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Urban Heat Islands of Small Towns
Reference
Oke, T.R. 1973. City size and the urban heat island. Atmospheric Environment 7: 769-779.

What was done
The author measured the urban heat islands of ten settlements in the St. Lawrence Lowlands that had populations ranging from approximately 1,000 to 2,000,000 people. He then compared his results with those obtained for a number of other cities in North America and Europe.

What was learned
Over the population range studied, it was found that the magnitudes of the urban heat islands of the cities investigated were linearly correlated with the logarithms of their populations. It was also determined from the urban heat island vs. population relationships defined by these data that at the lowest population value encountered, i.e., 1,000 inhabitants, there was already an urban heat island effect present on the order of 2 to 2.5C.

What it means
The urban heat island effect can greatly perturb a city's temperature record. Hence, in global temperature trend studies, it is necessary to remove whatever temperature increases may have been created by concomitant increases in the populations of the cities from which the temperature histories that make up the global average were acquired; and to make this correction, comparisons are often made between the temperature histories of these cities and nearby small towns, under the assumption that the small towns have negligible heat island effects. As this study clearly demonstrates, however, this assumption is vastly invalid. Consequently, there is ample opportunity for very large errors to occur in attempts to reconstruct true non-urban temperature trends, as towns with as few as 1,000 inhabitants create a warming of the air within them that is over twice as great as the increase in mean global air temperature believed to have occurred since the end of the Little Ice Age. This fact casts a pall of uncertainty over claims that the globe has warmed by even a fraction of a degree over the past century, and especially over the last two decades.


Reviewed 22 November 2000