"Proposition One is that, if things continue as they are, sometime before 2200 (and probably before 2050), we will run out of oil, the transport grid will collapse, global starvation will ensue, and unfettered toxic human waste, absent human guardians, will escape and cause the extinction of all vertebrate life forms."
"Ok, "I'm with you so far."
"Proposition Two is that we are at least a generation too late to bail things out by behavior modification."
"Still with you."
"Proposition Three is that there is no extant technology, and no reasonable extrapolation of any extant technology, that will bail us out."
"Hmmm. Nope, don't buy that one."
"There *IS* an extant technology?"
"An extrapolation, if we chase it. No one wants to."
"And that would be?"
"Bioweapon research is illegal, but everyone knows it is still happening. What we need is a plague that will knock the population down to sustainable levels at a rate that is slow enough to prevent societal collapse, and then burn itself out. Say, something that kills ten percent of the population every year for 20 years; that would knock us down to maybe 800 million, which is probably sustainable. You might want to let it run for 25 or even 30 years, just to be sure."
"It's triage. Right now terrestrial vertebrate life has a half life of about 70 years."
"Could you pull the trigger?"
"When the need arises — and it does — you must be able to shoot your own dog. Don't farm it out — that doesn't make it nicer, it makes it worse." -- Robert A. Heinlein, in the character of Lazarus Long, "Time Enough for Love"