Today's approximately 4,000 operational US nuclear bombs and warheads collectively comprise a maximum combined yield of about 1,398 Mt.https://twitter.com/wellerstein/status/838062645001474048 …
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I have played with numbers a bit (various methods) and got more like 400-1,000 Mt, but my methods are admittedly inexact.
Not knowing exactly how many B61-7/-11s and B83-1s are operational is the trickiest variable, esp. since B83-1 is 1.2 megatons.
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