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According to Wikipedia over 2000 nuclear tests have been performed since the Manhattan Project. If nuclear war would bring about a nuclear winter, why didn't testing do? Were they too much spread out in time to cause any real climate damage?

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In part because many tests were underground tests. –  Volker Siegel Jul 19 at 13:45
    
Looks like we're lucky to be alive bbc.co.uk/science/horizon/2000/snowballearth_transcript.shtml "...but if the freeze continues, such as might happen in a nuclear winter, they could advance down to about where Texas is today. Once the freeze had reached that point so much of the Earth would be covered in white ice that over half the solar heat that normally warms the planet would be reflected back into space. At that point there wouldn't be enough heat left to warm up the Earth. Once this happens there could, in theory, be a runaway freeze, a freeze that nothing can stop. –  Count Iblis Jul 19 at 21:13
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@CountIblis: That is actually quite a terrible article for the BBC. They keep saying an unquantified "the laws of nature" and I don't think that they know what it means. –  dotancohen Jul 20 at 4:59

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up vote 15 down vote accepted

A nuclear winter would be a result of large amounts of smoke blocking light from the Sun. The smoke would be from the fires started by nuclear bombs on cities, not directly from the bombs.

Most bomb tests have been underground, and the above ground tests were mainly done where there wasn't much to burn, for example in the Nevada desert so they didn't generate any significant amount of smoke.

Burning cities and volcanos affect the weather in much the same way but for slightly different reasons. Volcanos produce sulpher dioxide aerosols while burning cities produce carbon dispersions, but both reduce insolation. In both cases their effect on the weather is only marked if they manage to get a significant amount of fine material past the stratopause. Anything in the mesosphere gets rained out too quickly to cause much change, but once into the stratosphere smoke/dust can persist for several years and reduce the light reaching the ground. Rock vaporised by a nuclear explosion will condense quickly and is unlikely to reach the stratosphere.

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But I assume a large amount of rock was vaporized in the atmospheric tests? And - but that's another question - how does a burning city compare to an average volcano eruption? –  Gx1sptDTDa Jul 19 at 15:04
    
@Gx1sptDTDa: I've updated my answer to respond to your comment. –  John Rennie Jul 19 at 15:27

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