A recent story on NPR’s Morning Edition piqued my interest in the urban heat island (UHI) phenomenon, which has been recognized since the late 1980s. In the radio piece, the rather startling claim was made that cities are heating up at rates twice as fast as those measured for non-urbanized regions of the planet. The US city of Atlanta, GA is the focus of the NPR story, and Georgia Tech’s Urban Climate Lab director, Brian Stone Jr., is interviewed throughout, so I decided to take a look at the UCL website. The data on urban temperature trends are limited to fifty of the most populous US cities, but nevertheless support the need to develop mitigation strategies to counteract the impact of climate change on cities. For those who are interested in the comparison of warming trends in US cities vs. nearby rural areas, the climate trend map for the years 1961 through 2010 is here. Phoenix, Atlanta, Denver, and Greensboro are among the cities with the most alarming warming patterns.
Atlanta skyline; photo by Chuck Koehler via Wikimedia Commons, under Creative Commons Attribution 2.0 Generic license
Many of us at OT live in large cities, whether in urban or suburban districts, and I was curious about how widespread the UHI phenomena of accelerated warming trends are worldwide. Turns out (not surprisingly) that there’s a huge amount of peer-reviewed literature available, and a variety of consequences for human health, as well as planned and implemented mitigation strategies, for cities across the globe. Wilby (2008) used data from studies by the London Climate Change Partnership to examine existing and projected changes in UHI intensity and ozone pollution in the city. UHI intensity is typically represented by a temperature gradient between an urban station (St. James’s Park in this case) and a rural reference station (Wisley, Surrey – 30 km from the city center), and the average nocturnal UHI for London is +2.2C in August. The factors that contribute to the UHI effect include building materials that retain more solar energy than do surfaces covered with vegetation, lower wind speeds, reduced evapotranspiration (due to a combination of fewer plants and relatively impermeable surfaces), and a concentration of anthropogenic heat flux (air conditioning, transportation, cooking). From long-term records, it’s clear that spring and summer nocturnal UHI intensities in London have increased since the 1960s. Using several different models, Wilby (2008) predicts that, by the 2050s, nightime UHI intensities for London will increase by another 0.5C for the months of May through October, and that there will be more frequent intense UHI episodes on a background of more persistent and extreme European heatwaves. This would mean, for example, that Londoners will experience an average of seven intense UHI episodes each August, compared with the current average of five.
What about the UHI effect in the large cities of Asia, Africa, and South America? Peng and colleagues (2012) analyzed seasonal and diurnal variations in UHI intensity for 419 global big cities, with populations over one million. Rather than relying on air temperature differences measured between urban and rural stations, the researchers used a land surface temperature (LST) data set from satellite remote sensing (MODerate-resolution Imaging Spectroradiometer = MODIS-Aqua) to compare urban and suburban areas, and thus calculate a Surface Urban Heat Island Intensity (SUHII). SUHIIs were computed for nighttime and daytime, and for winter and summer periods. Other parameters, including vegetation, climate, density, and albedo, were defined for urbann and suburban areas.
With the exception of a few cities surrounded by desert (e.g. Jeddah in Saudi Arabia and Mosul in Iraq), annual mean daytime SUHII is positive. Medellín, Colombia has the highest daytime SUHII (7.0C), followed by Tokyo and Nagoya in Japan, São Paulo in Brazil, and Bogatá in Colombia – all with daytime SUHIIs over 5C. Nighttime SUHIIs for most cities are between 0 and 2C, with Mexico City holding the record at 3.4C. Average daytime SUHII over cities in developed countries is higher than that over developing countries, whereas for nighttime SUHII there is no significant difference. The two major sources of energy that contribute to SUHII are downward net solar radiation and anthropogenic heat flux. During the day, evapotranspiration from urban vegetation has a cooling effect and reduces SUHII, whereas albedo and the thermal properties of urban surfaces influence nighttime SUHII. Interestingly, human metabolic heating accounts for only a very small fraction of anthropogenic heat flux – so don’t worry about going for a long run or playing tennis after work.
Obviously, the combination of global warming, an increase in extreme climate events, and accelerated urbanization requires that city planners consider measures to reduce SUHIIs. I hope to discuss a few of these strategies in subsequent posts.
Peng S, Piao S, Ciais P et al. (2012) Surface urban heat island across 419 global big cities. Environmental Science and Technology 46, 696-703.
Wilby RL (2008) Constructing climate change scenarios of urban heat island intensity and air quality. Environment and Planning B: Planning and Design 35, 902-919.