Whenever the Tsar Bomba comes up, the potential for it to have ignited the atmosphere is always brought up. What exactly, however, does that mean? (Apologies if that sounds sarcastic; it's a genuine question!)

My guess is that the heat/EM could induce a state change in atmospheric gasses into plasma. Would this be a chain reaction that ultimately encircles the world? Also, what are the implications of that (locally or globally)?...

It might get brought up because before the first atomic bomb was detonated, one of the physicists who worked on it (Edward Teller) suggested the possibility that detonating the bomb would ignite Earth's atmosphere. There's a bit more information here: http://www.sciencemusings.com/2005/10/what-didnt-happen.html

There is an old Soviet sci-fi book from 50s by Alexandr Kazantsev, "The Burning Island", where he depicted an island in the Pacific Ocean producing some catalyst that made self-sustaining exothermal reaction between nitrogen and oxygen possible; the reaction had sucked out all oxygen from Earth atmosphere. The book has several scientific discussions, but the essence of the catalyst is avoided.

There is another sci-fi book by the same author, "Faetians", where he depicts the fifth planet of the Solar System, Faeton (a hypothetical planet between Mars and Jupiter that could be the source of the asteroid belt), destroyed by nuclear fusion of its ocean hydrogen after several powerful nuclear explosions. The author gives even less explanations there.

So, I think the atmosphere explosion is a myth rooted in sci-fi books, at least in Soviet culture (I have not yet encountered such plots in American literature/movies of that time, but maybe I'm just not familiar enough with it).

> There is an old Soviet sci-fi book from 50s by Alexandr Kazantsev, "The Burning Island"

Hah! I remember reading this as a kid and absolutely loving it.

Of course, most of the geopolitical stuff whooshed right over my head.

None that i can think of, but i recall one or two stories that poke at the concept of a nuclear winter.

Every transformation (chemical, nuclear) has an activation energy.

When you're exploding bombs using a new principle you might cross the activation energy of stuff that you'd rather keep around.

This is only a problem if it's possible for the transformation to be self-sustaining. That is, the act of transforming atmosphere causes yet more atmosphere to transform. Only in that situation would the whole planet's atmosphere burn.

Is that possible?

Let's make an analogy with trees. You can't burn a living tree outright. Or so you'd think. But once there is enough energy around a forest fire can jump from crown to crown faster than you'd imagine possible outrunning the ground fire. This will continue until the fire runs out of fuel or the conditions change enough to put it out.

http://www.thefreedictionary.com/crown+fire

There is so much energy in the system that the burning crown of one tree dries out the matter in the crown of the next to the point where it can be combusted in an extremely short time, contrary to ordinary conditions when lighting 'wet wood' appears not to work.

The whole point was that Teller thought that the reaction might be self sustaining once the activation energy was reached.

Think about it: someone made the call to ignore that possibility and pull the trigger on deployment even if they weren't sure about the outcome. Possibly the single most irresponsible deed in the history of man.

> someone made the call to ignore that possibility

Actually, they did the calculations and verified that it wouldn't happen. It was Richard Hamming who did the calculations. https://en.wikipedia.org/wiki/Richard_Hamming#Manhattan_Proj...

I said to myself, "What have you done, Hamming, you are involved in risking all of life that is known in the Universe, and you do not know much of an essential part? [uncertain variable]" I was pacing up and down the corridor when a friend asked me what was bothering me. I told him. His reply was, "Never mind, Hamming, no one will ever blame you."

I assume it's similar to fear that fusion bomb detonated under the sea would cause a chain reaction with the hydrogen, turning all water into plasma.

Fission of atmospheric hydrogen.

Share of hydrogen in the atmosphere is almost negligible; nuclear fusion of ocean water is a bit more likely, although it still has to meet too many conditions.

My understanding was you can actually get a chain reaction in the world’s oceans, but it would take significantly more energy to kickstart than any atomic bomb designed.

PS: Fusion is one of the few equations where you will see X^4th power so you vary quickly go from it's fine to big boom. However, igniting the world’s oceans would take enough energy to kick off that we would be dead either way.

You mean fusion.