[hoodoof]

The U.K., U.S. and Russia have already bombed the world with hundreds of atmospheric nuclear weapons in the mid 20th century.

In effect there was a nuclear war, it's just that all the bombs were exploded on "friendly territory" like Australia, the pacific islands and Nevada.

[D-Coder]

Atmospheric tests do not produce the same particulates as setting entire cities on fire.

[hoodoof]

So it was sort of OK "nuclear war on ourselves"?

[Teever]

I believe you misunderstand the cause of a nuclear winter.

The cause isn't the nuclear radiation per se. It is the extreme destruction of many large urban centres simultaneously. The problem is that all of those cities burning simultaneously would send up a tremendous amount of ash and smoke that would block the sky for quite a while.

This would cause a reduction in the amount of sunlight that would be available for growing crops and combined with the almost total destruction of global economy could result in the destruction of the entire human race through mass starvation.