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Comment by jbay808 | Hacker News Reader

Yes, and that history of chemical changes is punctuated by global mass extinction events!

Just saying, it's not like there's a law of nature that keeps Earth habitable for humans. We should be aware of that. I think you're shifting the goalposts -- earlier you seemed quite confident that there wasn't a period within hundreds of millions of years that wouldn't have been habitable by humans. Have I changed your mind about that?

I don't think anyone who worries about climate change as an existential threat is thinking of sea level rise, a few degrees C and a few hundred ppm of carbon doing us in. Those could make life quote difficult at most. But it's the feedbacks that might run away from there, which are very hard to bound. How quickly might a few hundred ppm become a few thousand, if methane hydrates start to off-gas? Etc.

How confident can you be that it won't happen, when the Earth has carried itself away before?

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Sorry, not convinced that the earth has been uninhabitable for humans in the past few hundred million years. What conditions made it uninhabitable everywhere on the planet?

Earlier posters made it seem like the problem with climate change was that earth was moving out of a narrow temperature band that supported human life. We live in the arctic, jungles, deserts, ice ages, mountaintops. We will be fine. The problem is the transition’s impact on our existing civilization (sea level change, new weather patterns), not the final temperature.

Let me put it this way: if we discovered earth 100 years in the future, even with our current climate change rate, we would rejoice about how easily it could be colonized (vs Mars, the moon, etc).

The climate will get hotter, and it’ll still be quite livable in places. This is the equivalent of people who think earth is in the “just right” orbit, if it were 10 feet off in either direction the planet would freeze or burn. And yet the elliptical orbit of the earth around the sun varies by millions of miles, and we don’t light on fire when climbing a ladder. We don’t die because someone left the AC 5 degrees too hot or cold.

Thanks, I appreciate your thoughtful responses so far.

http://burro.case.edu/Academics/USNA229/impactfromthedeep.pd...

If you can, please let me know what you think of the image on page 6. Mind you this is an unproven theory about the end permian mass extinction. But it's an interesting perspective on what could cause the whole planet to become uninhabitable, rather than just shifting the comfortable zone farther to the poles. I'm curious about your thoughts on, if it did happen that way, how long humans would last in such conditions.

Humans are hardy and adaptable, but so are insects, birds, rats, and many other species. Somehow the end permian extinction killed up to 96% of all species, including even insects and the formerly ubiquitous trilobite, and apparently nowhere on Earth was spared. Land vertebrates were almost wiped out.

I also appreciate the optimism about mars colonies, but we actually haven't demonstrated the technology or robustness to even sustain a colony on Antarctica without outside support. If we did set up a mars colony, I'm skeptical that it could last even a hundred years on its own. Technology is a powerful tool but supporting cement factories, steel mills, silicon fabs, power plants, farms, and everything else, in a self-sustaining way, really requires a habitable planet. I don't think it would be possible to sustain in an underground bunker for very long.

Thanks for the reference, that is interesting. I think that's what I was looking for in a theory of why things will be bad: it's not the temperature change itself (a few degrees is fine), or the atmospheric CO2 levels (still far less than 1%), or even the water levels (we can move, if painful economically), but the release of H2S, which kicks off a chain reaction of extinction, which may impact food supply etc.

I hardly know enough to comment on whether that is likely. But I think it's a much better argument about why the Earth will be uninhabitable for us specifically.

(That said, it doesn't really matter if the earth eventually becomes uninhabitable or not, we want to avoid the cost of having to move everyone for even minor sea level rises.)

Thanks for replying. Yeah I fully agree with you. The more probable impacts of climate change are bad enough that they should be the motivation. I think as an existential threat, it's an unlikely outcome but not one that we can totally rule out... Like other existential threats.

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