What was done
The authors studied the characteristics of urban heat islands in several cities in Australia with populations ranging from approximately 1,000 to 3,000,000 people.

What was learned
The maximum urban-rural temperature differences of the Australian cities were found to scale linearly with the logarithms of their populations.  The authors noted that the same was true for cities in Europe and North America, but that the heat islands of Australian cities were generally less than those of similar-size European cities (which were less than similar-size North American cities) and that they increased at a slower rate with population growth than did European cities (which increased slower than did cities in North America).  The regression lines of all three continents essentially converged in the vicinity of a population of 1,000 people, however, where the mean urban-rural temperature difference was approximately 2.2 ± 0.2 °C.

What it means
The results of this study suggest that very small towns, with populations measured in mere hundreds of inhabitants, likely have urban heat islands that are on the order of the entire amount of global warming that is believed to have occurred since the end of the Little Ice Age.  With such small aggregations of people having such a dramatic impact on air temperature, it is ludicrous to believe that on top of the natural warming experienced by the earth in recovering from the Little Ice Age we can confidently discern an even more subtle increase in background temperature caused by concomitant increases in greenhouse gas concentrations.  Changes in population, which have generally been positive nearly everywhere in the world over this period, could easily explain whatever tiny bit of warming is left after the natural component of warming (which must be substantial, relatively speaking) is subtracted from the total amount of warming recorded by the totality of earth's thermometers over the past century or so.