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How much has Chicago’s temperature risen since 1900?

Dear Tom,
Earth’s temperature has risen about one degree since 1900. How much has Chicago’s temperature risen?
James Longstadt, South Bend, Ind.
Dear James,
Chicago’s weather records show that temperatures here have risen about two degrees since 1900, but that rise is not necessarily due to global warming. Local non-meteorological factors have contributed. Cities are “heat islands” and Chicago’s urban growth since 1900 has speeded the warming. Also, the weather observation site has been moved from sites in the Loop to the University of Chicago, then to Midway Airport and now to O’Hare. In addition, there have been changes in thermometers and in their sensitivity, and changes in observation techniques. Each of these factors has disrupted the continuity of our temperature data, making it very difficult to compare one portion of the data set with another.

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