Background: Extreme heat events are known to be increasing in frequency in large U.S. cities and are responsible for a greater annual number of climate related fatalities, on average, than any other form of extreme weather. In addition, low density, sprawling patterns of urban development have been associated with enhanced surface temperatures in urbanized areas.
Objectives: This study examines the association between urban form at the level of the metropolitan region and the frequency of extreme heat events over a five decade period.
Methods: The authors employ a widely publish metric of urban form (a “sprawl” index) to measure the association between urban form in 2000 and the mean annual rate of change in extreme heat events between 1956 and 2005.
Results: Their results find the rate of increase in the annual number of extreme heat events between 1956 and 2005 in the most sprawling metropolitan regions to be more than double the rate of increase observed in the most compact metropolitan regions.
Conclusions: The design and management of land use in metropolitan regions may offer an important tool for adapting to the heat-related health effects associated with ongoing climate change.
The number of very hot days is increasing worldwide, but the rate of increase is more than double in the most sprawling metropolitan regions compared with more compact cities, according to a team of Atlanta-based scientists. This was true regardless of the urban regions’ climate zone, population size or rate of growth.
The study examined the number of very hot days in 53 U.S. metropolitan regions between 1956 and 2005. The annual number of very hot days increased by 14.8 days on average in the regions with the most sprawl and by 5.6 days in the least sprawling cities. A metropolitan region, as defined by the U.S. Census Bureau, may include many counties surrounding the city. The Atlanta metropolitan region, for example, has 20 counties.
“These findings show that the pace of climate change is greater in sprawling cities than in others, which has not been shown before,” says lead author and urban planner Brian Stone of the Georgia Institute of Technology. “Because severe heat kills more people on average per year than any other type of dangerous weather, residents of sprawling cities may be more vulnerable to this significant health threat posed by climate change.”
Sprawl and land use regulations appear to influence the frequency of very hot days through their effect on a city’s trees and other vegetation. The team found that between 1992 and 2001, the rate of deforestation in the most sprawling metropolitan regions was more than double that of compact regions. Other studies have shown that the loss of vegetative cover is one of the main reasons that cities become much hotter than surrounding areas.
Extremely hot days were identified using a city-specific heat stress index that the National Climatic Data Center (NCDC) developed for 187 U.S. cities. The NCDC calculated the average apparent temperatures, which include temperature and humidity, for those cities between 1961 and 1990. Very hot temperatures are defined as those above the 85th percentile, with the 100th percentile being the highest temperature reached during the study period for that city. Temperatures above the 85th percentile are associated with more heat-related deaths, other studies show.
To compare cities’ development patterns, the researchers used a widely publish metric developed in 2003. Called the sprawl index, it incorporates land use data from the 2000 census to quantify factors such as population density, the proximity of commercial and residential buildings, and street network patterns. The researchers categorized a region as one of the most sprawling if it was in the top 25 percent of the index and as one of the least sprawling if it was in the bottom 25 percent.
Examples of sprawling metropolitan regions include Atlanta, Tampa and Grand Rapids, whereas Chicago, Boston and Baltimore are more compact. “Factors that affect whether an area remains compact include, among others, local land use regulations and the timing of a city’s growth,” Stone says. “Boston grew when street-cars were popular, and Atlanta developed during the era of the automobile.”
The study is published online on June 23 ahead of print in the peer-reviewed journal Environmental Health Perspectives (EHP). Other authors of the study are Jeremy Hess, Rollins School of Public Health at Emory University, and Howard Frumkin, National Center for Environmental Health at the U.S. Center for Disease Control and Prevention.
The full article, “Urban Form and Extreme Heat Events: Are Sprawling Cities More Vulnerable to Climate Change than Compact Cities?”, is available online at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2957923/.
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