How long would the air on a dead world stay breathable?

[malloyd]

It's actually extremely difficult to measure the oxygen production rates of the ocean phytoplankton (which pretty much everybody agrees is all that matters to estimating this, nobody thinks they are less than 90% of the oxygen cycle, and most people peg it above 99%). In part because even the places you can measure the outgassing of the oceans, a significant part of that is because the water is getting warmer and hence dumping oxygen that has long been dissolved in it. But yeah, nobody has it so high that you'd observe much change in centuries, and most estimates would run to hundreds of thousands to millions of years.

However. There's a complicating factor. Stick humans in a sealed environment and they will never actually run out of oxygen, because they die of CO2 poisoning first. Oxidize all the biomass on the planet and depending on whose estimates you use you are actually getting pretty close to toxic levels, and that perhaps [could] happen in a couple centuries, though it probably would take hundreds of times that. There's still lots of oxygen in the air, but you die anyway if you try to breathe it.

[agentdenton]

Originally Posted by whswhs

When I was researching GURPS Future History, I got the impression that GRBs ordinarily wouldn't inflict radiation poisoning on most living organisms:

Little direct radiation reaches the surface, but the atmosphere is chemically altered, being flooded with nitrogen compounds for several years (roll daily vs. HT to avoid 1 point of toxic damage).

That doesn't seem as if it would have a lot of effect on deep oceanic life, especially anaerobic life.

Hmm, I'd assumed if it caused one of the 5 mass extinction events it might have been thru radiation at least in part.

You might need another event to create a total sterilization event that leaves the planet and atmosphere intact enough to make oxygen depletion an issue.

Honestly it's hard to imagine something that would utterly sterilize a planet like earth to the last microbe and leave the atmosphere intact. Some type of intense prolonged neutron bombardment might. Would any natural source cause the requisite one?

Niven positited a massive supernova chain reaction in the core of the galaxy that would generate a wave radiation that would in 30,000 or so years sterilize earth, but niven's not as brilliant as he thinks he is. (A quasar isn't as brilliant as niven thinks he is.)

[whswhs]

Originally Posted by agentdenton

Hmm, I'd assumed if it caused one of the 5 mass extinction events it might have been thru radiation at least in part.

That was what I assumed, too, but when I researched it it appeared to be otherwise.

[agentdenton]

Originally Posted by whswhs

That was what I assumed, too, but when I researched it it appeared to be otherwise.

Ah yes, the perils of the completely reasonable assumption..

[sjard]

<Moderator>
Moved thread to Roleplaying in General as that would appear to be a more appropriate location.
</Moderator>

[Agemegos]

Originally Posted by malloyd

However. There's a complicating factor. Stick humans in a sealed environment and they will never actually run out of oxygen, because they die of CO2 poisoning first. Oxidize all the biomass on the planet and depending on whose estimates you use you are actually getting pretty close to toxic levels, and that perhaps [could] happen in a couple centuries, though it probably would take hundreds of times that. There's still lots of oxygen in the air, but you die anyway if you try to breathe it.

Good point, but I think the carbonate-silicate cycle might remove the excess carbon dioxide from the air, because CO₂ concentration is equilibrated by feedback through temperature.

[ericthered]

Originally Posted by Agemegos

Good point, but I think the carbonate-silicate cycle might remove the excess carbon dioxide from the air, because CO₂ concentration is equilibrated by feedback through temperature.

How long does that take though? And won't other processes be drawing the oxygen out of the air as well?

[thrash]

Originally Posted by malloyd

However. There's a complicating factor. Stick humans in a sealed environment and they will never actually run out of oxygen, because they die of CO2 poisoning first. Oxidize all the biomass on the planet and depending on whose estimates you use you are actually getting pretty close to toxic levels, and that perhaps [could] happen in a couple centuries, though it probably would take hundreds of times that. There's still lots of oxygen in the air, but you die anyway if you try to breathe it.

For what it's worth:

The NIOSH recommended exposure limit for CO2 is 9,000 mg/m3. To reach that level with the biomass figure I quoted above, the excess CO2 would have to be confined to the lowest ~2,500 m of the atmosphere. This doesn't seem likely, given the mixing due to weather, etc. A lethal dosage would require an order of magnitude higher concentration.

Carbon monoxide is lethal in much lower dosages, but I don't remember how much is produced relative to carbon dioxide in ordinary combustion (e.g., forest fires).

[agentdenton]

This whole conversation reminds moi of something from 40k, an overblown weapon (like all 40k weapons) called the life eater virus.

It's used for planetary extermination and is a supervirus that simply consumes organic matter and reproduces at an impossible rate. As all living organisms are broken down in minutes by it the decomposition releases vast amounts of oxygen and flammabel gasses, which makes the atmosphere highly flammable and creates global firestorms as the air itself burns.

https://wh40k.lexicanum.com/wiki/Virus_bomb

Yeah, it's 40k grimdark and completely impossible but this topic just reminded me of it so much and I guess it meets the OP issue for something that would wipe out all life but leave the planet intact.

[Rupert]

Originally Posted by agentdenton

This whole conversation reminds moi of something from 40k, an overblown weapon (like all 40k weapons) called the life eater virus.

It's used for planetary extermination and is a supervirus that simply consumes organic matter and reproduces at an impossible rate. As all living organisms are broken down in minutes by it the decomposition releases vast amounts of oxygen and flammabel gasses, which makes the atmosphere highly flammable and creates global firestorms as the air itself burns.

https://wh40k.lexicanum.com/wiki/Virus_bomb

Yeah, it's 40k grimdark and completely impossible but this topic just reminded me of it so much and I guess it meets the OP issue for something that would wipe out all life but leave the planet intact.

May as well just go straight to the grey goo and be done with it.