There's faith!
But no, humans are not triggering global warming through main force--quite impossible--but by triggering cascades of events.
And no, no one knows where the cascades stop. It is safe to say the Earth will not get as warm as Venus. It is even safe to say single-celled organisms will survive. Beyond that, I haven't heard an argument that was anything other than wish-fullfillment.
By the way, # 3 IS the Permian, under the most recent hypothesis.
Right now, scenerio # 1 is within reach.
Meanwhile, for small change in climate, I think Miguel and Dodo have covered the basic idea. And, as they point out, it is not like it hasn't happened before. Indeed, a dirty secret of archaeology is that civilizations always collapse, seemingly from causes that are self-engendered. I think we are seeing that now.
What do I think is most likely?
Civilization collapses, spectacularly. Humans survive in smaller numbers, under more arduous conditions.
How arduous? Undecided at this time. Which is why what we do now is so important. The Fates are kind.
...It is safe to say the Earth will not get as warm as Venus. It is even safe to say single-celled organisms will survive. Beyond that, I haven't heard an argument that was anything other than wish-fullfillment.
An argument that says that, absent some specific argument to the contrary, it is implausible that small perturbations will produce effects far larger than the effects of numerous small perturbations in the past doesn't strike me as wish fulfillment.
When the proposed effects are far larger than those resulting from past perturbations that are themselves far larger, this line of argument looks even less like wish fulfillment. Note that "implausible" doesn't mean "known with certainty to be impossible".
Regarding large plants, #3, and the Permian/Triassic extinction, I perhaps got carried away when I read that amphibians survived. Regarding the plausibility of our bumping the Earth into a repeat performance, the currently popular theories of what happened all involve huge cosmic or geological causes (The Great Dying). None of these involves the climate just getting bumped off track a bit.
Regarding "civilizations always collapse", it's a good thing that the world has several, even if they do clash.
Regarding the question, "How arduous?", if the primary expected problem is higher temperatures, and people know an inexpensive way to cool the Earth, what would you expect to see happen? I think it's more than likely that the problems caused be greenhouse gases in mid-century, however large they may be, won't be those of high temperature per se.
If this is true, then scenarios that assume otherwise will misdirect our efforts. Words and ideas I offer here may be used freely and without attribution.
