Some of the proponents of global warming theory, 30 years ago were promoting the idea of a nuclear winter. That nuclear explosions would make the planet cooler for years or longer blocking out all sunlight. Scientists who said this is unreasonable were ostracized because they were promoting nuclear weapons.

Unfortunately even without nuclear weapons human beings find lots of ways to kill each other.If you're talking about defense, it actually makes more sense for the U.S. to have nuclear weapons than it does for it to have a large conventional military. There is essentially no conventional military threat to the U.S.,--since no country could fight a conventional military war across the Atlantic or Pacific, and Canada and Mexico are hardly threats--but there is a nuclear threat.

But from a climate perspective, a large nuclear war could seriously mess up the climate.

EDIT: Wow, I have nothing but thumbs down when no one else has a single one! With all the hawkish answers, you'd think I was suggesting unilateral disarmament.

I want to comment on a few things in other answers:

Raisin Caine starts off his answer with "If you look at the statistics for war..." It is statements like that that make me think that his Ph.D. in statistics must be one that you order off late night television. Nuclear weapons have only been used twice in war, there is no set of statistics to base any conclusions off of, and there is certainly no evidence that they've done a very good job at "keeping the peace".

Jim Z says "...he may be more than just stupid. He really may not care if the Mullahs get the bomb." this is absolute nonsense. If he really believes this, it is just more evidence of a serious detachment from reality.

Mike implicitly pooh-poohs the idea of nuclear winter, but gives no evidence to support this viewpoint. The cooling that the planet has experienced from volcanic eruptions such as Pinatubo and El Chichon provide strong evidence that a large scale nuclear war could have a large effect on the climate.

It is a myth that a nuclear war would kill all life on earth. It is really pretty simple math to figure out. It could certainly devastate civilization and kill a good portion of humanity but it wouldn't even kill all people. It would certainly shorten their lifespan for a few generations and mutation rates would increase as well cancer but humanity would survive.

Nuclear disarmament would only encourage other countries that they might win a war. Obama, in his naivete or anti-American cynicism or both, seems to have no care at all about America's interests as he willy nilly disarms us, disses our friends, and negotiates with and encourages our enemies.

I agree Raisin and Iran can't be count on being rationale yet Obama is allowing them to have nukes while pretending he isn't. He stops Israel from attacking them while he stalls and dithers. I guess I should point out that maybe he really is just stupid since he did claim there were 57 states even though he apparently was born in the 50th but it sometimes seems to me that he may be more than just stupid. He really may not care if the Mullahs get the bomb.

Probley because if there's an all out war. Nuclear would destroy everything everywhere, rather than some areas. Mike

So the US disarms. Do you even think other countries will do likewise? Even with Reagan and his, "Trust but verify," statement, we now find out that the trust in Russia was unfounded.

Hello! Do you really think the world would be safer if the US disarmed? Quite the contrary, it would make all the US citizens sitting ducks.

Theres no logical reason to disarm nukes.

If you look at the statistics for war, the nuclear bomb has been one of the most life-saving inventions ever. Assuming rational players, the threat of mutuallly assured destruction is great at keeping the peace.

Unfortunately, it only takes one person in power to NOT be a rational player, to end all life on the planet.

Personally, I don't think we should disarm. Nuclear weapons are close to a 70 year old technology. They are not even too terribly difficult to make, it is the material that is hard to obtain. It is certainly something I wish mankind would have never invented. Fortunately, our system is currently set up so that the US is guaranteed to have at least one rational player. Since this is the case, mutually assured destruction works for the rational players. For the non-rational players, we need to keep the weapons out of their hands.

That being said, I think having enough weapons to blow the world up 4 times might be a little bit of overkill. Enough to blow up the world once should be sufficient.