A Different Sort of Climate Change Is on the Move in Sprawling Cities

The chamber of commerce of its namesake city probably doesn't advertise too much the climax of the actual phoenix myth. The bird with a tail of gold and spirit of fire symbolizes regeneration through pretty much all tellings of the story, but what is being regenerated from is this: burning alive in its own nest. The great phoenix is always in the end reduced to ashes, and it's from these ashes that a new phoenix is born. After another 500 or 1,000 years of hanging out and feasting on frankincense and odoriferous gums, this phoenix too burns alive, leaving yet another pile of ashes. Ashes to ashes, but only after an agonizing death by combustion.

Burning alive in your nest is as good a metaphor as any for July and August in the Southwest United States' fastest growing city. In these months, temperatures often top or at least brush 110°F, well into the extreme danger zone for heat exhaustion and its deadly kin, heat stroke. The climate of southern Arizona is, obviously, hellish hot by default, but this is only exacerbated by the urbanization of the desert: heat loves sprawl as it loves concrete and asphalt.

A new study out from researchers at Arizona State University, published in the journal Science of the Total Environment, seeks to quantify the future of this urban heat effect, as the Phoenix metropolis continues to push outward into the Sonoran Desert, but also as arid and semi-arid climate cities in general continue to expand across the globe.

First off, the effects of heat are far more real than one might think given the relative violence of weather events like hurricanes, tornadoes, and snow storms. "Extreme heat is the leading weather-related killer in the United States," said ASU's David Hondula, the study's lead author, in a press release. "In Maricopa County [Phoenix], we see more than 100 premature deaths and hundreds of excess emergency department visits as a result of high temperatures each summer. Understanding how different urban development strategies will impact the health risks associated with heat can help long-term planners and public officials make more informed decisions that lead to sustainable and healthy cities."

The researchers looked at three different scenarios for Maricopa County and the relative changes in climate for each one: high growth, low growth, and growth with mitigation efforts in place (white sunlight-deflecting painted roofs). Using data collected between 1983 and 2007 relating temperature to mortality, the ASU team was able to come up with models predicting mortality increases in the future given the county's urbanization trends.

The results aren't quite what one might expect. Looking at the high-growth future of Phoenix, the researchers came up with a range of possibilities depending on the high to low spread of possible future temperatures. As sprawl worsens, daytime temperatures will actually decrease, but with the powerful trade-off of much higher nighttime temperatures. Overall, however, more people will die. The study's conclusion is that nighttime minimum temperatures are more important for mortality than daytime maximum temperatures.

"Future urbanization will lead to slightly lower summer daytime maximum temperatures in the urban core of Maricopa County compared to the surrounding natural landscape, because of the high heat retaining capacity of the built environment," said Matei Georgescu, one of the study's co-authors. "Continued growth would enhance this effect in the future leading to further declines in daytime highs and associated declines in health risks. The tradeoff is that nighttime temperatures increase significantly with urbanization, and this nighttime warming is much greater than the expected daytime cooling."

The smallest increase in heat-related mortality was actually a net decrease of about 46 deaths per year. This corresponded to low growth and adaptive measures (the white roofs). "The greatest health concern comes from large expected increases in nighttime temperatures which could be mitigated by lower-growth scenarios," Hondula said. "The next step is to look more closely at the conditions people experience on hot days, to ultimately determine if high maximum temperatures, minimum temperatures, or some combination of the two is the real culprit leading to adverse health events."

The ASU results have ramifications well beyond the Phoenix inferno, particularly in light of global warming. The heat island effect is more complex than it might seem on its face, and that's something planners need to be anticipating.

"Our research shows that if we are interested in combating possible heat-related health impacts arising from urbanization, it would be best to focus on mitigation and adaptation options where the benefits are realized during the nighttime hours and help prevent minimum temperatures from increasing," Hondula added in an email. "We do not expect urbanization to lead to more heat-related deaths associated with increasing maximum temperatures because, in places like Phoenix, urbanization actually can reduce daytime highs."