Not substantially. The fallout produced would obviously be substantially larger than any one nuclear explosion, but it's still a small amount on a planetary sca

Not substantially. The fallout produced would obviously be substantially larger than any one nuclear explosion, but it's still a small amount on a planetary scale. If you naively scaled up the contamination area of the castle bravo test from 15 Mt to 15 Gt, it would be 3% of earth's surface area. Further, the range that fallout can travel is limited by the half lives of the particles, not the size of the explosions, so it does not scale like this. We also have experience with large amounts of fallout released into the atmosphere. For comparison, in a 12 month period from 1961 to 1962 there were 340 Megatons worth of nukes detonated in atmosphere. While I would recommend against detonating a few times that much all at once, it's a few percent increase in global cancer rates scenario, not a nuclear hellscape scenario.