The simulations have been run, and it's official: even today, a nuclear war between the US and Russia would plunge the planet into a nuclear winter, with clouds of soot and smoke covering the planet.
Globally, temperatures would plunge by around 9 Kelvin (9 degrees Celsius or 16 degrees Fahrenheit) on average, due to the lack of sunlight reaching ground level.
The new model matches one of the best existing models we have, published in 2007. Both reports predict a nuclear winter of several years, upwards of a 30 percent global reduction in precipitation over the first few months, and a cloud of smoke enveloping first the Northern Hemisphere, and then the Southern Hemisphere, too.
"Here we repeat the [2007 nuclear war scenario] using an alternative state-of‐the‐art modern climate model run at higher resolution and with a more explicit simulation of stratospheric chemistry and aerosols," write the researchers in their paper.
"The use of nuclear weapons in this manner by the United States and Russia would have disastrous consequences globally."
There are some differences from the 2007 simulation though. The new report says smoke coverage would last longer, according to the Whole Atmosphere Community Climate Model version 4 (WACCM4) that the scientists used.
If both the US and Russia released their entire nuclear arsenal on each other's countries, the model shows, around 150 teragrams or megatonnes of soot would be released by the nuclear explosions and ensuing fires.
It would cover the Northern Hemisphere in one week, and the whole planet within a fortnight, reducing surface light levels. Then, it would take around three years before surface light was back above 40 percent of its original level.
The team used data from forest fires, volcanic eruptions, and previous nuclear bomb detonations to map the huge changes to the planet's climate that would follow, which would include "devastating" agricultural losses, shifts in wind patterns, and the collapse of the summer monsoon season.
This blanket of cloud, scattering and absorbing solar radiation, would take around a decade to disperse, the new simulation shows. You'd better hope that your nuclear bunker is well stocked with long-lasting food.
However, the levels of smoke released into the atmosphere would be "orders of magnitude smaller" than those that helped wipe out the dinosaurs, so the scientists leave open the possibility that humans could hang on to life – but it's not a hypothesis we really want to be testing.
While it's an interesting modelling experiment that takes into account a wide range of factors, it's also a frightening prospect, considering the starting point of the scenario is a cache of real, currently existent nuclear weapons.
Hence, the researchers argue that nuclear bombs are not weapons that the world's nations should have any access to.
"To completely remove the possibility of an environmental catastrophe as a result of a full‐scale nuclear war, decision makers must have a full understanding of the grave climatic consequences of nuclear war and act accordingly," conclude the researchers.
"Ultimately, the reduction of nuclear arsenals and the eventual disarmament of all nuclear capable parties are needed."
The research has been published in the Journal of Geophysical Research: Atmospheres.