It may feel as if the world is ending, but it’s already happened — five times over the planet’s 4.5 billion-year history, to be precise.

From the Ordovician-Silurian extinction event 440 million years ago to the dinosaur-killing Cretaceous-Tertiary extinction 65 million years ago, the Earth has experienced five mass extinction waves when more than 75 percent of the species on the planet were snuffed out. Forget threatened species — these were the moments when the lights almost went out on all life on Earth.

What nearly all of those extinction events have in common is severe climate change on a geologically rapid time scale. During the End Permian event 251 million years ago — when an estimated 96 percent of species on Earth were killed off — a colossal volcanic eruption near what is now Siberia blasted vast amounts of CO2 into the atmosphere. It led to a spike in global temperatures and climate disruption that most species couldn’t endure.

When it comes to extinction — including our own — we should be worried about sudden climate change that occurs too rapidly for us to survive. Human-caused climate change will cause unprecedented suffering, but even under the worst-case scenarios it seems unlikely to unfold fast enough to definitively wipe us out.

But as new research demonstrates, there is something else that may: nuclear winter.

The long night

In a study published Monday in the journal Nature Food, researchers led by Lili Xia and Alan Robock at Rutgers University modeled the climatic impacts of a nuclear war, and then attempted to quantify the effects on global food production.

The results were grim: a full-scale nuclear war between the US and Russia with their current number of warheads could lead to as much as 150 million tons of soot being injected into the atmosphere, thanks to massive fires ignited by the explosions. All that soot would quickly spread around the globe and block incoming sunlight, putting the equivalent of a shade over the planet and leading to drastic global cooling. In the cold and dark, crops would wither and die, as would the livestock that depend on them.

As a result, the researchers project that global calorie production could drop by as much as 90 percent, leaving an estimated 5 billion people dead from famine in what is now known as nuclear winter.

“This would produce climate change that is unprecedented in human history,” Robock told reporters in a briefing on Monday. “In a US-Russia nuclear war, more people would die [from starvation] in India and Pakistan alone than in the countries actually fighting the war.”

While the length and severity of the projected nuclear winter is related to the number of warheads used in an exchange, the researchers find that even a “limited” nuclear war between India and Pakistan — two nuclear-armed countries that have repeatedly clashed over the past 75 years — would have global effects on the climate. The fires from such a war could release as much as 47 million tons of soot into the atmosphere, with a worst-case scenario causing global calorie production to drop by as much as 50 percent and leading to 2 billion deaths around the world.

The new study took advantage of recent computational progress in the latest climate models, only forecasting what would cause rapid cooling rather than long-term warming. Models of soot-forced cooling were fed into the National Center for Atmospheric Research’s Community Land Model. That allowed the researchers to estimate the effect that cooling and the severe damage to the atmosphere’s ozone layer caused by nuclear explosions and soot would have on major crops like rice and wheat, as well as livestock pasture and global marine fisheries. “We were able to quantify how much food would be available for every country,” said Robock.

It’s important to note that these numbers are estimates of the unimaginable, with significant uncertainty. Even with the best computer models, it’s difficult to know exactly how the climate would respond to nuclear war, harder to predict how cooling would precisely impact food production, and even tougher to say how human society would respond to what would be the most catastrophic event our species would have ever experienced.

But we do know from the past that we would likely see significant cooling in the event of a nuclear firestorm. A massive volcanic eruption on Indonesia’s Mount Tambora in 1815 — the largest such event in human history — led to cooling so extreme that the following year was known as the “year without a summer,” as crop failures and famines led to global starvation. More recently, the 1991 eruption of Mount Pinatubo in the Philippines injected 15 million tons of sulfur dioxide into the atmosphere, causing a temporary reduction in average global temperatures of 0.5 degrees Celsius.

The return of nuclear winter — and nuclear fears

The Nature Food research is the latest paper in a long-running series of studies to examine the possibility of “nuclear winter.” While scientists were concerned about the effects that nuclear war would have on the climate from the earliest days of the Cold War, the term was first raised in studies published in 1983 by a team of researchers including the celebrity scientist Carl Sagan. Even before the research had come out — though after the studies had been accepted for publication — Sagan published an article in the popular magazine Parade hyping the threat from nuclear winter.

The original nuclear winter research had enormous political influence, and was enormously controversial, as the historian Jill Lepore described in a 2017 piece in the New Yorker. While his administration pushed back against the research, President Ronald Reagan was largely persuaded by the argument, as was then-Soviet Premier Mikhail Gorbachev; Reagan noted that a nuclear war “could just end up in no victory for anyone because we would wipe out the earth as we know it.”

Within a few years of the publication of the original studies, the number of nuclear warheads in the world began to decline, from over 60,000 to around 10,000 today — and, with it, the fears of nuclear war.

But as Russia’s invasion of Ukraine this year demonstrates, we may have forgotten about nuclear war, but nuclear war hasn’t forgotten about us. More countries possess nuclear weapons now than during the Cold War. International arms control treaties have begun to crumble, even as philanthropies have withdrawn from the nuclear realm. Delegates have been meeting this month at the United Nations in New York for the Non-Proliferation Treaty Review Conference, the cornerstone of the nuclear arms control regime, but little progress is expected even as global military spending is reaching a record high and international tensions have tightened.

If the threat of nuclear war isn’t as high as it was during the worst days of the Cold War, it is worse than it has been in years — and any risk of a disaster as horrific as the one outlined in nuclear winter research is too high to endure. Earlier this month, the Future of Life Institute, a Cambridge, Massachusetts-based think tank on catastrophic risks, gave its annual award to the scientists behind the original nuclear winter theory, and warned that this threat was not yet behind us.

“The latest nuclear winter research confirms that Reagan was right when he said that a nuclear war cannot be won and must never be fought,” said Max Tegmark, a physics professor at MIT and one of the founders of the Future of Life Institute. “In these turbulent times, the more decision-makers understand about nuclear winter, the less likely they are to make reckless decisions that may cause it.”