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Predicting the Past: It's Really Not That Difficult
Volume 4, Number 4: 24 January 2001

A few weeks back we received an e-mail message alerting us to a news story about the publication of a climate modeling study in Science magazine. The popular article briefly described the gist of the study and matter-of-factly concluded it demonstrated that "recent global warming is man-made and will continue."

Such statements always catch our attention. The title of the popular piece was also intriguing: "Hotter earth is confirmed by computer." Wondering how that could possibly be - since computers are not normally used as temperature sensors - we went to the source of the story, the research report of Stott et al. (2000), where we found, as might have been expected, that the media blurb was more than a little presumptuous in its declaration of both the warming and its cause.

What the researchers in question had actually done was use a coupled ocean-atmosphere general circulation model to calculate earth's mean near-surface air temperature from 1860 to the present for four different sets of initial conditions. These calculations were driven by what is supposedly known about anthropogenic forcings of climate and natural forcings of climate. When only natural forcings were employed, the model predictions matched up well with the observational temperature record, except for the last 35 years, when the calculated temperatures fell somewhat below the measured temperatures. When only anthropogenic forcings were used, the model predictions also matched up well with the observational temperature record, except for the preceding 35-year period, when the calculated temperatures also fell somewhat below the measured temperatures. When both sets of forcings were used, however, the calculations and observations generally agreed over the entire length of the study.

The climate alarmist spin on these results is that the new-and-improved general circulation model must faithfully represent earth's climate system, since when it is driven by what is claimed to be the full set of major climate forcings, it "hindcasts" quite well what is claimed to have happened, temperature-wise, from 1860 to the present. Hence, when the authors hold natural forcings constant and project another century forward in time, based on presumed future increases in anthropogenic forcings - due, primarily, to the burning of fossil fuels - they feel they are obtaining a valid picture of what our descendents can reasonably expect to encounter over the course of the next hundred years: an additional 2.5C temperature increase that is typically proclaimed to be catastrophic in terms of both its magnitude and rate of occurrence.

Although this exercise masquerades as news, that appellation is somewhat misapplied in the present instance. We can go fully two decades back in time, for example, and find a nearly identical exercise that was performed by Hansen et al. (1981). In this earlier incarnation of the current hubris over man's ability to alter earth's climate, the researchers compared the mean near-surface air temperature of the globe from 1880 to 1980 with model predictions based on the very same sets of forcings employed by Stott et al. (2000): anthropogenic, as represented by ever-increasing atmospheric concentrations of greenhouse gases, and natural, as represented by variable solar and volcanic activity. And the results? Phenomenal, of course, with a respectable match-up of predictions and observations over the entire course of the 100-year record.

Even more astounding was the following year's study of Gilliland (1982). Using the same greenhouse gas, solar and volcanic forcing functions as both Hansen et al. and Stott et al., this enterprising scientist's climate model explained in excess of 90% of the variance in the historical temperature record of the same time period, which is to be compared with Stott et al.'s more recent accomplishment (or great step backward) of only explaining "more than 80%" of the observed variance.

But why should any of these incredible results amaze us? There are enough adjustable variables in most climate models, as Ramanathan (1981) remarked fully two decades ago, that "it is relatively easy to obtain the desired result." In the case of Hansen et al., for example, a specific magnitude of CO2 forcing was picked from among the then-favored range of values that gave just the result that was needed by the modelers. Ditto for the solar forcing factor; and if you don't believe us, check out the paper for yourself. The researchers openly admit that the hypothetical solar radiation relationship they used was chosen because "other hypothesized solar radiation variations that we examined degrade the fit." This same guaranteed-to-work approach was also evident in their modeling of volcanic dust effects, when Hansen et al. selected the Lamb (1970) dust veil index to represent the intensity of historical volcanic activity; for this index has been severely criticized (Self et al., 1981; Newhall and Self, 1982) as being largely derived from the very measurements it is supposed to explain!

Much the same problems call the fantastic results of Gilliland into question; and the study of Stott et al. is not immune to them either. The latter investigators, for example, openly state "there are considerable uncertainties in some of the forcings used in this analysis," and they admit "the good agreement between model simulation and observations could be due in part to a cancellation of errors." The commentary on their paper by Zwiers and Weaver (2000) also describes these and a number of other problems, causing one to wonder if there is any virtue at all in conducting such exercises.

Although some would indeed say these climate modeling efforts are worthless, that appellation - like the word news - is incorrect; for without such studies, there would be no exciting stories to tell about the virtual world of catastrophic global warming spawned by the evil activities of man. But thrice-told tales? Over a span of nearly twenty years? And with no improvements to show?

Clearly, the struck-from-the-same-mold studies are beginning to get just a little old. Yet the climate alarmists know they can still milk them to their political advantage. And milk them they do, and ever so effectively, as evidenced by all the attention this one received. Furthermore, if they've been fooling the public for nigh unto twenty years now, you can be certain they'll do it again in the not too distant future, and then again, sometime after that. When will we ever learn ... that it's easy to predict the past?

Dr. Craig D. Idso
Dr. Keith E. Idso
Vice President

Gilliland, R.L. 1982. Solar, volcanic, and CO2 forcing of recent climate changes. Climatic Change 4: 111-131.

Hansen, J., Johnson, D., Lacis, A., Lebedeff, S., Lee, P., Rind, D. and Russell, G. 1981. Climate impact of increasing atmospheric carbon dioxide. Science 213: 957-966.

Lamb, H.H. 1970. Volcanic dust in the atmosphere; with a chronology and assessment of its meteorological significance. Philosophical Transactions of the Royal Society of London, Series A 266: 425-533.

Newhall, C.G. and Self, S. 1982. The volcanic explosivity index (VEI): An estimate of explosive magnitude for historical volcanism. Journal of Geophysical Research 87: 1231-1238.

Ramanathan, V. 1981. The role of ocean-atmosphere interactions in the CO2-climate problem. Journal of the Atmospheric Sciences 38: 918-930.

Self, S., Rampino, M.R. and Barbera, J.J. 1981. The possible effects of large 19th-century and 20th-century volcanic eruptions on zonal and hemispheric surface temperatures. Journal of Volcanology and Geothermal Research 11: 41-60.

Stott, P.A., Tett, S.F.B., Jones, G.S., Allen, M.R., Mitchell, J.F.B. and Jenkins G.J. 2000. External control of 20th century temperature by natural and anthropogenic forcings. Science 290: 2133-2137.

Zwiers, F.W. and Weaver, A.J. 2000. The causes of 20th century warming. Science 290: 2081-2083.