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Consider an Earth like world except much hotter with extremely hot and wet tropical rain forests. Assuming 100% humidity, how hot might the alternate Earth tropics get and still support tree like organisms and vegetation? And could humans survive un-aided in such an environment?

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  • $\begingroup$ Humans that have evolved on that planet, or humans that are colonizing it? I'm not sure how we can give you a factual answer for "how hot can it get?" when the only reference we have is Earth. What's the percentage of ocean to land? The closer you get to the sun, the more water there needs to be. Terrestrial jungle plants likely start dying at 130° - but they didn't evolve on a hotter world. I'm beginning to think this Q is opinion-based. $\endgroup$
    – JBH
    Sep 5, 2022 at 20:57
  • $\begingroup$ define hot, 40 degrees is pretty hot on earth. $\endgroup$
    – John
    Sep 5, 2022 at 21:01
  • $\begingroup$ Humans have evolved on the planet, but are a "high altitude" species effectively equivalent to colonists. The question is how hot can it get in a jungle assuming as much rain and moisture as is required? Is it reasonable to suppose that this temperature might easily far exceed what would be tolerable to a human? jungles at 60 or 70 degrees C? Or is anyone aware of any serious issues that might prevent such a high temperature jungle? $\endgroup$
    – Slarty
    Sep 5, 2022 at 21:19
  • $\begingroup$ @JHB I'm beginning to think that temperature could easily be sufficient to overcome any humans. But I can't think of any specific reason why vegetation shouldn't evolve in a high temperature tropical forest at 40, 50 or even 60 degrees C. I may even post an answer... $\endgroup$
    – Slarty
    Sep 5, 2022 at 21:31

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I'm assuming that "un-aided" means no A/C. Per https://en.wikipedia.org/wiki/Wet-bulb_temperature:

Even heat-adapted people cannot carry out normal outdoor activities past a wet-bulb temperature of 32 °C (90 °F), equivalent to a heat index of 55 °C (130 °F). The theoretical limit to human survival for more than a few hours in the shade, even with unlimited water, is a wet-bulb temperature of 35 °C (95 °F) – equivalent to a heat index of 70 °C (160 °F).

With 100% humidity, 32°C will force your humans to rest somewhere cooler, and 35°C will kill them. You should move your estimate 3° cooler if they've just come down from the mountains.

As for plants, according to the introduction in this paper published by Rowan F. Sage and David S. Kubien, the upper limit for photosynthesis in heat adapted plants is 40-45°C.

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  • $\begingroup$ Nice, +1, but I think that the weather.gov table is a bit on the safe side. This summer I stayed at 35-36 °C with around 70-75% humidity (Corse, to be specific), and yep, it was unwieldy, but survivable quite well with good hydration. $\endgroup$
    – Rmano
    Sep 6, 2022 at 7:07
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    $\begingroup$ @Rmano At 75% humidity, temperature limits are ca. 4 degrees higher, i.e. it would 34 ("possible") resp 36 °C (highly likely). The weather.gov limits are actually accurate, but two factors likely saved you: (1) it takes several hours until things get dangerous, what counts is essentially the average temperature in that time, and (2) you were instinctively driven to spots cooler than 36°C so your body got a manageable average temperature. (Good hydration is already factored into the weather.gov tables.) $\endgroup$
    – toolforger
    Sep 6, 2022 at 8:58
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    $\begingroup$ It's also a matter of training/acclimatization. weather.gov needs their data to work for any US citizen (their target audience), which includes people from Washington state on holiday in Florida. What is annoying weather to most Floridians could catch an unexperienced person (or their unprepared circulatory system) very badly $\endgroup$
    – Hobbamok
    Sep 6, 2022 at 9:00
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    $\begingroup$ The concept you are looking for is called wet-bulb-temperature, see en.wikipedia.org/wiki/Wet-bulb_temperature . Essentially 32 °C at 100% humidity is not fun but survivable, 35 °C at 100% humidity is lethal to humans after a few hours. $\endgroup$
    – quarague
    Sep 6, 2022 at 11:18
  • $\begingroup$ @quarague thanks, I'll correct my answer with that. $\endgroup$
    – BlockStop
    Sep 6, 2022 at 12:12

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