Read here. The 2007 IPCC report went out of its way to diminish the importance of the urban heat island (UHI) effect on global warming. In typical Climategate-style research, the IPCC had to strengthen its case that human CO2 was the primary cause of recent global warming, and the only way to accomplish this was to marginalize and reduce the impact of other factors, including UHI.
A Japanese scientist, unconvinced by the unsubstantiated and non-scientific UHI claims of the IPCC, studied the issue further and has determined that the UHI effect, even in sparsely populated areas, is "statistically significant."
"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.".....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."...that numerous studies have detected heat islands in small settlements "with a population of 1000 or less,"...where statistically significant trends on the order of 0.04°C per decade have been observed." [Fumiaki Fujibe 2011: International Journal of Climatology]
Additional peer-reviewed and UHI postings.