Phil Jones [1] correctly identifies as the most important question "whether the 0.6 degrees C global warming during this century can be unequivocally related to human-induced changes in the chemical composition of the atmosphere." He does not address this question, but merely cites two studies [2,3] that have been used by the Intergovernmental Panel on Climate Change (IPCC) to arrive at the ambiguous conclusion that "the balance of evidence suggests a discernible human influence on global climate." [4] These two studies, however, are severely flawed, [5,6] further weakening the already weak IPCC conclusion.
Jones clearly demonstrates that different analyses of proxy temperature data arrive at very different Northern Hemisphere (NH) climate histories since 1400. (See fig.1 of ref.[1]) For example, Briffa et al [7] show little variability, while Jones et al [8] show decadal and century-scale fluctuations of about 0.1-0.2 degrees C; Mann et al [9] exhibit an intermediate climate noisiness. (Some of the disagreement between these three curves can be explained by their use of different seasonal climatologies.) The most significant differences occur in the last 150 years: Jones shows a warming of nearly 0.6 degrees C between 1840 and 1940, in good accord with instrumental records, followed by the slight cooling, which is seen also by thermometers. [10] Mann shows a pre-1940 warming of 0.3 degrees C, beginning around 1900, while Briffa shows a warming of as little as 0.15 degrees C, beginning only around 1910 -- distinctly at odds with the instrumental record.
Jones, wisely, does not comment on the widely publicized, but misleading statement that the present century is the warmest in the past 600 years; this fact has been used to imply -- erroneously -- that the warming in this century must be due to human activities. But if the record had been extended to the past 1000 years, then the 11th century might have emerged as the warmest. Certainly, as shown by ocean core data [11], the North Atlantic region has experienced far warmer periods than the present during recorded human history -- e.g., about 2500 and 3000 years ago--greater even than predicted for a doubling of greenhouse gases.[4]
Returning to the fundamental question posed by Jones, all the available evidence suggests that the major warming during the early part of the century is mainly of natural origin [12,13], unrelated to human activities [14]. In principle, the "fingerprint" method, which compares observed and calculated geographic temperature patterns, can detect a human influence if the pattern correlation increases with time as the GH gas content of the atmosphere increases. If one were to accept the specific fingerprint analysis used by the IPCC [15], however, one would observe that this correlation decreases between 1900 and 1940 -- thereby denying the existence of an appreciable human contribution to the global warming of this century.
S. Fred Singer May 4, 1998 The Science & Environmental Policy Project 4048 University Drive, Fairfax, VA 22030 Tel: 703-934-6940 Fax: 703-352-7535 e-mail: ssinger1@gmu.edu
References and Notes
1. P.D. Jones, It was the best of times, it was the worst of times, Science 280, 544-545 (1998)
2. B.D. Santer et al., Clim. Dyn. 12, 77-100 (1995)
3. B.D. Santer et al., Nature 382, 39-46 (1996)
4. IPCC WGI. "Climate Change 1995: The Science of Climate Change," J.T. Houghton, L.G. Meira Filho, B.A. Callander, N. Harris, A. Kattenberg and K. Maskell (eds.). Cambridge University Press, Cambridge, UK, 1996.
5. S.F. Singer. "Hot Talk, Cold Science: Global Warming's Unfinished Debate" The Independent Institute, Oakland, CA, 1997. See p.9. Comparing fig 8.10 of the IPCC report [3, p.433] with corresponding figure in Ref 2, it is evident that the increasing trend of the pattern correlation, claimed by the IPCC to support a "human influence," depends entirely on an arbitrary choice of time interval, 1940 - 1990; during most of this interval global temperatures were actually decreasing.
6. P.C. Michaels and P.C. Knappenberger, Nature 384, 522-523 (1996). The positive temperature trend shown in [3], and used in fig 8.7.c of the IPCC report [4, p.428] to support a "human influence," depends entirely on the choice of the restricted time interval 1963 to 1987; had the full available record been used, the trend would have been close to zero. See also, G.R. Weber, Nature 384, 523-524 (1996), and reply by B.D. Santer, Nature 384, 524 (1996)
7. K.R. Briffa et al., Nature, in press
8. P.D. Jones et al., The Holocene 8, in press
9. M.E. Mann, R.S. Bradley, M.K. Hughes, Nature 392, 779-787 (1998)
10. There is little dispute about a cooling trend between about 1940 and 1975; there is an ongoing dispute about the direction and magnitude of the trend since 1979 [5]
11. L.D. Keigwin, Science 274, 1504-1508 (1996)
12. E. Friis-Christensen and K. Lassen, Length of the Solar Cycle: An Indicator of Solar Activity Closely Associated with Climate. Science, 254, 698-700 (1991). Their conclusion is based on the strong correla-tion between temperature and "solar activity" in the past 100 years
13. J.E. Penner et al., AGU fall meeting, Dec 1997; Clim. Dyn. (in press). Their conclusion is based on the large and uncertain radiative forcing, both negative and positive, of atmospheric aerosols.
14. This is also the view of Prof. Bert Bolin, former IPCC chairman: "The temperature increase early this century is not likely to have been due to human emissions of greenhouse gases." (from position paper entitled "Comments Regarding Criticism and Misconception of the IPCC Assessments," Stockholm, June 2, 1997)
15. IPCC fig 8.10 [4, p.433]
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