There is a strong argument to be made that there is a deadline at some point where we will need to have colonized other planets before we extinct ourselves through war.
Though sadly interplanetary warfare seems much easier than extraterrestrial colonization.
Please, humans destroying themselves through war is nothing compared to a large rock from space hitting the planet at 20,000 miles per hour. The amount of energy in the entire world's nuclear stockpile would be dwarfed by the energy of that happening. Hell, I wouldn't be surprised if the 2004 tsunami in the South Pacific contained more energy than the sum of all of earth's weapons.
The point is if you're going to talk about the threat of extinction, at least provide a plausible case for the argument. The negativity of "humans are going to extinct themselves" isn't inspiring. If we're going to kill ourselves, why even bother? If you want to make a case for us getting off this death trap of a rock, one of "we should work together to fight things out of our control" is much more inspiring than "I want to get off the planet before all you other idiots kill yourselves."
The sudden release of Energy isn't what would cause extinction from nuclear weapons, it's the persistent fallout which would make the entire surface of the planet uninhabitable for many years.
"Based on new work published in 2007 and 2008 by some of the authors of the original studies, several new hypotheses have been put forth.[16][17]
A minor nuclear war with each country using 50 Hiroshima-sized atom bombs as airbursts on urban areas could produce climate change unprecedented in recorded human history. A nuclear war between the United States and Russia today could produce nuclear winter, with temperatures plunging below freezing in the summer in major agricultural regions, threatening the food supply for most of the planet. The climatic effects of the smoke from burning cities and industrial areas would last for several years, much longer than previously thought. New climate model simulations, which are said to have the capability of including the entire atmosphere and oceans, show that the smoke would be lofted by solar heating to the upper stratosphere, where it would remain for years.
Compared to climate change for the past millennium, even the smallest exchange modeled would plunge the planet into temperatures colder than the Little Ice Age (the period of history between approximately A.D. 1600 and A.D. 1850). This would take effect instantly, and agriculture would be severely threatened. Larger amounts of smoke would produce larger climate changes, and for the 150 teragrams (Tg) case produce a true nuclear winter (1 Tg is 1012 grams), making agriculture impossible for years. In both cases, new climate model simulations show that the effects would last for more than a decade."
Hiroshima was about 15 kT, so your number gives 345 megatons. Tsar Bomba was about 50 Mt, which is more than the energy of the tsunami.
According to http://en.wikipedia.org/wiki/TNT_equivalent , "The total global nuclear arsenal is about 30,000 nuclear warheads with a destructive capacity of 5,000 megatons or 5 gigatons".
> From what I can tell that's a lot more than the global supply.
Where are you getting that from? Wikipedia says 17,000 nukes (4,100 active), and IIRC modern weapons tend to have ~30x the energy of the one that was dropped on Hiroshima.
It would be far cheaper just to build strong bunkers than to build a long-term self-sufficient colony on another planet. Not that a nuclear war would be anywhere near enough to cause extinction in the first place.
I'm all for colonizing other planets because it's cool, but lets not pretend there is any practical reason to do it.
Though sadly interplanetary warfare seems much easier than extraterrestrial colonization.