North Korea's fourth test of a nuclear weapon — whether it was a hydrogen bomb or not — calls attention to a well-known but sobering fact: There are a terrifying number of nuclear weapons in the arsenals of major powers around the world.
But worse, unprecedented and widespread devastation doesn't require the unlikely scenario of all those powers unleashing all the firepower at once, according to a recent study published in an American Geophysical Union journal.
In fact, that study found that a "limited, regional nuclear war" using 100 "small nuclear weapons" — the size of the bomb dropped at Hiroshima — could cause a nuclear winter that would last decades.
In the researchers' scenario, the aftereffects of a nuclear war between India and Pakistan alone would eliminate between 20% and 50% of the ozone layer that protects us from the sun's radiation over populated areas; at the same time, surface temperatures would become colder than they've been for at least 1,000 years.
Those combined effects "could trigger a global nuclear famine," according to the paper.
The doomsday scenario
For this study, which is an updated version of a model these researchers calculated previously, scientists computed what would happen if India and Pakistan each launched 50 nuclear weapons at cities in the other nation.
In that scenario, the researchers estimated the effects of 15 kiloton (kt) bombs, which are considered "small" by modern standards. (The US and Russia both possess weapons 1,000 times more powerful than these.)
Still, bombs in this scenario would be as powerful as the "Little Boy" dropped on Hiroshima, enough to devastate a city.
But global nuclear winter could be caused by the aftereffects of the weapons.
The bombs would ignite firestorms in the cities they hit, tearing through every available source of fuel — buildings, vehicles, fuel depots, vegetation, and more. These firestorms are what would make the use of these weapons in cities different from the nuclear tests of far more powerful weapons that have already occurred.
The flames would release even more energy than the bombs themselves, sending smoke into the stratosphere. Those black-carbon aerosols would then spread around the globe.
In the stratosphere, the fine soot would cause temperatures to skyrocket more than 70 Kelvin (K, equivalent to Celsius, C). Normally below-freezing, the stratosphere would stay more than 30C above normal for five years and take two decades to recover.
This would cause ozone loss "on a scale never observed," allowing a torrent of ultraviolet (UV) radiation from the sun to penetrate the atmosphere and reach the ground, damaging human health and transforming the DNA of crops and other species all over the planet — both on land and at sea.
But it gets worse.
Earth's UV-pummeled ecosystems would also be threatened by suddenly colder temperatures.
Using the most up-to-date climate and atmospheric models available, the researchers calculated that global temperatures would decrease over the next five years and wouldn't return to normal for more than 25 years.
Expanded sea ice would prolong the cooling process, since ice reflects warm sunlight. The radiation and sudden changes to ocean temperatures could devastate sea life, a significant source of food for the world.
Average temperatures around the world would drop 1.5C. That means that in populated areas of North America, Europe, Asia, and the Middle East, changes would be even more extreme (see above graphic). Winters there would be about 2.5C colder, and summers between 1 and 4C colder, reducing critical growing seasons by anywhere from 10 to 40 days.
Those sudden blows to the food supply and the potential "ensuing panic" could cause a "global nuclear famine," according to the authors.
They write that the "perils to human society and other forms of life on Earth" evident in these results should motivate the elimination of nuclear weapons around the world.