Worst-case climate scenario? Nuclear winter.

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The worst-case climate scenario we face isn't global warming of 4 or even 5 degrees Celsius. It's a nuclear winter that would trigger global cooling up to 12 or 13 degrees C.

That would happen within weeks of the start of a nuclear war, as smoke from burning cities blotted out the sun. The result would be a massive famine as the ocean's food chain collapsed and global crops failed.

In most scenarios, hunger would spread around much of the globe and kill hundreds of millions of people, said Alan Robock, a climatologist at Rutgers University and co-author of two new studies on agriculture collapse and ocean destruction. How bad it would get would depend on the size of the nuclear exchange, but even a "smaller" nuclear war — say, between India and Pakistan — would cause enough global cooling to starve hundreds of millions.

In a war that involved Russia and the U.S., which have more powerful weapons and larger stockpiles, the death toll would likely exceed half the world's population.

Robock is among a number of experts who think an aggressive posture in response to Russian President Vladimir Putin's nuclear threats isn't a deterrent but only puts the world in more danger. Daniel Ellsberg, the famous whistleblower who stole hundreds of pages of nuclear secrets along with the Pentagon Papers, argues the same thing in his 2017 book "The Doomsday Machine."

Talking about annihilation on this scale can make people feel helpless, but it shouldn't. Policy changes could drastically reduce the risk of nuclear apocalypse.

One set of measures should target global warming. That's because climate change caused by emissions could increase the risk of nuclear war by increasing political instability. "Extreme heat, extensive droughts, terrible disasters, and rising seas are creating wave after wave of challenge to societies around the world … thus affecting the social, economic, and political domains of nations — and thereby influencing relations among nations," wrote Christine Parthemore, head of the Council on Strategic Risks, in a speech for last year's United Nations climate summit, COP26.

Another set of reforms should make current nuclear policy less threatening. A way to start would be for more countries, including the U.S., to adopt a "no first use" policy — a pledge never to use nuclear weapons except in retaliation for a nuclear strike.

Last month, the U.S. failed to adopt a such policy in the latest version of the Nuclear Posture Review, a report issued by each new president since the Clinton administration. This seems not just dangerous but immoral. Putin appeared to be threatening first use of nuclear weapons during the Ukraine invasion, a threat the world correctly found monstrous. So why would the U.S. reserve the right to start a nuclear war?

Even in the absence of a no-first-use policy, the U.S. should take its land-based intercontinental ballistic missiles off what's called hair-trigger alert. Right now, if there's a warning of a Russian nuclear attack, the president has about 10 minutes to decide whether to strike back and thereby end the world as we know it, Von Hippel said.

It's a fallacy to equate an aggressive stance with the willingness to use nuclear weapons, said Pavel Podvig, an independent researcher in Geneva who was born in Russia and has studied nuclear security issues at the Massachusetts Institute of Technology, Stanford University and Princeton. The principle of deterrence, he said, rests on showing you have the capacity to kill more people than your opponent. But if the West tries to deter Russia this way, it needs to demonstrate the consequences of a global calamity would be worse for Russia, and that's not likely to be the case.

"I don't think there's any reason to believe that it's mutually assured destruction that prevents Russia's using nuclear weapons," he said. "I believe that it's the fact that any use would be almost universally considered completely unacceptable."

He said he doesn't think Putin will employ so-called tactical weapons in Ukraine because that notion was conceived to attack large concentrations of troops, "and this is not that kind of war." But he worries that even just talking about it is normalizing the use of nuclear weapons. "The message should be that if you use nuclear weapons, you're in criminal territory."

Americans are starting to realize how much we should demand climate stability, and decades of activist pressure is finally starting to move the needle on climate change. We should also be demanding a nuclear posture that does everything possible to prevent a nuclear threat to our atmosphere. And we should demand that our lives not be considered collateral damage in a war that can't be won.