Thank your for this piece. I am more pessimistic than the 1 billion figure but I guess the time frame is the question.

Dr. William Reese is way more pessimistic at 6 billion lost souls.

https://thetyee.ca/Analysis/2019/09/18/Climate-Crisis-Wipe-Out/

My own hope (can't call it prediction, not scientific at all) is that 4 billion can survive above the 40th parallel past 2060 or 2070 if fusion is 20 years away and won't always be.

I hope your estimate, terrifying as it is, is more accurate than Dr. Reese's or my nightmare.