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PostPosted: Sat Apr 13, 2019 1:25 pm 
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I've always liked the idea of once-fertile worlds that now lie in (biological) ruins, maybe with a few signs of a sapient species here and there. Probably ever since I saw the table in 2300.

(also I love the Dying Earth stories and am sometimes grateful that Vance had absolutely no understanding of how stars work)

And I've been trying to learn a bit more about the inevitable fate of our own world to get an idea of how that process will happen -- aside from catastrophic events.

So, no matter what in another 4-5 billion years, we're gonna be star food when the sun hits its giant phase. But from a couple things I've seen recently, it looks like life on Earth has maybe 600-ish million years left? That seems like such a short time on the cosmo scale.

Can anyone fill in some info for me on how and why that happens? Is it due to the crust continuing to thicken? The sun getting gradually hotter? Tidal forces? Leprechauns?


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PostPosted: Sat Apr 13, 2019 7:59 pm 
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I think the "imminent" loss of habitability in 500-1000 million years is largely due to the increase of solar luminosity with age. Without constructive intervention from humanity or its descendants, the increase would just overwhelm the Earth's natural feedback cycles and eventually push it into a runaway greenhouse effect.

Makes you wonder though. We have fossils that show that complex multicellular life has been around on Earth for only 600 million years (maybe 1 billion at a big stretch?), and then after another 600 million to 1 billion years it'll die out (though I'm sure bacteria etc would find a way to last til the earth is consumed by the dying sun in 5 billion years or so). So in a 10 billion year history, complex life might only be around on Earth for 1 billion years - but then maybe it'll pop up on Mars (again?) for a while too. But that could go some way to explain the apparently rarity of life.

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PostPosted: Sat Apr 13, 2019 11:07 pm 
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On of the things is solar brightening and the regulation of surface temperature by the carbonate-silicate cycle.

Carbon dioxide is released into the atmosphere from the geochemical carbonate reservoir by volcanoes. It is removed from the atmosphere by the weathering of silicate rocks and subsequent reactions, sinking to the sea-floor as carbonate ooze and being returned to the geochemical carbonate reservoir by subduction. This process depends on there being liquid water at the surface. Increasing carbon dioxide tends to raise the temperature, which melts ice and also increases the rate of weathering and the carbonate-silicate reaction. So when the world heats up CO2 gets scrubbed, lowering temperature. When the world cools off scrubbing is diminished (or ceases, if the world freezes over), that raises the temperature. So there is a negative-feedback loop that keeps Earth's surface at an equable temperature.

Back in the early days of life on Earth, the Sun was about 24% less bright, which would make Earth's equilibrium temperature 7% lower, i.e. about 20°C lower and cold enough to go into runaway glaciation. The carbonate-silicate cycle wound the concentration of CO2 up high enough to thaw the surface. Over time the Sun has gradually got about 30% brighter, raising Earth's equilibrium temperature. The have been wobbles, but the trend has been for the CO2 level to diminish continually, preventing the world from getting much hotter.

That long-term adjustment has a limit at the point where CO2 reaches zero. The natural level that kept Earth equable (before industrial-scale burning of fossil fuels) was down to 250 or 280 parts per million. 0.025–0.028%. As the Sun continues to get brighter we will reach the point where the concentration of CO2 that maintains an average surface temperature of 288K reach zero and then carbonate-silicate stabilisation will not be able to maintain a steady temperature any more.

Up until now the brightening Sun reduced atmospheric CO2, with little effect on temperature. In a comparatively short time (600,000,000 years?) CO2 will be close to zero, and the brightening Sun will raise temperatures without further diminishing CO2. Earth will turn into a steam-bath, then a pressure-cooker.

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Last edited by Agemegos on Mon Apr 15, 2019 6:43 am, edited 2 times in total.

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PostPosted: Sun Apr 14, 2019 10:39 am 
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So how much more luminous will the sun be in 600m years? To give the Earth a BBT approximating that of Venus it'd need to be about 1.9x what it is now, assuming I did the math right.

But I'm supposing Venus wasn't that warm when it started to undergo the change however long ago.


