What if, sometime within a decade after the Tsar Bombs test, both the Soviet Union and United States test a 1000 megaton bomb? The Soviets use the same location as the Tsar Bomb, the United States tests in Northern Alaska. Both are airdropped if possible, otherwise it's a ground blast.
What affects would this have on the world's environment? Would both sides keep trying to increase the yields of their tests? How would the international community react?
Alt. Scenario:
What if one of the tests is 10,000 megatons? What if the Soviets do it? What if it's a US bomb? How would the citizens of each feel about this?
What impact would this have on the area it was tested, and the world? What would the rest of the world think?