How would Earth's atmosphere change in the absence of life?

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axemaster
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Hey guys, I'm writing a story where Earth suffers a catastrophic event, destroying all surface and ocean life. The surface is basically bathed in intense radiation.

To be more specific, Earth is attacked by aliens, and ships destroyed during the battle in orbit crash into the planet below, spreading massive amounts of radioactive debris (from reactors, nuclear weapons etc). The aliens might even deliberately "salt the fields" with radioactive materials to ensure complete destruction.

My question is this: if a person (who could survive the radiation) returned to Earth, say 30 years after this event, would the atmosphere still be breathable? What would happen to the composition of the atmosphere, especially oxygen and CO2 content? I assume that O2 would decrease, but what would be the main pathways for that to happen, and how long would it take?
 
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Organic material would burn at some point, reducing the O2 content of the atmosphere a bit. Geological processes are more effective, but they need way longer than 30 years.

There are ~1018 kg of O2 in the atmosphere (WA). It is hard to find numbers for the flow, but we can use the CO2 numbers as upper estimate: about 8*1014 kg of CO2 cycle each year, which means plants generate at most ~6*1014 kg of O2 per year. Even if we neglect that animals won't need oxygen any more, and that my estimate is very conservative, the oxygen will stay in the atmosphere for (at least) thousands of years.
 
Very interesting, thanks for the info. I should have realized that O2, being ~20% of the atmosphere, wouldn't be significantly changed.

I was also wondering how much the CO2 would increase, if you burned all the plants. The current CO2 mass in the atmosphere is 1.8*10^15 kg. I looked up plant biomass, and it appears to be around 400 billion tonnes (4*10^14 kg). Do you know how to estimate the CO2 production from that?

EDIT: I suppose it's enough to know that even 100% conversion of plant mass into CO2 would only be about 20% increase in CO2. It's kind of impressive to consider that burning literally everything would have such a small effect.
 
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Even less: using the composition of humans as an example, we have 20% carbon (by mass). This would form CO2 with roughly three times the mass of a carbon atom, so we get 60% of the biomass as CO2, assuming a human has a typical composition for biomass in general.