system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:We've had over 2000 nuclear explosions due to testing; Why haven't we had a nuclear winter?
gpt
Because the "nuclear winter" idea presumes that they would all go off at once (at least, close enough together). And it also assumes a number of other issues, like that huge fires would erupt and that few people who be around to fight them. This would result in huge amounts of ash and dust and smoke in the air, less foliage to block chilling wind, etc. 

One or two isolated tests won't have that same effect.