In the past month, a disagreement has broken out into the open between the United States and its ally, the Republic of Korea, over the seriousness of the threat posed by North Korea’s nuclear-weapons program. This dispute, centering on whether Pyongyang can mount nuclear weapons on ballistic missiles—the United States says yes, and South Korea says no—reflects first and foremost the two sides jockeying for position over whether Seoul should introduce an advanced U.S. missile-defense system known as Terminal High Altitude Area Defense (THAAD). While THAAD is intended to deal with the threat from Pyongyang, Beijing opposes its deployment, given concerns that the system is really aimed at its own missile forces. But the dispute also reflects a bigger problem—namely, South Korea’s unwillingness to come to grips with the reality that the nuclear-weapons threat from the North is poised for significant expansion.

That expansion will benefit from accomplishments achieved between 2009 and 2014, banner years for Pyongyang’s nuclear-weapons and ballistic-missile programs. Aside from the obvious manifestations—two tests of nuclear devices and three of long-range rockets—North Korea has conducted a host of other activities intended to lay the foundation for the future growth of its nuclear deterrent. For example, in 2011, Pyongyang unveiled a new modern plant able to produce highly enriched uranium that can be used to expand its nuclear-weapons stockpile. More recently, that plant has doubled in size, possibly meaning it can produce twice as much of this material. On the missile front, aside from modernizing its main launch facility to test larger rockets, the North is also gearing up to eventually deploy a road-mobile intercontinental ballistic missile able to reach the west coast of the United States.

What do these developments mean for North Korea’s nuclear future? We have been looking at this prospect at the U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies and have concluded that the threat is going to grow, perhaps quite dramatically, by 2020. Of course, predicting the future of North Korea’s nuclear and missile programs is difficult, given inherent uncertainties in prognosticating about the most secret programs in an already secretive country. But there is also more evidence about these programs today than at any time in the past, in part because of the availability of commercial satellite and on-the-ground photography able to spot new developments, but also because as these programs move towards fielding operational weapons through visible activities, such as testing, it is easier to observe their progress.

Given these uncertainties, any prudent analysis has to construct different scenarios ranging from a “worst case” outcome for the North Koreans—little or no growth—to a “worst case” outcome for its neighbors and the international community—a great deal of growth—with a middle scenario that essentially reflects their current program and where it is heading. We did that working with David Albright, a well-known nuclear nonproliferation expert and head of the Institute for Science and International Studies, starting with a baseline of ten to sixteen nuclear weapons in Pyongyang’s stockpile, based on our understanding of how much fissile material the North had produced by the end of 2014. And the projections were not just for numbers of bombs, but also looked at Pyongyang’s ability to build smaller, lighter weapons (allowing the North to place them on top of missiles) with greater yields (that would cause more damage). (See Graph)

The outcome is sobering. Whichever scenario takes place, the North Korean nuclear threat will grow at an alarming rate, although just how alarming remains unclear. Even in a worst-case scenario for the North Koreans—they do not conduct any more nuclear tests, their ability to produce nuclear material for more bombs remains limited and their efforts to acquire foreign technology are unsuccessful—Pyongyang’s stockpile grows 100 percent, from ten to twenty weapons. In the worst-case scenario for the rest of us, North Korea steps up its yearly nuclear-test program, operates plants at full capacity to produce bomb-making material and is successful in acquiring foreign technology. The result is a stockpile numbering one hundred nuclear weapons by 2020, as well as significant progress in miniaturizing warheads to place on missiles and in increasing the explosive yields of those weapons. In the third, most likely “mid-range estimate”—based on Pyongyang continuing its infrequent nuclear tests, a more reasonable rate of fissile-material production and limited success in foreign cooperation—North Korea could still produce fifty nuclear weapons.

While it may seem surprising, Pyongyang could have more difficulty building new advanced missiles to carry these weapons than nuclear bombs, where the basic designs and production infrastructure are largely in place. North Korea’s current inventory of almost 1,000 ballistic missiles able to reach most targets in Northeast Asia, while largely based on decades-old Soviet technology, are well tested and reliable. Pyongyang clearly has ambitions to build weapons beyond its current systems. The North is developing new mobile missiles that can reach the United States, as well as key American military bases on Okinawa and Guam. It even appears to be exploring basing ballistic and cruise missiles on submarines and surface ships, a move that may seem surprising, but has been explored (often successfully) by other nuclear powers, including smaller nuclear countries like Israel and Pakistan. Overall, Pyongyang’s objectives are clear—build missiles that are better equipped to survive attacks intended to destroy them and with longer ranges able to threaten new important targets.

