Read here. The Urban Heat Island effect is known to cause significant warming in temperature datasets. Yet leading IPCC climate scientists never seem able to find it - always just beyond their grasp, or is it their comprehension, as a source of warming. Instead, it's been much more rewarding and convenient to rely on CO2-caused rationale for warming trends. Climategate's Phil Jones was a leading practitioner of missing the obvious UHI effect, as was shown also in his South African research on temperatures.
"Hughes and Balling report that the mean annual air temperature trend of the five large cities averaged 0.24°C per decade, while the mean warming rate of the 19 non-urban centers was a statistically insignificant 0.09°C per decade over the 1960-1990 period, which values are to be compared to the overall warming rate of 0.31°C per decade that was derived by Jones for the entire country....suggest that urbanization has influenced the Jones (1994) records for South Africa over the 1960-1990 period of apparent rapid warming....and that their analyses suggest that "half or more of this recent warming may be related to urban growth, and not to any widespread regional temperature increase."