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PostPosted: Mon Apr 15, 2019 3:16 am 
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Matt Wilson wrote:
So how much more luminous will the sun be in 600m years? To give the Earth a BBT approximating that of Venus it'd need to be about 1.9x what it is now, assuming I did the math right.

But I'm supposing Venus wasn't that warm when it started to undergo the change however long ago.

I don't think Venus was ever a garden world.

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PostPosted: Mon Apr 15, 2019 3:25 am 
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Edited: the original text gave the impression that these things were a sequence, whereas I meant to list them as separrate processes.

  • Stars brighten during their main-sequence lifetimes. Garden worlds respond by their CO2 levels falling to keep temperature constant until they run out of CO2, then they warm up until they enter a runaway Greenhouse state.
  • Water vapour leaks slowly through the atmospheric cold trap (or rapidly, if their isn't one). Water vapour about the ozone layer is dissociated by hard UV, producing hydrogen, which escapes to space by Jeans escape or is stripped by the solar wind. The Garden world desiccates.
  • The planet's internal heat runs out, which puts and end to mantle convection. There might be a phase of episodic supervulcanism for a while. The magnetic field fails and perhaps the atmosphere starts getting stripped by the solar wind. Ionising radiation reaches the surface and kills stuff. Outgassing of CO2 ends, the CO2 is gradually removed by weathering, things chill off and freeze over.

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Last edited by Agemegos on Tue Apr 16, 2019 12:18 am, edited 1 time in total.

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PostPosted: Mon Apr 15, 2019 5:42 pm 
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But within what timeframe does this happen?

From the point when CO2 runs out, how long does it take before temperatures rise too high to support the planet´s dominant life forms? Thousands of years? Millions of years? And once you are at that point, how much more time does it take to get to the endpoint where the world is a desiccated, frozen post-garden?

Also it just occurred to me: We are currently producing excess CO2 in non-trivial amounts. Could a civilization about as advanced, or maybe a little more advanced, as ours potentially stave off the point where the CO2 runs out by intentionally producing large amounts of excess CO2? Not forever, but perhaps long enough to put some sort of Plan B (STL colony ship to wherever?) into action?

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PostPosted: Tue Apr 16, 2019 12:15 am 
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Sir Chaos wrote:
But within what timeframe does this happen?


That depends on how fast the star is brightening, which depends mostly on its mass. 1% stellar brightening produces about ¼% increase in the black-body temperatures of planets, moons, and asteroids. For habitable ones with black-body temperatures about 270 K ¼% increase is about 0.7K. The Sun is brightening about 30% over 4.5 billion years, which is 1% per 150 million years. So temperature rises about 1K per 200 million years, until you hit runaway greenhouse, then it rises a couple of hundred degrees in centuries.

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From the point when CO2 runs out, how long does it take before temperatures rise too high to support the planet´s dominant life forms? Thousands of years? Millions of years?


About a billion years for Earth, maybe? Longer for a late K star, much shorter for an early A.

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And once you are at that point, how much more time does it take to get to the endpoint where the world is a desiccated, frozen post-garden?

That's not where that branch leads. It leads to a pressure-cooker.

Quote:
Also it just occurred to me: We are currently producing excess CO2 in non-trivial amounts. Could a civilization about as advanced, or maybe a little more advanced, as ours potentially stave off the point where the CO2 runs out by intentionally producing large amounts of excess CO2? Not forever, but perhaps long enough to put some sort of Plan B (STL colony ship to wherever?) into action?

A civilisation could produce it CO2, but it would harm, not help. The problem is that you reach the point where it would take a CO2 level below zero to maintain an equable temperature, which is impossible, so temperature rises. Adding CO2 will only make it rise faster.

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PostPosted: Tue Apr 16, 2019 5:18 pm 
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The Epona Project was all about designing life on a world which was chronically short of CO2, because IIRC the geological recycling systems had ground to a halt.
http://worldbuilders.info/epona/


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PostPosted: Tue Apr 16, 2019 7:16 pm 
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Epona! Wow, is that still going?

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