Climate Change Prediction

In 1975, Science published my paper “Climate change: Are we on the brink of a pronounced global warming?” ([1][1]). This paper was prompted by the observation that despite an appreciable buildup of carbon dioxide in the atmosphere, between 1945 and 1975 there had been no perceptible global warming. Rather, a slight cooling occurred. I suspected that this response failure might be the result of a natural cooling that countered the expected greenhouse gas warming. As, at that time, few quantitative climatic records extending back beyond 100 years existed, I was struck by the analysis of the Camp Century Greenland ice core oxygen-18 record published by Dansgaard and his colleagues in 1973 ([2][2]). It showed prominent 80- and 180-year cycles. So, I made a large leap of faith and assumed these cycles to be global. I then proceeded to match the amplitudes of the expected exponential temperature rise resulting from the carbon dioxide buildup with that for Dansgaard's combined 80- and 180-year cycle in such a way that the sum of the two curves cooled slightly between 1945 and 1975. I then extended both curves into the future and made the prediction that, once the natural cooling had bottomed out, the two trends would join forces and we would experience a substantial warming. As the minimum in the Dansgaard curve came in 1975, a prominent warming was predicted for the 1980s and 1990s. At my request, a colleague, Arthur Greene, matched the updated Hansen-Lebedeff ([3][3]) global temperature record with the graph given in my 1975 paper. As can be seen in the figure below, my prediction was pretty much on target. Since 1976, the mean global temperature has undergone a steady climb. Does this mean that Dansgaard's cycles are influencing global climate? As no convincing verification of the existence of climate cycles with these periods has appeared during the 23 years that have elapsed since my paper was published, the answer is likely “no.” However, the documentation of a strong 200-year and a weak 88-year cycle in the atmosphere's carbon-14 content ([4][4]) suggests that the solar wind and its associated perturbation of Earth's magnetic shielding has fluctuated on roughly these time scales. But the solar energy output fluctuations associated with sun spot activity are generally thought to be too small to cause temperature changes of the required amplitude. Further, the absence of evidence for comparable fluctuations in the elevation of snow lines in the European Alps or of the sea ice cover surrounding Iceland on these time scales during the period 1600 to 1850 suggests that the warming experienced during the last century was a one-shot event, rather than a continuation of a persistent cycle. If nothing else, this historic piece of luck reminds us that a very important issue in the global warming debate remains unresolved. Until a satisfactory explanation has been established for the pronounced demise of the Little Ice Age during the period 1870 to 1940, adequate room for maneuvering will exist for those who doubt that the buildup of carbon dioxide and other greenhouse gases constitutes a substantial threat. 1. [↵][5]1. W. S. Broecker , Science 189, 460 (1975). [OpenUrl][6][Abstract/FREE Full Text][7] 2. [↵][8]1. W. Dansgaard, 2. S. J. Johnsen, 3. H. B. Clausen, 4. N. Gundestrup , Medd. Gronland 197, 1 (1973). [OpenUrl][9] 3. [↵][10]1. J. Hansen, 2. S. Lebedeff , Geophys. Res. Lett 15, 323 (1988). [OpenUrl][11][CrossRef][12][Web of Science][13] 4. [↵][14]1. C. P. Sonett, 2. M. S. Giampapa, 3. M. S. Matthews 1. P. E. Damon, 2. C. P. Sonett , The Sun in Time, C. P. Sonett, M. S. Giampapa, M. S. Matthews, Eds. (Univ. of Arizona Press, Tucson, 1992). 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