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Global Warming and Urban Heat Islands
Reference
Fujibe, F. 2011. Urban warming in Japanese cities and its relation to climate change monitoring. International Journal of Climatology 31: 162-173.

Background
The author writes that "in the context of global climate change, urban warming can bias results obtained for background monitoring, as many of the observatories that have been in operation for a long time are located in cities." Nevertheless, and in spite of this fact, Fujibe notes that the IPCC (2007) has suggested that "the globally averaged temperature trend is hardly affected by urbanization."

What was done
Unconvinced of the validity of the IPCC's assertion, the Japanese researcher reviews what is known about the subject based on research conducted in Japan.

What was learned
Fujibe reports that "the recorded rate of temperature increase is a few degrees per century in large cities and tends to be larger at night than during the daytime." In some cities, in fact, Fujibe writes that "the increase in annual extreme minimum temperature exceeds 10°C per century." And at the other end of the population spectrum, Fujibe notes that numerous studies have detected heat islands in small settlements "with a population of 1000 or less," as reported by Tamiya (1968), Tamiya and Ohyama (1981), Sakakibara and Morita (2002), Sakakibara and Kitahara (2003) and Sakakibara and Matsui (2005), where statistically significant trends on the order of 0.04°C per decade have been observed.

What it means
The take-home message of Japanese research into the urban heat island phenomenon, according to Fujibe, is that "urban warming can be a biasing factor that may contaminate data used for monitoring the background temperature change," with sparsely populated sites with population densities of 100-300 people per square kilometer "showing a statistically significant anomalous trend of 0.04°C per decade."

References
IPCC. 2007. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the IPCC, Solomon, S., Qin, D., Manning, M., Chen, Z. Marquis, M., Averyt, K., Tignor, M. and Miller H.L. (Eds.). Cambridge University Press, New York, New York, USA.

Sakakibara, Y. and Kitahara, Y. 2003. Relationship between population and heat island intensity in Japanese cities. Tenki 50: 625-633.

Sakakibara, Y. and Matsui, E. 2005. Relation between heat island intensity and city size indices/urban canopy characteristics in settlements of Nagano basin, Japan. Geographical Review of Japan 78: 812-824.

Sakakibara, Y. and Morita, A. 2002. Temporal march of the heat island in Hakuba, Nagano. Tenki 49: 901-911.

Tamiya, H. 1968. Night temperature distribution in a new-town, western suburbs of Tokyo. Geographical Review of Japan 41: 695-703.

Tamiya, H. and Ohyama, H. 1981. Nocturnal heat island of small town, its manifestation and mechanism. Geographical Review of Japan 54: 1-21.

Reviewed 30 March 2011