The team assumed an engagement between two emerging countries in the subtropics, and that the attacks with 15-kiloton devices were focused on large metropolitan centres.
The explosions would ignite huge firestorms, fuelled by all the wood, plastics and petroleum products commonly found in such locations.
These would send thick black smoke into the upper troposphere, where heating by the Sun would drive the particles even higher into the stratosphere.
The new simulations, run on a Nasa supercomputer, show that for five million tonnes of black smoke sent skyward, the result would be a global cooling of 1.25C, as the material spread out and blocked light from reaching the ground.
Some regions would probably go back to cold conditions not experienced since medieval times and a climate phase sometimes referred to as the "Little Ice Age" when Europe in particular was hit by very harsh winters
Faced with the possible annihilation of 100 million people, the prospect of harsh winters is the least of my worries.