'Nuclear winter' from a US-Russia conflict would wipe out 63% of the world's population

A nuclear bomb test in Mururoa atoll, French Polynesia, in 1971

(Image credit: Alamy Stock Photo)

More than 5 billion people — roughly 63% of the world's current population — would die of famine in the aftermath of a full-scale nuclear war between the United States, Russia and their allies, a new study has revealed.

According to the researchers, the conflict would create widespread fires that could eject up to 165 million tons (150 million metric tons) of soot into Earth's atmosphere, leading to crop declines in the food-exporting U.S. and Russia that would send global calorie production plummeting by as much as 90%.

The study, published Aug. 15 in the journal Nature Food, is the latest in four decades of landmark research that has attempted to sketch out the threat of a nuclear war. Of the world's approximately 12,705 nuclear warheads, Russia has 5,977, and the United States has 5,428, according to the Stockholm International Peace Research Institute’s latest report. The country with the third most nuclear warheads is China, with 350. India and Pakistan have 160 and 165, respectively.

The most immediate effects of any nuclear war, at least for those in a targeted city, have been popularly known since the dropping of the U.S. atomic bomb "Little Boy" on the Japanese city of Hiroshima on Aug. 6. 1945. The single bomb killed an estimated 140,000 people within five months of its detonation and destroyed or severely damaged more than 60,000 of the city's approximately 90,000 buildings. Six eyewitness accounts, compiled by journalist John Hersey and published in 1946, tell of the attack's instantaneous devastation and its immediate aftermath. First, the light of the bomb appeared as a blinding, "noiseless" flash as bright as the sun; then the shock wave arrived, hurling bodies under toppling buildings. In the aftermath, the clear, black shadows of the vaporized dead were seen stenciled on the walls and streets, and the survivors who were closely exposed to the blast emerged naked, their skin "sloughed off" by the heat of the explosion, to wander the ruined city in stunned bewilderment.

Studies sprung up as soon as 1947 to document the suffering following the attack, which for many would last a lifetime. Radioactive fallout, a byproduct of the nuclear fission reaction that gave Little Boy its cataclysmic power, had blanketed the area. In Hiroshima and Nagasaki, which was bombed three days later, increased rates of cancer, cataracts and other health conditions persisted for years in survivors who had been close to the bombs' epicenters.

But it would take four more decades for scientists to begin to learn and discuss the most lethal and frightening outcome of even a small-scale nuclear war: so-called "nuclear winter". In this doomsday scenario, radioactive dust and smoke would block out a significant portion of the sun's light. With temperatures dropping, many of the world's crops, smothered by the gloom, would die, creating a global famine and wiping out billions of people.

To model how this apocalyptic event would affect the planet's ability to sustain life, the researchers calculated the amount of soot that would be generated from six potential nuclear war scenarios: ranging from five scenarios based on a "limited" war between India and Pakistan over the Kashmir region, which would produce 5.5 million to 52 million tons (5 million to 47 million metric tons) of soot depending on the conflict's scale, to full-scale global nuclear war involving the U.S. and Russia, which would produce countless conflagrations veiling the sky with 165 million tons (150 million metric tons) of soot.

With the soot quantities in hand, the scientists plugged the data into the National Center for Atmospheric Research's (NCAR) Community Earth System Model, a forecasting tool that simulates changes to Earth's sunlight, temperature and precipitation. These changes were then fed into NCAR's Community Land Model, which gave the scientists a country-by-country breakdown of the dramatic reductions a nuclear winter would bring to corn, rice, soybean, wheat and fish harvests.

The researchers found that in the worst-case scenario of a nuclear war between the U.S. and Russia, temperatures on Earth's surface would drop by as much as 29 degrees Fahrenheit (16 degrees Celsius, or more than three times the temperature difference between now and the last ice age) and that 5 billion people would perish. In the most extreme war between India and Pakistan, global calorie production could drop by 50%, causing 2 billion deaths.

"The important thing to know is the amount of smoke being put into the atmosphere," study co-author Owen B. Toon, a professor of atmospheric and oceanic sciences at the Laboratory for Atmospheric and Space Physics who worked with Carl Sagan on the 1983 paper credited with introducing the concept of "nuclear winter" to public consciousness, told Live Science. "The energy released from these fires is 100 to 1,000 times the energy released by the weapons themselves. It doesn't rain in the stratosphere. So when that much smoke gets up there, it will stay there for years."

Toon, Sagan and their collaborators were first drawn to the topic of nuclear winter after taking note of a surprising revelation about what might have killed the dinosaurs. In 1980, a separate team of scientists discovered that an asteroid had struck Mexico's Yucatán Peninsula at the end of the Cretaceous period, about 66 million years ago. As is common knowledge today, the asteroid wiped out the non-avian dinosaurs. But it wasn't the energy from the collision itself that killed around 75% of Earth's animals, including the dinos — it was the cooling cloud of dust and debris that the impact released.

Using rudimentary atmospheric models and satellite data, Toon, Sagan and their colleagues applied this insight to nuclear conflicts. They found that small-scale thermonuclear wars, using as few as 100 1-megaton nuclear warheads, could start enough fires to send a thick layer of jet-black smoke into the atmosphere, causing land temperatures around much of the world to plummet to 5 to minus 13 F (minus 15 to minus 25 C) within just one or two weeks. They forecasted a cooling effect that would last up to two decades. "The possibility of the extinction of Homo sapiens cannot be excluded," their study concluded.

Sagan had been drawn to questions of humanity's long-term survival through his interest in the Drake equation, the famous formula that enables scientists to guess the potential number of intelligent alien civilizations living in the Milky Way. Concerningly, the first estimates made by the inventor of the equation — the astrophysicist Frank Drake — suggested that the advanced extraterrestrial civilizations occupying our galaxy could number anywhere between 20 to 50 million. This made Sagan ponder an idea known as the Fermi paradox: If that were the case, why hadn't we encountered them yet?

While the overall quantity of the world's nukes has dropped sharply since the end of the Cold War, the number of countries that possess the weapons has increased, and bilateral peace treaties between the U.S. and Russia discarded by Russian President Vladimir Putin and then-U.S. President Donald Trump are unlikely to be renewed during Russia’s ongoing invasion of Ukraine. China, meanwhile, could be planning to quadruple its nuclear arsenal to more than 1,000 by the end of the decade, according to an assessment by the U.S. Department of Defense.

"All of the nuclear-armed states are increasing or upgrading their arsenals, and most are sharpening nuclear rhetoric and the role nuclear weapons play in their military strategies," the Stockholm International Peace Research Institute wrote in its latest annual report, which put global annual military spending at a record high of $2.1 trillion for 2021, its seventh consecutive year of increase.

"If nuclear weapons exist, they can be used, and the world has come close to nuclear war several times," Robock said. "Banning nuclear weapons is the only long-term solution. The 5-year-old U.N. Treaty on the Prohibition of Nuclear Weapons [which bans the development, testing, production, stockpiling, stationing, transfer, use and threat of use 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."

Ben Turner is a U.K. based writer and editor at Live Science. He covers physics and astronomy, tech and climate change. He graduated from University College London with a degree in particle physics before training as a journalist. When he's not writing, Ben enjoys reading literature, playing the guitar and embarrassing himself with chess.