Excerpts From:
Http://www.mindfully.org/Nucs/Sagan-Nuclear-Consequences31oct83.htm

There has been a systematic tendency for the effects of nuclear weapons and nuclear war to be underestimated. The yield of the first nuclear explosion near Alamogordo, New Mexico, on July 16, 1945, was underestimated by almost all those who designed and constructed the weapon. The extent of fallout from early thermonuclear weapons tests was underestimated; the impairment or destruction of satellites by nuclear weapons explosions in space was a surprise; the depletion of the ozonosphere by high- yield bursts was unanticipated; and nuclear winter was for many—ourselves included—an astonishment. What else have we overlooked?

At least Dr. Sagan had the gumption to ask "What else have we overlooked?"
The question from Dr. Saini may have lead to such overlooked questions, unfortunately it was "inappropriate" to consider those questions at that time.

DR. SAINI: The second one had to do with some unforeseen effects of nuclear weapons detonations and the relationship to the Van Allen radiation belt. I was wondering if you knew about that and would comment on what seems to be one of the most disturbing aspects of our current situation; that is, the militarization of space.

DR. SAGAN: The imminent introduction of weapons into space is a policy question which is inappropriate for this meeting. It is certainly true that if you explode a nuclear weapon at the appropriate altitude, you have injected charged particles into the Van Allen radiation belt. But I do not think that has any climatic effects of the magnitude we are talking about here?

Dr. Sagan made an obviously unsubstantiated remark that nuclear explosions in the upper atmosphere may not make a great difference in the climatic effects of the lower atmosphere.
Dr. Sagan and his team of scientists were mainly interested in the effects of small particles of soot, black smoke and such, particles of light absorbing/blocking nature that would be lifted high into the atmosphere in a nuclear war.

Had they also considered and calculated the effects involved in the, even smaller, particles such as the radioactive atoms of various gasses and the kinetic energy that they can impart on their surrounding molecules, I think that they would have come to a very surprising, and perhaps even more frightening, conclusions.

Johannes