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Human activities are causing large, and readily observable, changes |
An expert
in signal processing research at AT&T Bell
Laboratories has developed a new method for analyzing temperature records
and concludes that human-induced global warming began in "about 1940," in
a paper delivered in December to the American Geophysical Union.
According to the February issue of Scientific American magazine, David J. Thomson has examined historical temperature data from several locations with long past records, such as one in England where temperatures have been kept since 1651. However, instead of looking at temperature averages, he has analyzed the "annual cycle," or timing of the seasons. The annual cycle shifts by about one day per century as the result of a slight, steady natural shift in the earth's rotational motion. But since 1940, it has shown a much larger rate of change that appears to be due to greater radiational heating, consistent with the theory that a rise in heat-trapping "greenhouse" gases in the atmosphere will cause more solar heat to be retained by the earth. According to the abstract of "Precession, Global Temperature, and Carbon Dioxide," Thomson's paper, "Since about 1940, the phase of the annual cycle has been advanced by several days by carbon dioxide. This phase shift appears to result from an increase in the direct radiation component of temperature . . . " Thomson also points out in his abstract that the seasonal change is operating independently of increases in global average temperatures, ruling out changes in the Sun's output as the cause: "[L]ack of coherent variations between the average temperature and the amplitude of the annual cycle eliminates solar variability as a major factor in the last century's warming. "Taken together," Thomson's abstract adds, "these effects force one to conclude that changes in carbon dioxide resulting from human activities are causing large, and readily observable, changes both in the average temperature and in the annual cycle." Further details about Thomson's analysis are not yet available, sources said, because he is awaiting publication of an article on his work in a major scientific journal.
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Tom Gray writes on environmental issues for the American Wind Energy Association.
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