With the steady escalation of both multilateral and U.S. sanctions against it, North Korea is threatening once again to ratchet up its response. The week began with U.S. President Donald Trump telling the U.N. General Assembly on Sept. 19 that North Korean leader Kim Jong Un was on a "suicide mission" and the United States would "totally destroy" North Korea if necessary to protect itself and its allies. Trump followed up his remarks by signing an executive order on Sept. 21 that will allow the U.S. Treasury Department to go after entities trading with North Korea. On Sept. 22, Kim responded by promising countermeasures.
Kim’s vague threat was sharpened by North Korean Foreign Minister Ri Yong Ho, who speculated to reporters in New York that Kim might be considering carrying out "the most powerful detonation of an H-bomb in the Pacific." Ri said he did not know what Kim was considering and that the nature of the response was entirely Kim’s decision.
These threats don't necessarily suggest that North Korea would immediately detonate a hydrogen bomb in the Pacific Ocean, but it's not impossible that it could. Ri's allusion to Kim's power to choose a course of action is similar to North Korea's August threat against Guam, which was followed by missile tests but none along the lines of those that had been outlined.
A North Korean nuclear test in the Pacific likely would involve launching a nuclear-tipped ballistic missile over the ocean and exploding the warhead at a high altitude. Such a test also likely would involve flying the missile and its warhead over Japan. A flight over Japan would showcase the likely flight path of an intercontinental ballistic missile launched toward the U.S. mainland as well.
North Korea could try to minimize collateral damage from an atmospheric nuclear test by testing at a very high altitude — perhaps as high up as the edge of space — in a remote location of the Pacific where there is little maritime traffic. A high-altitude test also could allow North Korea to get around the limitations of its still rudimentary re-entry technology.
Even with the measures North Korea could take to minimize the damage of an atmospheric nuclear explosion, the risk of such a test is always substantial. An accident or miscalculation could result in a nuclear explosion at a location and altitude that differs from the original intent. Depending on the exact yield of the warhead, a very high-altitude nuclear test demonstration could also result in a significant electromagnetic pulse effect that would damage or at least disrupt radar, satellite and radio networks.
Because of the Pacific Ocean's vast size and the likely remoteness of the detonation zone, dispersal and dilution would serve to limit the overall human exposure to radioactive material. While there is a large volume of trans-Pacific shipping traffic, a high-altitude test would unlikely have a long-term effect on shipping beyond the impact of the electromagnetic pulse. There also is a low probability that a surface-level test would even have an impact on shipping lanes, given the Pacific's size.
The closer a test occurs to the surface, the more damage any nearby infrastructure would suffer, and the higher the environmental damage and radioactive fallout will be. A surface-level test would have a destructive, but localized impact on sea life, or on any unlucky passing vessel. A nuclear test that occurs over a known fishing zone could affect the fishing industry. Human exposure to radiation would ultimately be limited, but the test would create a perception of exposure that could negatively affect demand.
Even a North Korean nuclear test over the Pacific that results in few or no casualties would be deemed highly provocative. No country has carried out an atmospheric nuclear test since China in 1980, and that was in the remote Lop Nur basin of Xinjiang province within its own territory. Such a test today would mark a violation of long-established bans on nuclear testing beyond just the Comprehensive Test Ban Treaty.
A Pacific test would isolate North Korea further internationally and would likely invite additional economic sanctions and diplomatic pressure, including reluctant powers such as Russia and China. An impending North Korean live nuclear test also would provide added impetus to the argument that Japan and the United States should shoot down North Korean missiles, despite the risk that the intercept attempt could fail. Intelligence forewarning permitting, the United States could even attempt to destroy the missile on its launch pad before it could fly. And a nuclear test over the Pacific would reinforce the narrative that North Korea is not an entirely rational or predictable nation.