Apart from examining the medical effects of nuclear weapons the Tribunal sought to address the recent and spectacular conclusions of a number of atmospheric and biological scientists who now believe that nuclear warfare can result in such drastic changes to the global climate that the continued existence of many life-forms would be questionable. The Tribunal recognized that the recent predictions of the so-called 'nuclear winter effect' needed special attention for three primary reasons:
The Tribunal received written evidence from four experts one of whom, Professor Ian Percival FRS of Queen Mary College London appeared before the Tribunal in person for cross-examination. The Tribunal was presented with papers from Professor Carl Sagan of Cornell University reporting the original research work which predicted a nuclear winter and the biological effects; a paper from Dr. Norman Myers on the environmental consequences; and a paper from Mr. Jonathon Porritt, also on the impact on the biosphere.
During the course of the oral proceedings Professor Percival outlined the main causes of the nuclear winter and its effects. He explained that one major effect of a nuclear war would be that dust and smoke from large fires would be carried high into the atmosphere where it would be carried around the globe acting as a barrier to sunlight causing darkness and extreme cold at the Earth's surface. He explained that if the dust and smoke reached the comparatively stable stratosphere as the computer models predicted then it could remain air-borne for many months and even up to one or two years resulting in severe biological consequences.
The precise nature of these consequences was outlined in some detail in the other written evidence. Basically the occurrence of very low light levels (down to a few percent of normal) combined with protracted periods of cold would effectively obliterate agriculture as plant photosynthesis would cease. In addition damage to the Earth's ozone layer caused by massive quantities of nitrogen oxides generated as a by-product of nuclear explosions would result in an increased level of ultraviolet light reaching the Earth once the dust had cleared, posing a further threat to any life-forms which might have survived the cold and the dark. In addition the Earth's surface and atmosphere may be contaminated not only with enhanced levels of long-lived radioactivity but also with hazardous chemical pollutants resulting from the combustion of plastics and other materials prevalent in a modern highly industrialized society.
If such predictions are correct then the consequences for human survivors are clear. With failing agriculture and no relief from the cold or dark the long term survival of the human race appears to be in some doubt on the basis of these startling scientific predictions.
The Tribunal attempted to ascertain what minimum level of nuclear exchange could give rise to the nuclear winter. Results from calculations by Professor Sagan and others indicate that sub-zero temperatures averaged over a hemisphere would exist for up to 3 months after only a 100 megaton city attack corresponding to a small 'theatre' exchange.
The Tribunal was also interested in assessing the certainty of the nuclear winter predictions or the lack of it. Under cross-examination Professor Percival accepted that the nuclear winter theory depends very much on estimates of the extent of fires, the amount of soot and smoke generated, the height to which soot and smoke particles rise, and the rates at which they fall out. Professor Percival accepted that there were many imponderables in the models but that the best judgment could be reached by examining a variety of studies.