By B. Rose Kelly, Woodrow Wilson School
Heat waves are among the deadliest and most common of environmental extremes. As the earth continues to warm due to the buildup of greenhouse gases, heat waves are expected to become more severe, particularly for cities, where concrete and a dearth of trees create what’s known as the urban heat island effect.
Using a global climate model, a team led by Princeton University researchers measured how severely heat waves interact with urban heat islands, now and in the future, in 50 American cities across three climate zones.
Currently, eastern and southeastern American cities are more severely affected by heat waves — in terms of relative temperature increase — than arid and semiarid western cities. This is because of the amount of impenetrable, concrete surfaces and lack of moisture in eastern and southeastern cities compared to their rural surroundings. In contrast, both rural and urban dry environments experience similar temperatures, and both have less annual rainfall than their eastern and southeastern counterparts.
By 2100, however, this is expected to flip, the researchers reported in the journal Environmental Research Letters. Arid cities such as Phoenix will become more susceptible to heatwaves compared to surrounding rural areas, while cities on the eastern seaboard will be less severely affected by heatwaves compared to theirs. This is because future arid cities will remain water-limited due to the lack of permeable surfaces in cities, while their rural neighbors are projected to be no longer “dry” due to higher rainfall. The overuse of air conditioners also emits heat into the urban heat islands, playing a significant role.
The findings highlight the importance of heat-mitigation strategies and infrastructures such as green roofs, said first author Lei Zhao, a postdoctoral research associate in Princeton’s Program in Science, Technology, and Environmental Policy (STEP), which is based at the Woodrow Wilson School of Public and International Affairs. Zhao also is a research scholar associated with the Princeton Environmental Institute's (PEI) Carbon Mitigation Initiative, which supported this research.
“Given that 50 percent of the world’s population currently lives in cities, and that percentage is projected to increase to 70 percent by 2050, there is a pressing need to understand how cities and landscapes are affected by heat waves," Zhao said. "Our study explains why cities suffer even more during extreme heat events and highlights the heat risks that urban residents face now and in the projected future.”
“Our findings underscore the importance of implementing heat-mitigation strategies today,” said co-lead author Michael Oppenheimer, the Albert G. Milbank Professor of Geosciences and International Affairs and the Princeton Environmental Institute. "They also highlight the need for more studies of this nature to give us a better idea of the cities and landscapes that are most affected now and also under additional greenhouse warming."
Princeton co-authors also included Jane Baldwin, a graduate student in the Program in Atmospheric and Oceanic Sciences who studies heat waves as a 2015-17 PEI-STEP Fellow under Oppenheimer; and PEI associated faculty Elie Bou-Zeid, associate professor of civil and environmental engineering.
The paper, “Interactions between urban heat islands and heat waves,” first appeared online as an accepted manuscript in Environmental Research Letters Dec. 6, 2017. The research was supported by PEI's Carbon Mitigation Initiative, a High Meadows Foundation Fellowship in STEP, and the National Science Foundation.