Temperatures would plunge in a bigger shift than the last Ice Age and crops would fail around the world after nuclear war erupted, a study has shown.
The after-effects would be long-lasting with oceans taking hundreds of years to recover, and fishing devastated, the researchers warn.
Professor Cheryl Harrison, of Louisiana State University, said: "It doesn't matter who is bombing whom. It can be India and Pakistan or NATO and Russia. Once the smoke is released into the upper atmosphere, it spreads globally and affects everyone."
In all the simulated scenarios, nuclear firestorms would release soot and smoke into the upper atmosphere that would block out the Sun, resulting in crop failure around the world.
In the first month following a nuclear exchange, average global temperatures would plunge by about seven degrees Celsius, a larger temperature change than in the last Ice Age.
Ocean temperatures would drop quickly and would not return to their pre-war state even after the smoke clears.
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As the planet gets colder, sea ice expands by more than six million square miles and six feet deep in some basins, blocking major ports including Beijing's Port of Tianjin, Copenhagen and St. Petersburg.
The sudden drop in light and ocean temperatures, especially from the Arctic to the North Atlantic and North Pacific oceans, would kill the marine algae, which is the foundation of the marine food web, essentially creating a famine in the ocean.
This would halt most fishing and aquaculture.
The researchers simulated what would happen to the Earth's systems if the US and Russia used 4,400 100-kiloton nuclear weapons to bomb cities and industrial areas, which resulted in fires ejecting more than 330 billion pounds of smoke and sunlight-absorbing black carbon into the upper atmosphere.
They also simulated what would happen if India and Pakistan detonated about 500 100-kiloton nuclear weapons resulting in 11 billion to 103 billion pounds of smoke and soot into the upper atmosphere.
Co-author Alan Robock, distinguished professor in the Department of Environmental Sciences at Rutgers University in New Jersey, said: "Nuclear warfare results in dire consequences for everyone.
"World leaders have used our studies previously as an impetus to end the nuclear arms race in the 1980s, and five years ago to pass a treaty in the United Nations to ban nuclear weapons.
"We hope that this new study will encourage more nations to ratify the ban treaty."
The study shows the global interconnectedness of Earth's systems, especially in the face of perturbations, whether they are caused by volcanic eruptions, massive wildfires or war.
Harrison said: "The current war in Ukraine with Russia and how it has affected gas prices, really shows us how fragile our global economy and our supply chains are to what may seem like regional conflicts and perturbations."
Volcanic eruptions also produce clouds of particles in the upper atmosphere and have induced sudden changes in climate at various points throughout history.
Harrison said: "We can avoid nuclear war, but volcanic eruptions are definitely going to happen again. There's nothing we can do about it, so it's important when we're talking about resilience and how to design our society, that we consider what we need to do to prepare for unavoidable climate shocks.
"We can and must, however, do everything we can to avoid nuclear war. The effects are too likely to be globally catastrophic."
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