Here’s what a nuclear war would do to the planet, according to scientists

Experts insist the odds of the conflict in Ukraine escalating to one that would lead to a nuclear event are remote. But despite that, 69% of Americans surveyed by the American Psychological Association say they “are worried the invasion of Ukraine is going to lead to nuclear war.”

Although the people who watch for signs of heightened activity in Russia say they’ve seen nothing to cause concern, comments from officials, such as United Nations secretary general António Guterres saying “the prospect of nuclear war is now back within the realm of possibility,” continue to stoke those fears. 

The focus of many people in that unlikely event is the initial blast, which would cause mass casualties and unimaginable devastation. When the U.S. detonated nuclear weapons over the Japanese cities of Hiroshima and Nagasaki 77 years ago, it leveled the cities and killed more than 100,000 to 200,000 people from the blasts or radiation sickness.

But, according to scientists, the horrors of a nuclear war could affect the whole planet. A number of studies have examined possibilities, with a special focus on the environment, and it’s not encouraging.

As you might expect, the extent of the nuclear exchange plays a tremendous role in the impact. One or two nuclear impacts would not have global effects, according to a 2012 analysis published in The Bulletin of the Atomic Scientists. But the impact of 100 weapons the size of the one that fell on Hiroshima would lower temperatures around the world to below those of the Little Ice Age that occurred from roughly 1300 to 1850. That would result in crop failures and famine on a wide scale.

In a separate study, from 2014, four U.S. atmospheric and environmental scientists modeled the aftereffects of a conflict that also explored the effects of a detonation of 100 warheads—in this case picturing what would happen if the battle took place over the Indian subcontinent. Here’s what they found:

Five megatons of soot and ash would fill the sky, bringing about a “nuclear winter.”
After one year, the average surface temperature of earth would fall by about 2 degrees. After five years, the earth would be three degrees cooler than it was. Twenty years down the road, it would warm up to one degree below where it was before the nuclear event. 
That might sound beneficial, given all the talk about global warming today, but lower temperatures mean less rain. Five years after the detonation, rainfall would be at 91% of current levels. After 26 years, we would still see 4.5% less rain than we did before the war. And the reduced rainfall would bring about global drought conditions.
Depending on the region, growing seasons for crops would be 10 to 40 days shorter, resulting in widespread famine.
The ozone layer would diminish due to the radiation, ultimately becoming as much as 25% thinner for the first five years after the event. After 10 years, there would be some recovery, but it would still be 8% thinner. This would result in a rise in skin cancer and sunburns. And the increased ultraviolet rays would put plant and animal life that survived the initial blast at risk.

It wouldn’t necessarily take 100 missiles for those theories to be tested, either. The United States’ modern B83 bombs are 80 times more powerful than the weapon that hit Hiroshima. Russia has tested weapons that are even more powerful. That said, experts say that if Russia were to use nukes in Ukraine, it would likely use “tactical” weapons, which are less powerful than the ones the United States used in World War II but could still cause widespread casualties from radiation.