Re: Cut the sh*t!
1) Urban stations are individually adjusted to remove the urban heat island effect, based on trends in other local weather stations which are not part of the network. (Denialists know this; this is part of the "The data is worthless because its adjusted" bleat of the denier. They assert that adjusted data is worthless, and likewise if the data is not adjusted then it is worthless, as here because of the UHIs)
"Extensive tests have shown that the urban heat island effects are no more than about 0.05°C up to 1990 in the global temperature records used in this chapter to depict climate change. Thus we have assumed an uncertainty of zero in global land-surface air temperature in 1900 due to urbanisation, linearly increasing to 0.06°C (two standard deviations 0.12°C) in 2000." http://www.grida.no/publications/other/ipcc_tar/?src=/climate/ipcc_tar/wg1/052.htm#2221
2) It's not a oneway process where the monitoring station has become hotter because of more pavement, fewer trees, etc.; in many cases the monitoring station's location has become a park, or has become shadowed by buildings, tree growth, etc. Thus the individual adjustments above.
"Using satellite night-lights-derived urban/rural metadata, urban and rural temperatures from 289 stations in 40 clusters were compared using data from 1989 to 1991. Contrary to generally accepted wisdom, no statistically significant impact of urbanization could be found in annual temperatures. It is postulated that this is due to micro- and local-scale impacts dominating over the mesoscale urban heat island. Industrial sections of towns may well be significantly warmer than rural sites, but urban meteorological observations are more likely to be made within park cool islands than industrial regions." http://www.researchgate.net/publication/252960119_Assessment_of_Urban_Versus_Rural_In_Situ_Surface_Temperatures_in_the_Contiguous_United_States_No_Difference_Found
3) Neither well established urban areas, nor areas which are currently becoming urbanized show a UHI trend that explains the scale of the AGW recorded:
"We show examples of the UHIs at London and Vienna, where city center sites are warmer than surrounding rural locations. Both of these UHIs however do not contribute to warming trends over the 20th century because the influences of the cities on surface temperatures have not changed over this time. In the main part of the paper, for China, we compare a new homogenized station data set with gridded temperature products and attempt to assess possible urban influences using sea surface temperature (SST) data sets for the area east of the Chinese mainland. We show that all the land-based data sets for China agree exceptionally well and that their residual warming compared to the SST series since 1951 is relatively small compared to the large-scale warming. Urban-related warming over China is shown to be about 0.1°C decade−1 over the period 1951–2004, with true climatic warming accounting for 0.81°C over this period." http://onlinelibrary.wiley.com/doi/10.1029/2008JD009916/abstract
