e significant climatic and environmental
change, but we cannot rule out interactions with other phenomena, such
as ozone depletion, which might produce utterly unexpected results.

We have come to realize that nuclear weapons can be as unpredictable as
they are deadly in their effects. Despite some 30 years of development
and study, there is still much that we do not know. This is
particularly true when we consider the global effects of a large-scale
nuclear war.

Note 1: Nuclear Weapons Yield

The most widely used standard for measuring the power of nuclear
weapons is "yield," expressed as the quantity of chemical explosive
(TNT) that would produce the same energy release. The first atomic
weapon which leveled Hiroshima in 1945, had a yield of 13 kilotons;
that is, the explosive power of 13,000 tons of TNT. (The largest
conventional bomb dropped in World War II contained about 10 tons of
TNT.)

Since Hiroshima, the yields or explosive power of nuclear weapons have
vastly increased. The world's largest nuclear detonation, set off in
1962 by the Soviet Union, had a yield of 58 megatons--equivalent to 58
million tons of TNT. A modern ballistic missile may carry warhead
yields up to 20 or more megatons.

Even the most violent wars of recent history have been relatively
limited in terms of the total destructive power of the non-nuclear
weapons used. A single aircraft or ballistic missile today can carry a
nuclear explosive force surpassing that of all the non-nuclear bombs
used in recent wars. The number of nuclear bombs and missiles the
superpowers now possess runs into the thousands.

Note 2: Nuclear Weapons Design

Nuclear weapons depend on two fundamentally different types of nuclear
reactions, each of which releases energy:

Fission, which involves the splitting of heavy elements (e.g. uranium);
and fusion, which involves the combining of light elements (e.g.
hydrogen).

Fission requires that a minimum amount of material or "critical mass"
be brought together in contact for the nuclear explosion to take place.
The more efficient fission weapons tend to fall in the yield range of
tens of kilotons. Higher explosive yields become increasingly complex
and impractical.

Nuclear fusion permits the design of weapons of virtually limitless
power. In fusion, according to nuclear theory, when the nuclei of light
atoms like hydrogen are joined, the mass of the fused nucleus is
lighter than the two o