And yet we DO have the historical record. I am no climate-change-denier (far from it), but the current concentration of CO2 is far from the historical maximum. We can say with some confidence that the new equilibrium point is not likely to be Venus. Reaching it _is_ going to cause mass extinctions and _will_ have very negative effects for humanity, but the planet has been through this before and is probably going to be okay in timeframes measurably longer than our existence as a species.

In geologic time frames, we are likely to be entering a domain where glaciation entirely disappears, including in Antarctica. The world was last here ~34M years ago, and sea levels were 200ft (60m) higher. I'm sure anyone reading this understands that this will have huge impacts on our day to day, including the drowning of most of the today's population centers.

(If anyone is curious, research the Eocene, the era that likely has the best predictive power for what will be happening to our climate over the next few thousand years). Just imagine it running in reverse, at a greatly accelerated rate.

And if you're wondering how we might avoid this fate, from an engineering perspective we need to recreate something like this: https://en.wikipedia.org/wiki/Azolla_event

So, i agree, not Venus. Not even as difficult as living under water. But the air would be hostile by default. So special equipment and housing would probably be required for long term survival. We'll all be settlers living on an earth-like planet.

co2 material safety data sheet says 5000 ppm is max allowable for an 8 hour work day. [1]

co2 levels (appear to) have a direct impact on cognition [2], specifically strategy and information usage.

Ecoene had levels around 2000ppm. It'll be interesting to see if we overshoot or undershoot. Sensitive people start noticing effects at around 1k, perhaps as high as 2k for insensitive people. 2-5k "Headaches, sleepiness and stagnant, stale, stuffy air. Poor concentration, loss of attention, increased heart rate and slight nausea may also be present." [3]

I don't think there have been any studies of long term exposure to 2k+ ppm (with no recovery time). as far as i know, nobody has to deal with that level of exposure for months or years. It's likely not good. But, as you say, not Venus.

[1] https://www.airgas.com/msds/001013.pdf

[2] http://ehp.niehs.nih.gov/15-10037/

[3] https://www.kane.co.uk/knowledge-centre/what-are-safe-levels... (not a great source, but plausible)

Maybe it's already been written but I would like to read a sci-fi novel where humanity realizes that our atmosphere is at risk and begins a race to capture it. The end result being that the planet's atmosphere is completely sucked up in to massive containers of liquid air. These are controlled by a few entities around the world, i.e. those who were most successful at liquefying air when it was free. Liquid air then becomes the main commodity about which all commerce is based. Who you get your air from determines your social status and vulnerability.

Keep in mind that solar output itself has steadily increased since the eocene.

I don't disagree with anything you just said, but many people take "history" to be distinct from "prehistory" (the latter ends and the former begins at the point in time when humans began to keep records of their affairs). So I caution you not to assume people are necessarily being disingenuous when they say something is at a historical high or whatever.

> I am no climate-change-denier (far from it)

Have we really reached the point where it's necessary to claim political correctness before embarking upon a discussion?

Do you think it's necessary in order to have your opinion taken seriously, or to avoid moral opprobrium?

I am having a very hard time understanding any climate scenario that would make Mars more habitable than Earth. Why should we spend a dollar getting to Mars?

In and of itself, the attempt to colonize Mars probably won't do much for the species. The value is in actually going through the paces: developing primary technologies, techniques, protocols, and doing research, especially observing how organisms (ultimately, humans) are affected by extraterrestrial life, etc.

If the worst comes to pass and the species needs to throw a hail mary, I have to think our chances of success would be improved (even if slightly) if we were to have these sorts of details ironed-out in advance.

There's also the terraforming possibility, even though that's still a long, long way away. It may turn out to be easier to create a habitable atmosphere from scratch than to make even relatively small changes to our own, since the Earth already has all these complex feedback loops. There's also not as much margin for error here because, as The Tick would say, this is where we keep all our stuff.

Not going to be a problem in the next 100 years, but ...

Suppose we started to really use Fusion power not just at our current energy needs, but enough to directly warm the planet. There are hard limits on just how much heat per m^2 you can dump before you start to cook the planet.

Such a civilization may prefer low atmospheric pressure and a planet further from the sun simply for cooling.

I agree. However, I doubt anyone who is actually investing resources in getting to Mars is doing so with climate scenarios as their sole motivator.