Why Has Arizona's Heat-Related Death Rate Increased Tenfold in Twenty Years?
The heat-related death rate in Arizona has increased roughly tenfold in the last twenty years. In Maricopa County, which is home to about 60 percent of the state’s population and includes the Phoenix Metropolitan Area, there were 645 heat-related deaths last year, setting a new record for the eighth consecutive year and shattering the previous record of 425 in 2022. And the death rate in Pima County was almost as high as Maricopa County, even though it was about seven degrees cooler.
In our white paper Why has Arizona’s heat-related death rate increased tenfold in twenty years?, we examine data from the Maricopa County Department of Public Health, the Pima County Medical Examiner’s Office, the National Weather Service, and other sources for clues as to why deaths have increased so dramatically. Our analysis indicates two main trends that are driving up deaths: 1) temperatures are rising, and 2) Arizonans are more vulnerable to heat because of social factors such as homelessness, drug use patterns, and social isolation.
There’s some good news: we can mitigate both trends via policy changes that can be feasibly accomplished at the state and local level. In other words, we don’t have to wait for large-scale policies to tame climate change to dramatically reduce deaths and create habitable, resilient cities.
ABOUT OUR RESEARCH
We examined temperature data from the National Weather Service, and death data from the Maricopa County Department of Public Health and the Pima County Medical Examiner’s Office, along with some data from other sources. Using the statistical software R, we conducted a negative binomial regression to model the relationship between daily median temperatures and heat-related deaths to roughly estimate how much temperature changes have increased deaths.
Vulnerabilities such as homelessness and drug use may be intertwined, and some factors—such as social isolation—are difficult to measure. To avoid a false sense of precision, we did not attempt to quantify the relative contribution of each vulnerability or model interactions between multiple variables. However, we attempted to isolate the contribution of increasingly hot summer temperatures relative to overall vulnerability. We consulted many other sources, such as academic studies and government reports, to inform our findings.
KEY FINDINGS
Temperature
Phoenix summer daytime temperatures have increased by about seven degrees in the last 100 years. Much of this increase— likely most of it—is caused by a severe “urban heat island effect” rather than global climate change. An urban heat island occurs when temperatures inside a city are consistently higher than surrounding rural areas because the city is covered by surfaces that absorb more sunlight and emit more heat than the natural surfaces they displaced. Although Tucson has not warmed as much as Phoenix (about four degrees in the past 100 years), there is evidence that Tucson has warmed faster in the past two decades and may unfortunately be catching up to Phoenix.
Our statistical analysis, which we conducted on several years of data, indicates that a one-degree increase in daily median temperature consistently corresponds with about a fifteen to twenty percent increase in heat-related deaths.
Since 2001, we estimate that warming alone is responsible for about a 40 percent increase in heat-related deaths in Maricopa County. The graph below shows that 40% increase in blue, prorated over the years.
Vulnerabilities
The gray portions of the bars in the graph show the rise in death rates that our estimated effect of rising temperatures does not explain. This suggests that death rates would still be several times higher now than in 2001 even if temperatures had stayed the same. We believe this portion of the rise in death rates is attributable to increasing population vulnerability to heat.
Our analysis reveals two general categories of victims with rapidly increasing death rates. The first group dies outdoors: these victims tend to be predominantly male and are often homeless. Outdoor deaths also often involve the use of one or more substances, most commonly methamphetamine, which appears to be particularly deadly when combined with extreme heat. The second group involves deaths that occur indoors, most often in a situation where the air conditioner isn’t working. Indoor victims tend to be older, skew more female, and are much less likely to have used any substances.
A common thread that ties the two groups together is social isolation. The first group may contain many who are isolated from the rest of society, perhaps estranged from friends or family. A homeless person may die in plain sight if bystanders don’t understand the gravity of their situation or are reluctant to get involved because of the person’s appearance or behavior. The second group may consist of many who are elderly and/or homebound, perhaps living alone with little human contact.
The rise in heat-related deaths has occurred alongside worsening homelessness, drug use, housing costs, and social isolation, suggesting possible causal links that are in many cases confirmed by data about the victims. For example, heat-related deaths in Maricopa County have increased dramatically since 2015, the same year that a sharp rise in unsheltered homelessness began—from under 1,300 in 2015 to nearly 5,000 in 2023. In 2015, eight homeless people died from heat-related causes; there were 291 such deaths in 2023. Dramatic increases in heat-related deaths involving methamphetamine and fentanyl have followed a similar trend in general overdose rates for those drugs. The rise in indoor deaths without air conditioning has occurred against a backdrop of skyrocketing housing prices that have outpaced incomes, perhaps leaving many unable to afford replacement units, repairs, or electricity bills. And all of this has occurred in a society that has become increasingly socially isolated by many measures, likely leaving many without help when they get in trouble.
POLICY IMPLICATIONS
Some action has been taken by policymakers to reduce heat-related deaths. For example, Maricopa County has recently expanded emergency homeless shelters with announced plans for more beds that may become available shortly. Both Phoenix and Tucson have stated plans to increase their tree canopy by 2030 as a heat-mitigation measure. And Arizona has recently passed laws designed to increase the supply of housing and relieve financial stress on lower-income residents.
Nonetheless, more action is needed. Expanded shelter capacity and other emergency measures may rely on one-time funding sources, some of it related to federal COVID-19 pandemic relief legislation. Phoenix’s heat response and mitigation director David Hondula compared the situation to a New England city depending on donations to buy snow plows rather than regularly budgeting for them. Implementation of heat mitigation efforts involving urban infrastructure, such as the plans to increase tree canopy, appear to be progressing very slowly in general.
Policymakers should ramp up efforts to address the major causes of the crisis. For example, cities can reduce the coverage area of unshaded pavement over time by integrating trees into right-of-ways as public green infrastructure and removing unnecessary pavement to mitigate urban heat island effects. Likewise, loosening land use laws can increase the supply of homes to help alleviate homelessness and financial stress that may be leaving people unable to afford air conditioning. Policies like these that make our cities more comfortable and affordable could improve many lives beyond those saved. Other emergency measures—more shelter beds, for example—will likely save many lives as soon as they are implemented.
