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How long can a planet sustain life?

  1. Sep 23, 2012 #1
    Dear Physics Forums community,

    This question has been bothering me for a few days, and I must apologize for the ambiguity of it. I'll try to explain it a little bit. I am not asking when the Sun will expand or the Earth cool down via radiative processes. So what I asked myself was: if you treat all organisms on a planet as a single system, how well does it perform "in the long run"? How fast the biosphere is depleting planet's natural resources? How efficiently all sorts of life are being recycled? If you exclude extintion events caused by asteroid impacts or radiation butsts, how the total biomass vs. time function can look like?

    I do realize that these questions aren't very clear, but I don't really expect definite answers. I don't have a good biology background, so it's hard for me to put it all together.
  2. jcsd
  3. Sep 23, 2012 #2
    Obviously this is arm chair philosophizing and things are impossible to quantify. Essentialy it's all about cycles and recycling, water cycles, oxygen cycles, carbon cycles. The big question is if all cycles balance or if there are leaks.

    Interesting is the big carbon cycle, since the amount if carbon in the biosphere is likely the limiting factor for the total possible biomass. Interesting are the very long cycles, coal forming, rock weathering, but especially oceanic micro organism making carbonate shells, remains of which we see in the white cliffs around the world.

    So this seems a leak in the carbon cycle, when carbon reacts with calcium and oxygen forming limestone, calcite etc.

    It's not a complete leak as volcanism returns some of that as CO2. So the question is, if this balances and if not, at what (much?) lower level will the cycle eventually balance? Enough to sustain life? Nobody's guess. Even if we could quantify the carbon loss today exactly, we have no idea about future mass flows. But I don't think we have to worry about that for our species.

    Just two cents, it may be totally different.
    Last edited: Sep 23, 2012
  4. Sep 23, 2012 #3
    Thank you Andre! I was afraid that my question was a total dead-end, but your reply is just what I needed!
  5. Sep 23, 2012 #4
    :smile: You're welcome, I like collecting thank you's :uhh:
  6. Sep 23, 2012 #5
    One more thing, burning fossil fuels also implies that we are indeed recycling old and lost carbon for the moment, bringing it back into the biomass carbon cycle.
  7. Sep 23, 2012 #6


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    Thinking only in terms of "cycles" is too restrictive. For example, consider what happened to life on earth when some pesky organism mutated and started doing photosynthesis for the first time.

    Most of the previously existing species were poisoned by the free oxygen that was released, but the ecosystem as a whole evolved and carried on just fine in a very different environment.
  8. Sep 23, 2012 #7
    To answer the OP the only thing we can say with reasonable confidence is "about three and a half billion years, at least". We have only educated guesses as to whether life even exists anywhere else, so all the modelling we do - no matter how elegantly constructed - is highly speculative.

    My own speculation would be that if it were not for increases in solar luminosity that will render the Earth uninhabitable in about a billion years that we could continue until internal heat stopped powering plate tectonics and the cycling Andre referred to ground to a halt. Still, that should take at least another billion years to prove fatal.
  9. Sep 29, 2012 #8


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    It should last until December 21, 2012. After that, all bets are off! :biggrin:
  10. Oct 1, 2012 #9
    It is an interesting thought about burning fossil fuels. I wonder if there could be another pesky organism mutation (hats off to AlphZero) which allows rapid fossil recycling. Or what if we are *interrupts himself* But then again, I plunge into chair philosophizing even deeper. Thanks again.
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