Omitted evidence

"The truth, the whole truth, and nothing but the truth."

One of the most common fallacies is the deliberate (or accidental) leaving out of evidence that would weaken the conclusion of an argument.  It is also one of the more difficult to detect.  The best advice is to become familiar with the area under discussion so that you will know when information is being omitted.

Examples

"Far more chlorine is added to the atmosphere from sea spray and volcanoes than from the breakdown of CFCs."  What is left out is that most of the chlorine from sea spray and volcanoes is washed out of the lower atmosphere and never reaches the ozone layer. (see Taubes)

One of the most frequent examples is to make it appear that the Antarctic ozone "hole" was first discovered in 1956 by leaving out several relevant facts.  Here is the version presented by Bolch and Lyons (page 85):

In 1956, G. M. B. Dobson, the Oxford professor for whom the Dobson unit of ozone measurement is named, first reported that in September-October 1956 at Halley Bay, Antarctica, atmospheric ozone was at a lower level than expected in the early spring, but that it returned to normal in November.  In 1985 a British group, again at Halley Bay, reported that beginning in 1975 short-lived declines in atmospheric ozone were observed in October, but levels returned to normal in November. . .  These reports are often taken to be empirical verification of man's destruction of the ozone.  On the contrary, what those reports say is that ozone seems to vary seasonally at certain locations-which is no more alarming than a report that in Boston many flowers seem to die in October and are reborn in May.

The authors want the reader to conclude that Dobson and the "British group" were describing the same phenomena.  But when the original documents are checked neither of them are refering to normal seasonal changes.  (And Bolch and Lyons do not give a reference for either Dobson (who actually did not report the results until 1968) or the "British group" (the British Antarctic Survey or BAS, see Farman, et al.) so that it is not possible to find this information without checking another source.).  Dobson was comparing the ozone levels over Antarctica with those found in the spring over the arctic, not with those found previously over Antarctica (for which there was no data) or seasonal changes. Bolch and Lyons fail to note that the levels reported by Dobson continued to be found through the 60s and early 70s.  The decrease noted by the BAS was from the historical levels in the spring, not a change from winter to spring levels.  The authors also ignored the several scientific studies of the ozone hole, such as the Airborne Antarctic Ozone Expedition (the results of which were reported in 1989 in the Journal of Geophysical Research), which showed conclusively that the hole was related to chlorine chemistry.  (These studies are discussed at length in Maureen Christie's The Ozone Layer.)  Bolch and Lyons end with a bad analogy, comparing the unusual changes over Antarctica with normal seasonal changes in Boston.  A better analogy would be if the flowers in Boston suddenly started to die in August.  

References

Bolch, Ben and Harold Lyons, Apocalypse Not : Science, Economics, and Environmentalism, 1993

Christie, Maureen, The Ozone Layer:  A Philosophy of Science Perspective, Cambridge University Press, 2000

Dobson, G. M. B., "Forty Years' research on atmospheric ozone at Oxford", Applied Optics, 1968, pp 387-405.

Farman, et al., "Large Losses of Total Ozone in Antarctica Reveal Seasonal ClO_x/NO_x Interaction", Nature, May 16, 1985, pp 207-210.

Journal of Geophysical Research, V. 94, 1989, D9 pp 11181-737 and D14 pp 16437-854.

Taubes, Gary, "The Ozone Backlash", Science, June 11, 1993, pp 1580-1583.

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Written by Jim Norton

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