Increasing global temperatures are expected to increase the occurrence of extreme heat events (EHEs) such as heat waves. These events are the cause of hundreds of deaths each year in the Unites States, particularly in the old, young, ill, and homebound. Heat related deaths occur most often in cities as a result of what is called the urban heat island effect, through which urban areas experience higher temperatures in comparison to surrounding rural area. This effect is caused by lack of vegetation, surfaces that reflect heat (pavement for example), and building layouts that trap heat. In the last half century, cities have been growing in a way that has increased area and decreased land-use density. This pattern is called urban sprawl. Urban sprawl has been associated with higher surface temperatures in cities, which has provoked research into the correlation between urban design and the probable occurrence and intensity of EHEs; which would be a direct link between human health and our activity towards the environment.
To determine if urban sprawl does in fact increase the occurrence of EHEs, a team of researchers from the Georgia institute of technology and Emory University looked at increasing rates of EHEs in different metropolitan centres over within the past five decades (1956-2005). To relate the data on EHEs to urban sprawl, the researchers used a sprawl index to quantify land-use in the cities studied. This index was based on connectivity, centeredness, density, mix of land uses. Data on EHEs was taken from a National Climatic Data Centre heat stress index, which describes an EHE as any time that apparent temperature is greater than the 85th percentile of the base period. Only cities which had recorded EHE data for at least 42 of the 50 year period were used in statistical analysis. Yearly changes in occurrences of EHEs were averaged for each city and the correlation with the sprawl index was measured (influence of population size and growth was controlled in analysis).
The study found that EHEs have been occurring more frequently in all cities looked at, increasing at a yearly rate, such that on average, a city experienced 10 more EHEs in 2005 than in 1956. Cities that fell in the top quartile on the sprawl index saw an increase in events by 14.8 days, whereas cities that fell in the bottom quartile only saw an increase of 5.6 days. These findings confirmed that there is some connection between urban sprawl and increased rate of EHEs observed.
The authors of the report suggest that a possible cause of this relationship is the effect urban sprawl has on vegetative land cover. Between 1992 and 2001, regions in the top quartile or the sprawl index more than double the area of forests compared to those in the bottom quartile.
The findings of this study tell us that we should be reconsidering the manner in which we expand and develop our urban areas, and should be taken into consideration for future development. Decreasing urban sprawl and adjusting to a more compact urban design can help decrease the number of EHEs. Some things that can be done to lesser the magnitude of the urban heat island effect include the use of more reflective surfaces, use of green roofs, integration and preservation of vegetation, and decreasing the dependency on vehicular travel. Not only will these strategies decrease temperature spikes in urban settings, but can also decrease pollution, promote physical activity and thus decrease obesity, and decrease road traffic injuries.
The study only looked at the occurrence of EHEs compared to the urban sprawl index and did not look at heat-related mortality. The authors note that incidences of heat-related deaths and injuries in the United States have remained fairly level despite rising temperatures. This could be explained by the presence of protective factors such as air conditioning in homes. However he authors note that if the effects of urban sprawl continue to increase, our ability to counteract them on a personal level stating that “as Shanghai’s urban heat island has grown, heat related mortality rates have increased. This finding suggests that there is the potential for a similar trend in association with urban sprawl (Tan et al. 2010) – a question that deserves further study”.
This study was funded by the national Centre for Environmental Health, and U.S. Centres for Disease Control and Prevention. It was published in the Journal Environmental Health Perspectives and can be found here: http://ehp03.niehs.nih.gov/article/fetchArticle.action?articleURI=info%3Adoi%2F10.1289%2Fehp.0901879