Correspondence/Rebuttal pubs.acs.org/est
Comment on “Surface Urban Heat Island Across 419 Global Big Cities”
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n their recent global analysis of urban climate, Peng et al.1 related the surface urban heat island intensity (SUHII) of 419 big cities to various factors such as city size, population density, and the difference of vegetation cover. Using remotely sensed land surface temperatures, they found a variety of relationships and conclude that urban vegetation is relevant for mitigating the SUHII. Both the study itself but even more so the discussion of results show weaknesses that are likely leading to confusion for an interdisciplinary readership that is not familiar with the subject. This criticism concerns three aspects.
3. INTERPRETATION OF POPULATION DENSITY Peng et al. did not find a relationship between population density and the SUHII difference between cities. They conclude “that metabolic heating, about 100 W per person, accounts for only a very small fraction of the urban anthropogenic heat flux” (ref 1, page 700). While this statement is true, it unfortunately misses the point of the relation of population density and the urban heat island. Population density merely is used as a proxy for anthropogenic heat fluxes stemming, for example, from transportation, heating, industry and the like as well as building density. Building density has implications especially for the vertical structure of a city and canyon geometries: High vertical structures provide more surface area and create multiple reflections.5 To conclude, the study itself touches a very relevant topic, as the urban heat island effect is an important aspect to consider in terms of climate change adaptation and enhancing human well-being in cities. A lot of work has obviously been done to acquire and prepare data. However, the analysis has severe shortcomings, such as the lack of controlling for latitude as well as the sample design regarding the relationship of city size and SUHII. What is more, the uncommented parts of the study design (developing versus developed countries) and the discussion (population density) likely mislead readers. A clarification of these issues is crucial, especially for an interdisciplinary audience. Nina Schwarz* UFZ − Helmholtz Centre for Environmental Research, Department Computational Landscape Ecology Permoserstrasse 15; 04318 Leipzig, Germany
1. DIVISION OF CITIES The authors differentiated cities belonging to developing and developed countries, respectively, and found a significantly higher annual daytime SUHII over developed countries than over developing countries. Unfortunately they do not provide any reasoning for this differentiation, nor do they offer an interpretation. I hypothesize that this seeming influence of development status onto SUHII is in fact the well-documented influence of latitude onto the SUHII.2 Thus, it would have been helpful to use latitudes or climate zones for the analysis instead of the development status of the respective countries, the latter probably leading to spurious relationships. Providing these results without putting them into context poses more questions than it answers. 2. EFFECTS OF CITY SIZE The authors did not find a strong relationship between city size and SUHII; less than 3% of variance in SUHII was explained on the global scale, and 16% for the European cities in the data set. The authors deem factors such as different countries, different climatic zones, and different economic development relevant for masking the effect of city size on SUHII. However, they only considered global big cities with more than one million inhabitants. Thus, they did not make use of the full range of possible city sizes. The different findings for the global sample and the European subset also point in this direction, as the latter consists of more small cities compared to the overall sample. Thus, the study design probably did not allow for finding a strong relationship between the two variables. The authors cite refs 3 and 4 as studies that found much higher relationships. While Oke4 in his review of the urban energy balance does not report a relationship between city size and urban heat island intensity, Imhoff et al.3 analyzed urban heat islands in the U.S. with a focus on the 38 largest U.S. cities. When investigating the relationship between city size and SUHI, however, they specifically added random cities for this part of the study. Following this procedure, their data set also included small cities having less than 10 km2 of more than 25% impervious surface and thus included a broader range of city sizes. © 2012 American Chemical Society
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AUTHOR INFORMATION
Corresponding Author
*Phone +49 341 235 1970; fax +49 341 235 1939; e-mail nina.
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Peng, S.; Piao, S.; Ciais, P; Friedlingstein, P.; Ottle, C; Bréon, F.M.; Nan, H.; Zhou, L.; Myneni, R. Surface urban heat island across 419 global big cities. Environ. Sci. Technol. 2012, 46 (2), 696−703. (2) Roth, M. Review of urban climate research in (sub)tropical regions. Int. J. Climatol. 2007, 27, 1859−1873. (3) Imhoff, M. L.; Zhang, P.; Wolfe, R. E.; Bounoua, L. Remote sensing of the urban heat island effect across biomes in the continental USA. Remote Sens. Environ. 2010, 114 (3), 504−513. (4) Oke, T. R. The urban energy balance. Prog. Phys. Geogr. 1988, 12 (4), 471−508. (5) Oke, T. R. The energetic basis of the urban heat-island. Q. J. R. Meteorol. Soc. 1982, 108 (455), 1−24.
Published: May 17, 2012 6888
dx.doi.org/10.1021/es301245j | Environ. Sci. Technol. 2012, 46, 6888−6888
