Rich, you are generally pretty logical, you seem to be derping over this article though. The author is claiming a drop of 1.1k, not a drop to 1.1k, which means a drop of around 1 degree Celsius, which is pretty serious, but not absolute zero. The reason given for this is not the nuclear warheads themselves, which create very little carbon and dust relatively, but the massive firestorms that would result from their detonation in Indian and Pakistani megacities, of which there are almost a dozen bigger than New York, some of them many times the size of New York. The Author assumes fairly logically that fires thus created would burn for a long time, given the annihilation of urban fire departments, the scale of the fires, and the relative inadequacy of rural fire departments. It is not unreasonable to assume that 100 burning Urban centers would put out an amazing amount of smoke, which could have serious climate effects.
If you remember what happened with the Iceland volcano a few years ago, he is talking about something like that, only spread across the subcontinent and lasting for months.