As climate change continues to bake the Earth, it is not merely the presence of greenhouse gases in the atmosphere that is heating our cities. In many cases, human-made infrastructure is exacerbating or even making our cities more uninhabitable.
Indeed, as the world warms, something called the "heat island effect" is a major threat to countless cities. The heat island effect is a phenomenon in which urban areas experience higher temperatures than the areas adjacent to them. It is typically caused by infrastructure, like buildings and roads, absorbing excess heat; they retain that heat that they absorb during the day and keep cities hot, even at night time. This is why the summer overnight low in cities like Phoenix, Ariz., is often 90° Fahrenheit or higher.
"Climate stress does not affect everyone equally, and those with more resources will be able to protect and sustain their way of life for much longer than other poorer, and more vulnerable populations."
For sprawling cities with lots of paved land, the heat island effect is going to be pervasive — and in the United States, this is true perhaps nowhere more so than in desert cities like Phoenix.
As a result — and in the not-too-distant future — Phoenix will likely be uninhabitable. Scientists can not say for sure when that will happen, but they do know what the signs will be — and they even have a vague sense of how a depopulation scenario might play out.
B.D. Wortham-Galvin, Director and Associate Professor at Clemson University's Master of Resilient Urban Design, observed that "what the science does say is that the temperature limit of human tolerance is around 95 degrees Fahrenheit. Above that temperature humans cannot shed heat well enough to maintain core temperature ... if you can't cool down that is when brain and organ damage begins."
One important metric is wet bulb temperature, or the temperature of a wet thermometer in a shaded area while water evaporates freely off of its surface. As University of Arizona geosciences professor Peter W. Reiners wrote for Salon last year, "It is important to understand that wet bulb temperatures of 95 °F (35 °C) are not conditions we can just get used to. Human bodies have fundamental physiological limits; our planet's perturbed, angry climate doesn't care about them."
Reiners added, "Air conditioning may save some, but increased demand and likelihood of outages in already strained power grids makes this a risky bet at best."
Dr. Andrew Ross, a professor at New York University and author of a book on Phoenix's environmental politics titled "Bird on Fire: Lessons from the World's Least Sustainable City," argued that social scientists might be in the best position to figure out when different groups of people will be unable to survive any more due to the heat island effect.
"Climate stress does not affect everyone equally, and those with more resources will be able to protect and sustain their way of life for much longer than other poorer, and more vulnerable populations," Ross wrote to Salon. "In addition to rising temperatures, factors like access to water supply, occupation of sheltered and higher land, and the financial ability to secure resources will be at play. There are already groups (the unhoused and those without A/C) who are suffering climate stress, so, for them, uninhabitability is not a future horizon."
In other words, the city's population might decline in a slow trickle, with the rich (meaning, those who can afford excellent insulation, constant air conditioning, and so on) leaving last. Dr. Juan Declet-Barreto, a senior social scientist at the Union of Concerned Scientists, echoed Ross' concerns about the plight of vulnerable communities.
"Climate stress does not affect everyone equally, and those with more resources will be able to protect and sustain their way of life for much longer than other poorer, and more vulnerable populations," Ross wrote to Salon.
"I don't think that Phoenix will be abandoned as you often see in post-apocalyptic or science fiction films or literature," Declet-Barreto wrote to Salon. "I think that without short-term adaptation measures to protect the most vulnerable, what will happen is that the gap between populations with social, economic, and technological resources to avoid the worst of extreme heat impacts, and those without those resources, will continue to widen."
Declet-Barreto pointed to Maricopa County as one example, where there are hundreds of heat-related deaths every year.
"A study I co-authored in 2013 revealed that a large proportion of heat-related deaths in Maricopa County ocurred among persons experiencing homelessness for example, and another 2016 study found that population vulnerability to heat, and not weather conditions, were responsible for the spike in heat-related deaths that year," Declet-Barreto explained.
In terms of when a point might be reached where Phoenix is downright uninhabitable, Ross told Salon that "some might say that a limit is reached when the temperature no longer falls below 100° F on a given night, but that would not be 'official.' For others, the point of no return might be when Arizona's reduced share of the Colorado water impacts city dwellers."
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While Phoenix is not quite at the point where it's beyond saving, only radical infrastructure changes are likely to do the trick.
"The expansion of concrete infrastructure from mid-20th century to today needs to be reversed; it is what is trapping the heat," Wortham-Galvin told Salon. "The green/landscape areas that were reduced need to be brought back as critical green infrastructure. Policy changes and new regulations about how housing, commercial, industrial, and institutional buildings are built must include green infrastrcuture at every scale. Vertical gardens, roof gardens, rain gardens are all easily implements, but they have to be part of a mandate, not just left up to the market."
"Without short-term adaptation measures to protect the most vulnerable, what will happen is that the gap between populations with social, economic, and technological resources to avoid the worst of extreme heat impacts, and those without those resources, will continue to widen."
Wortham-Galvin also called for reforms such planting trees throughout the city in public rights of way and publicly owned areas, as well as using alternatives to asphalt and finding nature-based solutions when designing and building future structures. Barring that, as Ross pointed out, it is difficult to imagine a future where Phoenix could actually be inhabited.
"The Hohokam, who preceded modern Phoenicians, sustained themselves as desert farmers for a thousand years, so it's plausible," Ross recalled, adding that the Hohokam did not have 4 million people drawing on the same comparatively scarce resources — and, in particular, did not have to rely as heavily on building new homes as their population continued to grow. "The larger question is which populations in other parts of the world will suffer if our carbon-intensive civilization continues to support unsustainable development in inhospitable locations. So, at what costs to others if places like Phoenix could be saved?"
Declet-Barreto struck a more hopeful note about Phoenix's future.
"Phoenix and the state of Arizona need to do their part to mitigate carbon emissions to curb climate change, but also need an ongoing commitment to heat adaptation by putting in place measures to help the population adapt to unavoidable heat conditions that will persist for a long time," Declet-Barreto explained. "The City of Phoenix is taking heat mitigation seriously by creating one of the first heat mitigation and response offices at the municipal level in the US. For example, Phoenix's climate action plan includes the creation of 'cool corridors' that consist of canopied vegetation to create shaded pedestrian corridors. These measures are being planned together with heat health and climate scientists, experts in diverse related fields, and, critically, community members with long-standing expertise and knowledge of their neighborhoods and most pressing issues."
If the city is unable to mitigate heat through drastic measures — will the last person to leave Phoenix please turn off the sun?
