Background
The putative warming of non-urbanized areas of the planet over the past century is believed to be less than 1°C.  Urban-induced heating in large cities, on the other hand, may be as great as 10°C.  Hence, since nearly all long-term temperature records have been obtained from sensors located in towns and cities that have experienced significant growth over this time period, it is extremely important that urbanization-induced warming - which can be a full order of magnitude greater than the background trend being sought - be removed from the original temperature records when attempting to accurately assess the true warming (or cooling!) of the natural non-urban environment.

What was done
The author used a series of measurements of soil temperatures obtained in a totally rural setting in central Illinois between 1889 and 1952 and a contemporary series of air temperature measurements made in an adjacent growing community, as well as similar data obtained from other nearby small towns, to evaluate the magnitude of unsuspected heat island effects that may be present in small towns and cities that are typically assumed by the IPCC to be free of urban-induced warming.

What was learned
The soil temperatures obtained in the totally rural setting revealed the existence of a temperature increase from the decade of 1901-1910 to that of 1941-1950 that amounted to 0.4°C.  This warming is 0.2°C less than the 0.6°C warming determined for the same time period from data of the U.S. Historical Climate Network, which is supposedly corrected for urban heating effects.  It is also 0.2°C less than the 0.6°C warming determined for this time period by eleven benchmark stations in Illinois with the highest quality long-term temperature data, all of which are located in communities with populations of less than 6,000 people as of 1990.  And it is 0.17°C less than the 0.57°C warming derived from data obtained from the three benchmark stations closest to the site of the soil temperature measurements and with populations of less than 2,000 people.

What it means
In the words of the author, his findings suggest that "both sets of surface air temperature data for Illinois believed to have the best data quality with little or no urban effects may contain urban influences causing increases of 0.2°C from 1901 to 1950."  He further notes - in a grand understatement - that "this could be significant because the IPCC (1995) indicated that the global mean temperature increased 0.3°C from 1890 to 1950."

Clearly, the meticulous efforts of this world-renowned climate specialist - Stanley A. Changnon - call all surface-based global air temperature records into question.  Therefore, until the challenge of very-small-town urban heat island effects is resolved, the climate alarmists' "unprecedented" global warming of the past century cannot be accepted at face value.  In all likelihood, it is artificially inflated, perhaps severely so.

Reference
Intergovernmental Panel on Climate Change.  1995.  Climate Change 1995, The Science of Climate Change.  Cambridge University Press, Cambridge, U.K.