nine consecutive days, the high temperature topped 110 degrees, and highs averaged near 110 for the month—several degrees above average—with nighttime lows often only dropping to around 90.  The heat wave was so severe that it made national news; the New York Times reported that a “relentless and lethal blanket of heat” had killed 20 people in Phoenix in just a week.  By the end of July, the death toll from the heat wave rose to over 30, most of them homeless.

In response to the deadly 2005 heat wave, Maricopa County began closely tracking heat-associated deaths and illnesses and took mitigation efforts such as setting up cooling and water distribution centers.  Yet despite these efforts over the past sixteen years, the alarming number of deaths from the 2005 heatwave now barely registers as a blip on a long term chart of heat-associated deaths in Maricopa County:  

Maricopa County heat-associated deaths

The graph likely understates the true figure for 2021:  In addition to the 252 heat-associated deaths in 2021 reflected in the chart, the county was still investigating 86 additional deaths at the time of its latest report to determine whether they were heat-associated.  It’s therefore possible that 2021’s final death total will surpass 2020’s record total of 323.  If it does, that would represent the sixth consecutive year in which heat-associated deaths have reached a record high in Maricopa County.   

It’s clear that the increasing summer heat in the Phoenix area is more than an inconvenience.  It is the cause of a dramatically worsening public health crisis that has defied the city’s present efforts to mitigate the harm.

The reasons for the deadly Phoenix heat are well understood.  Central and western Arizona has always been hot, with temperatures peaking in June and July. Monsoons arriving in July may cause daytime highs to drop but may not bring much relief, because the humidity they bring raises the apparent temperatureGlobal human activity has caused temperatures to increase: average temperatures in the region have been about two degrees warmer over the last 20 years than long-term averages and are increasing at a rate of about one-half to one degree per decade.  

Moreover, Phoenix has a significant urban heat island effect, making the problem much worse in the city.  As people have converted the area’s natural landscape of open soil, desert vegetation and croplands to engineered surfaces that store more heat during the day, an urban heat island effect has developed, making Phoenix much hotter than the surrounding desert.  Air-conditioning, which keeps things cool indoors for many, transfers more heat outdoors and has been found by Arizona State University researchers to add substantially to the heat in the city, particularly at night.  The city now regularly experiences summer nights where the temperature does not drop below 90 degrees, a phenomenon that was exceedingly rare just a few decades ago.  

The heat island effect tends to be most intense in low income areas, places where people tend to have fewer resources to escape the heat.  It’s a particularly brutal situation for homeless people.  Of the record number of people who died from the heat in 2020, more than half were homeless.  Other traditionally disadvantaged populations suffered disproportionately, including African Americans, American Indians, and the elderly.  A majority of the deaths occurred outdoors; for those that occurred indoors, most involved an air conditioning unit that simply did not work.  Substance use was a contributing factor in over half of the deaths; methamphetamine abuse combined with the heat proved to be particularly deadly.

What can be done?  Obviously, it should be a major aim of policymakers to reduce temperatures in the city or at least slow the rate of increase. The city should step up measures to increase the canopy of vegetation in the city.  In 2010 the city set a goal to increase its tree canopy to 25% by 2030, a substantial improvement upon the city’s estimated 11 to 13% vegetation cover at that time.  Increasing the tree canopy to 25% could decrease temperatures by up to 3.8 degrees.  But a 2017 study of satellite data indicated that vegetation cover in the city had actually declined.  Progress on such goals should be regularly tracked and efforts redoubled where reality falls behind the goal.  

Also, the city can replace current surfaces with materials that absorb less heat and promote policies that encourage residents to do the same.  For example, the city is testing a promising “cool pavement” treatment that can be applied to asphalt so that it tends to reflect sunlight rather than absorb it.   And pavement coverage could be reduced through policies that encourage excess parking spaces to be removed.  Finally, the city should take a leadership role in taking measures to slow the advance of global warming. 

Slade Smith

Slade is Assistant Director of the Applied Health Policy Institute and an Arizona attorney. He is a 2017 graduate magna cum laude of the University of Arizona James E. Rogers College of Law and co-recipient of the 2017 Ralph W. Aigler Memorial Award for his professional and scholarly contributions to the college. He is from Columbus, Ohio, and now lives in Tucson, where he regularly enjoys outdoor activities in southern Arizona's beautiful and varied desert and mountain landscapes.