For more than fifty years, since the entry into force of the Partial Nuclear Test Ban Treaty (PTBT), almost all nuclear-weapon possessing states have observed a moratorium on nuclear tests conducted above ground. France stopped after 1974, and China after 1980, ending the era of atmospheric nuclear tests. This choice is principally meant to prevent the release of radioactive gases and particulates from bomb debris (known as fallout) into the environment, where it could contaminate large areas after it settles to ground level. As observed by the late Herbert York, the first director of Lawrence Livermore National Laboratory, the PTBT also legitimized nuclear testing and had the consequence of making the “the continuation of uninhibited weapons development politically respectable.” (Quoted in Fehner, Terrence R., and Francis George Gosling. Battlefield of the Cold War: The Nevada Test Site. US Department of Energy, 2006.)
Unhappily, the foreign minister of North Korea, Ri Yong Ho, stated recently in New York that North Korea may test a hydrogen bomb over the Pacific Ocean rather than underground, as they have done previously, ending the global moratorium that has existed since before many of us were born. He did not state explicitly that any such test would involve a warhead mounted on a missile, but this would be a reasonable assumption. If the warhead is a thermonuclear bomb, it may also have an additional jacket of uranium to enhance the yield while at the same time producing more radioactive fallout. What might it look like? What might be the results?
The least provocative version of such a test might be a high-altitude airburst detonation in the region near where the last two North Korean missiles that flew over Japan landed. The geographical location of the detonation is not critical so far out into the ocean, so accuracy of the missile that deliver the warhead is also not critical. (The North Korean missile launched over Japan on August 29 may have landed hundreds of km from where it was expected, based on an analysis conducted by Dr. Marco Langbroek.)