Can the global warming during the early part of the century, from about 1880 to 1940, “be unequivocally related to humaninduced changes in the chemical composition of the atmosphere” [Jones, 1998]?
The IPCC [1996a] arrived at the ambiguous conclusion that “the balance of evidence suggests there is a discernible human influence on global climate,” based on “fingerprints” in the climate record—an increasing correlation (with time) between observed and calculated global temperature patterns. However, this positive trend in correlation depended entirely on the arbitrary choice of the time interval 1940–1990, during most of which temperatures were actually decreasing. A different choice of interval would have produced a zero or even a negative trend. Another piece of evidence cited in the IPCC report to support a human influence depended on showing an increasing temperature trend in the middle troposphere of the Southern Hemisphere (presumably related to the cooling effects of sulfate aerosols in the Northern Hemisphere). Again, this result is related entirely to the particular choice of time interval [Michaels and Knappenberger, 1996]; more complete datasets give a contrary result—a greater warming trend in the Northern Hemisphere.
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