Phoenix stands at a parched crossroads. Global scale climate change is forecast to bring hotter summers and more extreme heat to the Valley, but regional urbanization also will impact temperatures experienced by residents.

So how should Phoenix grow knowing that such growth could cause temperatures to increase in the future and bring added health risks? Should the city deploy mitigating technologies to help fight summer's heat? Would adopting a low-growth strategy reduce the adverse health consequences of hot weather?

New Arizona State University research examines the heat-health aspects resulting from urbanization and the challenge of sustainable future growth in Maricopa County.

A study released this week shows how urban development could be a factor in the number of lives lost due to heat in future summers. The study is described in the article, "Challenges associated with projecting urbanization-induced heat-related mortality" published in the current online issue of the journal Science of the Total Environment.

"Extreme heat is the leading weather-related killer in the United States," said David Hondula, a postdoctoral scholar in health informatics in ASU's Center for Policy Informatics and lead author of the study.

"In Maricopa County, 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."

In the research, the team tried to quantify the number of excess deaths attributed to heat in Maricopa County based on three future urbanization and adaptation scenarios and multiple exposure variables. Two scenarios (low and high growth projections) represent the maximum possible uncertainty range associated with urbanization in central Arizona; a third represents an adaptation strategy by simulating the deployment of white roof technology to the area.

The researchers - in addition to Hondula included Matei Georgescu and Robert C. Balling Jr., both of ASU's School of Geographical Sciences and Urban Planning - related temperature to mortality using historical data from 1983 to 2007.

Regional climate model simulations based on 2050-projected urbanization scenarios for Maricopa County generated distributions of temperature change, and from these changes in future excess heat-related mortality was estimated. They studied Maricopa County because it is a fast growing metropolitan area situated in a semi-arid region that experiences "chronic" heat during the summer months.

Overall, projections of heat related mortality ranged from a decrease of 46 deaths per year (-95 percent) to an increase of 339 deaths per year (+359 percent).

Projections based on minimum temperature showed the greatest increase for all expansion and adaptation scenarios and were substantially higher than those for daily mean temperature. Projections based on maximum temperature were largely associated with declining mortality. Low growth and adaptation scenarios led to the smallest increase in predicted heat related mortality based on mean temperature projections.

Because of the environment in which it is built, increases in overnight minimum temperatures in Maricopa County associated with urbanization were found to be of much greater concern for health impacts compared to increases in daytime maximum temperatures. The same would be true in many other cities located in semi-arid regions.

"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," Matei Georgescu said.

"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."

Hondula added that what this means for planners is that because heat impacts vary from day to night, projections of heat related health outcomes that do not consider place-based, time varying urban heat island effects are "neglecting essential elements for policy relevant decision-making."

"The manner in which the Sun Corridor develops over the next several decades will impact the regional climate and, if no new adaptation measures are introduced, change the health risks for Maricopa County residents associated with extreme heat," Hondula said.

"The greatest health concern comes from large expected increases in nighttime temperatures which could be mitigated by lower-growth scenarios."

"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," he added.