Almost two-thirds of the world's population would be wiped out by a nuclear war, new research has shown.
Scientists say more than five billion people would die of hunger after a full-scale conflict.
Vladimir Putin 's invasion of Ukraine has brought the threat of nuclear war and computer simulations have been created over what would happen in the event of nukes being used.
Lead author Professor Lili Xia, of Rutgers University, New Jersey, said: "The data tell us one thing. We must prevent a nuclear war from ever happening."
Models showed firestorms would release soot into the upper atmosphere that would block out the Sun resulting in global crop failure.
They shed fresh light on the exact impact if 'Mad Vlad' reaches for the button.
Nine nations, including the UK, currently control more than 13,000 nuclear weapons.
Even a clash between new nuclear states would "decimate food production and result in widespread starvation" the report warns.
The study shows the UK, US, Germany, France and China would be devastated and almost everyone dead by the second year
But places such as Argentina and Australia would have no deaths although livestock in other countries would perish and no trade would exist.
Professor Alan Robock, co-author added: "There still would be enough domestic production for them, but you can imagine there will be flotillas of hungry refugees from Asia on their way there.
"So it wouldn’t be necessarily peaches and cream just for Australia. The threat of using nuclear weapons to deter an attack is the threat of a suicide bomber.
“Because if you use them, everybody in your country will starve to death.”
The US team calculated how soot would travel from five smaller India-Pakistan wars and a large US-Russia one based on the size of each country's arsenal.
A climate forecasting tool estimated the impact on maize, rice, spring wheat and soybean country-by-country.
The researchers also examined projected changes to livestock pasture and marine fisheries.
A localised war between India and Pakistan, global average caloric production decreased seven percent within five years.
In the worst case scenario - involving the US and Russia - this would rise to 90% three to four years after the fighting ended.
Crop declines would be the most severe in the mid-high latitude nations, including major exporters such as Russia and the US.
It could trigger restrictions and cause severe disruptions in import-dependent countries in Africa and the Middle East.
These would induce a catastrophic disruption of global food markets. Even a seven percent decline would exceed the largest since records began in 1961.
Under the largest war scenario, more than 75% of the planet would be starving within two years.
Using crops fed to livestock as human food or reducing waste would have minimal benefits.
Professor Xia said: "Future work will bring even more granularity to the crop models.
"For instance, the ozone layer would be destroyed by the heating of the stratosphere, producing more ultraviolet radiation at the surface, and we need to understand that impact on food supplies."
Climate scientists at Colorado University are creating detailed soot models for specific cities - such as Washington DC.
Inventories of every building will provide a more accurate picture of how much smoke would be produced.
The study was, published in Nature Food. Earlier this year another US team found nuclear war between the US and Russia would trigger a 'Little Ice Age' lasting thousands of years.
In the first month following detonation, average global temperatures would plunge by about 13 degrees Fahrenheit.
This would surpass those in the most recent Ice Age that ended 11,700 years ago killing off the woolly mammoth.
Once the smoke is released into the upper atmosphere it spreads globally - and affects everyone.
Marine ecosystems would also be devastated both initially and in a new ocean state - resulting in long-term impacts to fisheries and other services.
Professor Robock, also from Rutgers, said researchers already have more than enough information to know a nuclear war of any size would obliterate global food systems - killing billions of people in the process.
He said: "If nuclear weapons exist, they can be used, and the world has come close to nuclear war several times.
"Banning nuclear weapons is the only long-term solution.
"The five-year-old UN Treaty on the Prohibition of Nuclear Weapons has been ratified by 66 nations, but none of the nine nuclear states.
"Our work makes clear that it is time for those nine states to listen to science and the rest of the world and sign this treaty."
