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Friday, May 31, 2013

Cities to get much hotter as heatwaves amplify Urban Heat Island Effect

A new study just published looked at the way Urban Heat Island effect interacts with heatwaves. It is not a simple addition of the heatwave increase in temperature added to the urban heat island temperature: heatwaves exacerbate and amplify the Urban Heat Island Effect so that the impact is magnified. This is a major energy use and health concern for people living in cities, particularly the poor and vulnerable.

"Not only do heat waves increase the ambient temperatures, but they also intensify the difference between urban and rural temperatures. As a result, the added heat stress in cities will be even higher than the sum of the background urban heat island effect and the heat wave effect." say the researchers, Dan Li and Elie Bou-Zeid, both from the Department of Civil and Environmental Engineering, Princeton University

As the climate warms heatwaves like the January extreme temperatures for Australia in 2013, the lethal heatwave in the US in 2011, or the Russian heatwave of 2010, will occurr more frequently with greater intensity. Their effect will be magnified in the cities causing an increase in energy use, as well as impacting population health through heat stress and heat related mortality.

The urban Heat Island effect has been known and measured for an extensive period. Dr. Ping Zhang from NASA Goddard Space Flight Center explains in this video the dynamics of urban heat islands.

The researchers from Princeton University in this new study identified that the added impact of heat waves on urban areas can be attributed to reduced surface moisture in urban areas and the low wind speed associated with heat waves.

Global Warming is set to exacerbate Heat related deaths in New York and other cities. Heatwaves in cities will also increase heat stress resulting in reduced wotk capacity. For those unprepared for extreme heat events, heat stress can approach and pass the physiological limits of human health in coping with temperature extremes.

"Given that heat waves are projected to become more frequent and that urban populations are substantially increasing, these findings underline the serious heat-related health risks facing urban residents in the 21st century. Adaptation and mitigation strategies will require joint efforts to reinvent the city, allowing for more green spaces and lesser disruption of the natural water cycle." warns the study.

White roofs, greater vegetation and increase in air conditioning

Painting roofs white and planting more trees and green spaces in our cities may help to slightly mitigate the Urban Heat Island effect, the more important long term activity to limit urban temperatures is to reduce greenhouse gas emissions. Painting roofs white may have a very local impact in moderating temperature, but on a wider regional level has a negligible effect according to Stanford University Professor of civil and environmental engineering Mark Z. Jacobson. He advocates rather than painting your roof white, install solar panels and reduce your carbon footprint.

In September 2012 researchers at Arizona State University (ASU) warned that changing the surface albedo such as using more white roofs while reducing local temperatures can result in consequences for the hydrological cycle, reducing regional rainfall. (Georgescu et al 2012)(Full Paper)

"We found that raising the reflectivity of buildings by painting their roofs white is an effective way of reducing higher average temperatures caused by urban expansion," said Matei Georgescu, an assistant professor in ASU's School of Geographical Sciences and Urban Planning. "However, increased reflectivity also modifies hydroclimatic processes and, in the case of the 'Sun Corridor,' can lead to a significant reduction of rainfall. Our maximum Sun Corridor expansion scenario leads to a 12 percent reduction in rainfall, averaged across the entire state. Painting roofs white leads to an additional 4 percent reduction in rainfall."

Research explained at the AGU 2010 Fall Meeting - Press Conference outlined that for every 1% extra vegetation in an urban area you have a 0.2 degree C reduction in temperature. Heatwave conditions like in Paris in August 2003 indicate it was not so much the maximum daytime temperatures during a heatwave that caused increased heat related deaths, but high nocturnal minimum temperatures which over several days increased physical stress on human bodies resulting in increased mortality.

While air conditioning might be used for relief widely in cities, this adds substantially to energy usage placing more demands on our energy grid, and increases outside air temperature through dumping waste heat through heat exchange, especially in high density built up areas. It sets up a vicious cycle of temperature increase due to Air conditioning use leading to increasing air cooling demand. (C. De Munck et al 2012)(PDF)

You can watch the 50 minute media conference from the AGU 2010 Fall Meeting on recent Urban Heat Island research. The press conference featured: Ping Zhang, Research Scientist, NASA Goddard Space Flight Center/Earth Resource Technology, Greenbelt, Maryland, USA; Benedicte Dousset, Researcher, Hawai`i Institute of Geophysics and Planetology, University of Hawai`i at Manoa, USA; and Cécile De Munck, Scientist, National Centre of Meteorological Research (CNRM), Météo-France, France.


  • Dan Li and Elie Bou-Zeid, Synergistic Interactions between Urban Heat Islands and Heat Waves: the Impact in Cities is Larger than the Sum of its Parts, (abstract), Journal of Applied Meteorology and Climatology 201, May 2013, doi:
  • Cecile De Munck et al (2012) - How much can air conditioning increase air temperatures for a city like Paris, France? (abstract), International Journal of Climatology, 33 : 210 - 227 (2013) DOI: 10.1002/joc.3415
  • ASU News, September 7, 2012 - Researchers emphasize need for evaluation of tradeoffs in battling urban heat islands
  • Graphic of Urban Heat Island profile courtesy the US EPA

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