The Urban Heat Island Effect (UHIE) is a phenomenon in which the concentration of structures and waste heat from human activity (mostly air conditioners and automobiles) results 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 Siberia, Alaska and Northern Canada 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. The greatest temperature difference is in areas of the world that are the least populated.