Nuclear Winter is up for debate in terms of its effects. Krakatoa, in 1883, blasted something like 5 Cubic Miles of Ash into the atmosphere; this caused a dip in temperature of roughly 1.2 degrees C with a five year gap to normalcy.
One thing I'd consider seriously about nuclear weapons is that exaggerating their effects to make their use less likely may be a deliberate aim.
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There is one point I'd insist on: Japan is no position to resist as is being speculated here. The Landing on the Beaches would be brutal, but it would also be final. Operation Downfall assumes that Japanese Morale would hold up in the face of the massive bombardment, starvation and privation being forced on a people.
Once the landings on Kyushu are hammered through, the case for surrender is impossible to ignore. It is one thing to face near starvation, another entirely to face the consequences. Millions are dying; what advantage does further resistance hold? Kyushu would have most of the IJA's last ability to resist--and probably against the wrong foe. The Soviets remember 1903, and their revenge will be dark and terrible...
The landings in 1946 will be the last great fight of WW2; all the cards will come to play: The USA's use of its nuclear arsenal, Japan's final deployment of its Kamikaze and Kaiten forces.