The Urban Heat Island Effect (UHIE) is a phenomenon in which cities give out heat (From mostly roads, rooftops, automobiles, and air conditioners) resulting in slightly warmer air over cities and suburban areas when compared to surrounding rural areas.
It has been suggested that UHIE has significantly influenced temperature records over the 20th century when there was rapid growth of urban environments. This, some Skeptics claim, is the reason for global warming.
However, the temperature map below, put out by the Goddard Institute for Space Studies, shows that temperatures are much warmer in Alaska and Siberia where there are very few and small urban centers.
Figure 1: A temperature anomaly map from the Goddard Institute for Space Studies for February 2016.