There is no scenario where 400ppm CO2 has ever led to any of the extreme events you are casually throwing out there as realistic possibilities. We have geological evidence of Earth's CO2 concentration and climate history going back 500 million years. We know it was close to 7000 ppm during the Cambrian explosion 500 million years ago. We know it was in the thousands of ppm for most of life's existence on this planet. We know the oceans didn't boil off.
What we know from geology is that not only is the planet extremely antifragile, life itself is as well and always adapts to the always changing climate. Nature has thrown many curve balls, including Laschamp events that caused the Earth's magnetic field to flip for a few hundred years, which led to mass extinctions in the southern hemisphere and some say, the end of the Neanderthals [2]. Or more recently, the Younger Dryas 12 thousand years ago, where temps returned to ice age levels before rapidly warming to above modern day levels [3], and also caused mass extinctions of many animals including wooly mammoths, American lions, saber tooth cats, giant ground sloths, and so much more [3].
And those two events happened with CO2 levels at less than half of what they are today. Of course, we wouldn't ever want CO2 levels to go below 150ppm, because at that level photosynthesis is not possible and all plant life chokes and dies (happens much earlier than that for higher elevation).
Thankfully that Younger Dryas episode, whatever caused it, we still don't fully understand it...subsided, CO2 levels rose back up with the warming temps to the Holocene optimum levels, that led to favorable conditions for civilization to flourish and for agriculture to take off.
We're still in an ice age, and we'll return to a glacial period once again, because volcanoes are ultimately in control of that process. We got a preview of that in 1816, the famous "year without a summer". Our job as humans is to adapt to whatever nature throws at us.
[1] https://www.tropicaltidbits.com/analysis/models/?model=gfs&r... [2] https://techthoroughfare.com/earth/laschamps-excursion-mass-... [3] https://www.climate.gov/sites/default/files/historictemperat... [4] https://en.m.wikipedia.org/wiki/Year_Without_a_Summer
I'm really unclear on the point here. A rapid transition to the climate of the Cambrian period would annihilate most civilization - The globe was entirely free of ice at the end of the Cambrian and temperatures were 7 degrees celsius higher than today.
While it's plausible for humans to adapt to significantly colder temperatures, it's difficult for Human's to adapt to higher temperatures in a short period. There are limits to what A/C systems can handle, and I'd doubt that people would enjoy required bathing times per day for temperature reasons.
And if you think our geological history provides the answers to what our future with climate change will look like, I implore you to get familiar with the anthropic principle[1]. We have never significantly altered our atmosphere this fast before, there is no precedent. And using our geological past as a dataset is inherently flawed. In order for us to observe that data, we necessarily would have had to survive/emerge from that history. How many other planets had similar geological records but then took a different turn? If you can show me that data, I will re-consider you anti-fragile position.
Humans are adapted to Holocene, so it seems best to not poke the bear. However, 100 years of pollution need to be undone for that.
And regrading your concerns about the modeling. I agree, the uncertainty is real and quite frankly that should scare you and everyone else. There is simply no way for humans to fully understand all of the different mechanisms at play. We are entering uncharted territory. We might get lucky and some dampening feedback emerges that we did not account for that moderates the climate impacts. Or we might get very unlucky and kick off a mechanism that results in our oceans boiling off killing all life. Most likely, we will end up somewhere in the middle of those two extremes and it will take quite a while before we get there. But once again, the uncertainty is massive. So we are essentially rolling the dice on the future of humanity all so a handful of fossil fuel executives can continue to line their pockets.
[1] https://mobile.twitter.com/PolarPortal/status/14112805173166...