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I’m running a one-shot campaign and I want it to be in a future where climate change has continued unabated but not so far that people can’t breathe on the surface.

Realistically, how hot can it get before oxygen production is reduced to the point where the air is no longer breathable?

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    $\begingroup$ Most, almost all, oxygen is produced by the plankton in the ocean. There is no way to get the ocean warmed up to 40 °C and still have people alive who would care. (And there is no way to get the ocean warmed up to 40 °C, at least not in the next two billion years. Unless a biggg asteroid hits us, but then it is over in an instant.) $\endgroup$
    – AlexP
    Jun 17, 2023 at 19:13
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    $\begingroup$ It's not the temperature of the ocean that kills things, it's the acidity. As atmospheric CO2 concentration increases, so does the amount of CO2 being dissolved into carbonic acid. In fact, that's where most of our CO2 goes. epa.gov/ocean-acidification/… $\endgroup$ Jun 18, 2023 at 1:01
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    $\begingroup$ Plankton have a fast life cycle, they'll be able to evolve. What will crash the system is when CO2 levels drop too low. $\endgroup$ Jun 18, 2023 at 3:09
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    $\begingroup$ Just to make an unpopular point: I've been listening to "experts" claiming climate change of one form or another would lead to irreversible ecological damage of one form or another "in the next 5-10 years" since the 70s. They've yet to be right. My point? The only realistic answer is "we don't know." Life has proven remarkably resilient. We have some ideas, but no assurance. If what you're looking to do is rationalize an imaginative idea, then a good question is, "what do we think would lead to...." $\endgroup$
    – JBH
    Jun 18, 2023 at 3:21
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    $\begingroup$ @Pelinore Comments are too short for decent instruction. Our sweet spot is "I have situation X and would like consequence Y. I've tried A, B, and C, but they didn't work because of Z. Are there any Real World examples of Y that I can use to model my world?" Unfortunately, it's pretty rare to find questions asked that way. $\endgroup$
    – JBH
    Jun 19, 2023 at 15:29

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The problem with climate change isn't actually the temperature, it's the speed of the change. The difference between a quarter stick of dynamite and a charcoal briquette is time. What's safe to burn over half an hour will blow your grill apart if released in a tenth of a second.

The planet can adapt to change over millions of years. Extinction events are quick, but they still make watching the hands of a clock move look like a dogfight. The "Great Dying", the P-T extinction, wiped out 80% of aquatic species, 70% of terrestrial chordata (that's us) species, and the vast majority of all biomass on the planet. It occurred because our CO2 levels went from around 200ppm to around 2000ppm over a period of 60 thousand years.

We're on track to hit 2000 ppm in a mere 300 years. We are currently destroying species about 2000 times faster than natural selection can spit them out. Those who are watching the hands of the clock call it the "Holocene Extinction Event," and it's going on all around us.

The ocean PH is currently around 8.1, down .1 since the beginning of the industrial revolution. It's really surprising how little PH difference it takes to break biochemical processes. For instance, going from PH 7.5 to 7.4 is enough to change the ionization state of histidine, one of our essential amino acids. Ocean acidification is already responsible for significant coral bleaching. We aren't going to die because it's too hot, we're going to die because the entire ecosystem collapses out from underneath us.

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    $\begingroup$ Any "scientist" that is predicting climate 300 years from now is a sensationalistic, populistic fear-monger... Climate models are extremely inaccurate in predicting events even a few decades in advance (remember the predicted ice-age that predated the global warming?). Also, 2000 ppm of CO2? Without massive volcanic eruptions that is practically impossible. There probably isn't enough fosil fuel left to reach that number at all... $\endgroup$
    – Negdo
    Jun 19, 2023 at 8:29
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    $\begingroup$ @AlexP I hate to be a pedant, but the bronze age literally ended because of a collapse of global trade meaning tin and copper were not in the same hands anymore. Bronze was generally seen as superior to iron at the time due to being much easier to work with. In fact, many bronze age civilisations had the ability to work iron, they just rarely used it, as it wasn't worth the effort when bronze was right there. So in a sense, the bronze age explicitly ended due to the lack of tin or copper (depending on area). $\endgroup$
    – Mash
    Jun 19, 2023 at 12:27
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    $\begingroup$ @Negdo, you're thinking of weather, not climate, and you missed the "it's not the heat, it's the speed" bit. Weather is unpredictable. Climate is inevitable. If you aren't familiar with the models, then you probably think that it's complex on the scale of ecology, but in reality it's no more complex than accounting. (that said, don't underestimate the complexity of accounting). $\endgroup$ Jun 19, 2023 at 14:42
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    $\begingroup$ @Negdo, Please cite your information sources. I'll cite mine. Let's start with climate.nasa.gov. They aren't the best, but they're good at bringing it down to the average person's level. Climate models are accurate to around .05 degrees per year, and even the most conservative ones trend up. Any complaint you can come up with, I guarantee a climatologist has heard it and addressed it. $\endgroup$ Jun 19, 2023 at 14:57
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    $\begingroup$ @Jedediah, That's a blatant falsehood. Scientists are constantly trying to disprove their own theories. That's the foundation of science. For instance, they sent a satellite up to measure the amount of heat being received from the sun, and the amount reflected by the atmosphere, simply because that was a possible way to disprove climate change. Spoiler: it didn't. If you want change your statement to "every experiment performed recently gave results that support climate change," then you'd be much closer to the truth. $\endgroup$ Jun 19, 2023 at 15:23
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The concept you need is called wet-bulb temperature, essentially the temperature you get when you put a wet piece of cloth on your thermometer. If this goes above 35°C humans can't survive without artificial cooling. As I understand it, any other warm blooded animal would have the same problem.

So what you want is some tropical regions where this temperature is surpassed on a regular basis. It is too hot there for humans to live or even go there exploring. Further away from the equator it is still unpleasantly hot but survivable. I think current models predict that this will start happening with around 5°C of global warning, if you use say 10°C you should get a thick strip of heat death around the equator with some barely survivable areas closer to the poles.

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Getting some mass dying on the way is likely, though at least some of plants and algae are going to not only make it, but thrive as their competition would be dead.

Paleocene–Eocene Thermal Maximum is not even related to any mass dying, and we already talk about +6C and polar regions with temperate forests. CO2 concentration reached at most 0.1%

Based on project RHO realistically humans should be able to live with 1% CO2

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I'd expect some non-linear extrapolation, but plausibly you should get some jungle planet. (heat maintained, so low temperature differences)

I'd say that weakest part would be humans having to live with temperatures above 40C, though with air conditioning (or mountains: -6C/1000m) it would be still possible. I'd suspect that quite a few algae are going to enjoy it.

Concerning organism adapting - we would need milenia of CO2 emissions, so civilization doing so, would be able to release some adapted GMOs.

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