Particularly important will be North Korea’s ability to overcome technological and engineering hurdles that even more advanced countries would find challenging. In this context, since the North is not self-sufficient in missile production, the level of external assistance—through illicit acquisitions or working with foreign governments—could be a critical factor in determining how much progress Pyongyang is able to make in technologies such as high-performance liquid-fuel engines, solid-fuel rocket motors, high-speed heat shields and reentry vehicles, guidance electronics, sophisticated machine tools and high-strength, lightweight materials. Whether the North will be successful in acquiring what it needs is an open question. But like other emerging nuclear powers, it may have a far-less-demanding definition of “success” in the development of new missiles than countries like the United States, where extensive tests are conducted before weapons become operational to ensure a high degree of reliability. For North Korea and others, the benefits of “sending a political signal” are sometimes just as important.

In view of these uncertainties, our study once again postulates three scenarios for the future based on different assumptions. In the first scenario, characterized by no new long-range rocket tests, as well as a failure to acquire technology overseas, Pyongyang’s development of new delivery systems slows, resulting in a force that remains essentially the same as today, with some marginal improvements, such as deploying existing short-range ballistic or cruise missiles at sea, probably on surface ships—an option that has also been at least explored by every country from the United States in the 1950s to Iran more recently—that would give it an additional capability for attacking targets in Northeast Asia.

In the second scenario, which is essentially a continuation of Pyongyang’s current development and deployment pathway, including the launching of a long-range rocket every three years, the result is a greater threat to targets in the region and the emergence of a more credible intercontinental threat by 2020. In the theater, greater numbers of sea-based systems would be deployed, and it is quite possible a new road-mobile intermediate ballistic missile called the Musudan—currently under development—able to reach U.S. bases in Okinawa and Guam would become operational. On the intercontinental front, even with very limited testing, the new KN-08 ICBM, also road-mobile, could be available on an emergency basis as it moves towards becoming an operational weapon. Pyongyang might also consider deploying an existing long-range missile based on the Unha space launch vehicle—the Taepodong—in hardened missile silos.

The last scenario assumes that North Korea accelerates the development and deployment of new systems, resulting in a more rapidly emerging regional and intercontinental threat. In the theater, the Musudan IRBM would achieve an earlier initial operating capability and deployments of missiles would increase. Pyongyang might also deploy its first operational ballistic-missile submarine armed with variants of land-based weapons, such as the Musudan IRBM. On the intercontinental level, the KN-08 ICBM would reach an initial operational capability with growing numbers deployed by 2020, though numbers would still probably be limited by the availability of critical components, particularly engines.

Of course, having a large number of missiles and nuclear warheads does not mean those warheads can be placed on those missiles, which brings us back to the current controversy. True, one suspects that the differing estimates by the United States and South Korea as to whether Pyongyang can place warheads on missiles is influenced by the politics of missile defense on the Korean Peninsula on both sides. That can not be proved, of course, but it’s clear the Koreans do not want to be rushed into THAAD deployments, probably because of Chinese sensitivities, while the United States wants to move forward.

However, even before the current THAAD debate heated up, political sensitivities appear to have been influencing public statements on this issue. For example, in late 2013, the U.S. commander in Korea stated that it was his personal opinion that North Korea probably could put a nuclear warhead on its missiles. Following that remark, the South Korean minister of national defense said that Pyongyang had achieved a significant level of miniaturization. But the South Koreans hedged their bets by also saying the North had not yet actually tested a missile with a warhead, neglecting to mention that only the Chinese have ever actually conducted such a test. As one American expert has pointed out, “Do we really want to insist that North Korea arm a missile with a live warhead and conduct a demonstration?” South Korea’s statements may be based on a technical assessment, but just as likely, Seoul does not want to alarm its public or admit how much progress the North has really made.