These contemplated scenarios could easily lead to 1 billion deaths or more from a catastrophic event. Something at that scale hasn't happened since the Flood. But that scale is explicitly laid out in the Book of Revelation near the beginning of the tribulation. One of the first shots over the bow in the tribulation will be the deaths of 1.75 billion people (at today's population) (Rev 6:8) not too long after that, another 1.75 billion people gone (Rev 9:15).
As I said there are a great many times in the history of man that catastrophic events have occured that seemed to be the end such as the Black Death which killed 1/4-1/2 of the earth’s population. As for me no mater how bad the scenario I would never presume to call it God’s Tribulation.