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So, I've heard that above around 35% O2 concentration in Earth-like atmospheres leads to wildfires that just never go out. I'm thinking of using this like California, with its regularly occuring fires that help clear the land for new growth, but on a global scale. Every couple of years, the O2 conc. is high enough for fires to never go out, so they spread and burn the world down but also use up a lot of that oxygen to the point that they may die again. Plants then regrow raising the O2 back up to the threshold starting the cycle anew.

If I set the oxygen concentration to or just below that threshold, will this be sustainable? If not, how could I get this to work?

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    $\begingroup$ I think forests don't have enough biomass to really affect O2 threshold that way. $\endgroup$ – alamar Dec 24 '19 at 7:58
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    $\begingroup$ I think forests don't have enough biomass to really affect O2 concentration that way. Maybe with very thin atmosphere, with extra large landmass (no oceans) and super-rich flora. Even then it's a stretch. $\endgroup$ – alamar Dec 24 '19 at 8:00
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    $\begingroup$ Well, in the Carboniferous period oxygen concentration remained above 30% for some 60 million years, of which for some 20 million years it was even above a whopping 35%... $\endgroup$ – AlexP Dec 24 '19 at 9:54
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    $\begingroup$ On a side note I'll add that increased O2 concentration cause spiders and bugs to grow much larger. In that case burning whole planet is quite appealing idea. $\endgroup$ – Guy with jewels' names Dec 24 '19 at 10:06
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    $\begingroup$ The location of the finale for The Player of Games has a world-encircling landmass which has a continuous firestorm that burns around the planet in a wave in time to re-ignite the mature regrown vegetation. $\endgroup$ – Starfish Prime Dec 24 '19 at 14:34
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Boom bust cycles.

  1. Oceanic and land based plants compete for CO2 and are limited by CO2 availability.

  2. Oceanic has an edge in that they cannot burn.

  3. When O2 gets high enough the land burns.

  4. Free from competition with land plants, and with all that land carbon now available as CO2, the ocean plants have a burst of growth.

  5. Ocean plants overgrow the availability of their noncarbon nutrients (iron, nitrogen, phosphorus) and their populations crash.

  6. Oxygen levels fall because land and water are now depleted of photosynthesizers.

  7. Land and water life begins to regrow. Land has an edge because there are more nutrients available in the soil than in open ocean. Land regrows first. Ocean starts later.

  8. Oceanic and land based plants compete for CO2 and are limited by CO2 availability.

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What would restrain fires from burning in the "low" periods? Any lightning could start one, and compost heaps that get too dry have been known to burst into flame, however rarely -- naturally accumulating dry organic matter might to do the same.

Such fires would use up the fuel and oxygen for the world-wide bursts, and create natural fire-breaks that would limit spread. Other natural firebreaks such as bodies of water, prevailing winds, desert, rainstorms, and bare rock would also prevent the necessary sparks from flying all over the world. Any condition, such as high winds, that would spread the fire far and wide during the high-oxygen period would also spread fire in the low-oxygen periods, if not so widely.

The variation in fire between the high and low oxygen periods would be continuous, not a step function. You would need some reason other than "low oxygen" to explain why fires were not a regional factor as the oxygen built up, and a major regional factor as it approached the high period.

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fire needs three things fuel, oxygen and temperature ... so there would also be a temperature / humidity component to the excellent cycle willk describes.

unless you want to use the planet's orbital eccentricity to regulate temperatures ....

coinciding with stage 3 would be a drop in temperature and humidity but first the ash fertilisation kickstarts stage 4

stage 5 would see a second longer drop in temperature especially along the high latitude coast as albedo changes, further encouraging step 6

stage 7 would be spotty because glacial melt would be encouraged by land growth changing the albedo but the new river deltas would be the kickstart for ocean growth

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