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If nothing else wipes out human existence prior to this, at what point will the Sun make Earth uninhabitable for humans?

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    $\begingroup$ You're already assuming we don't destroy the Sun in the interim. It's a damn good source of raw materials. $\endgroup$ – Patrick Stevens Aug 11 '16 at 18:33
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    $\begingroup$ @PatrickStevens Not all that good. It is mainly hydrogen and that isn't exactly rare. All those materials are very hot and at the bottom of a deep, deep gravity well. At the very least, we aren't going to be horning in on the sun until Jupiter is gone. $\endgroup$ – Shane Aug 11 '16 at 19:28
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    $\begingroup$ @corsiKa He covered that with, "If nothing else wipes out human existence prior to this". $\endgroup$ – JBentley Aug 11 '16 at 20:11
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    $\begingroup$ Relevant: Distant future of the Sun and Earth revisited and this answer to the question What will happen to life on Earth when the Andromeda and Milky Way galaxies collide? Notably: We only have 1, maybe 2, billion years before water boils off the surface, at which point I think we can make the claim that Earth no longer sustains life. $\endgroup$ – Adam Davis Aug 12 '16 at 16:59
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    $\begingroup$ Are you assuming that "humans" will be the same then, in thousands of millions of years, as we are now, which we have only been for a few hundred thousand years? $\endgroup$ – a CVn Aug 12 '16 at 17:10
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The Sun is gradually getting larger and brighter. In fact, as called2voyage pointed out, its brightness is increasing by 1% every 100 million years. You can see how the Sun will change in the future from this graph: (Source)

enter image description here

According to this paper, within 1 billion (short scale) years from now, the ever-increasing luminosity will have made Earth nearly uninhabitable. The average temperature will have reached 47°C, compared to its current 15°C. Essentially no water will be left either, except at the poles. This may allow for simple life to survive for a while.

By 3.5 billion years, Earth will no longer resemble its current self. Its oceans, magnetic field and ozone layer and plate tectonics will be no more. Its surface temperature will skyrocket to roughly 1,330°C, hot enough to melt surface rock. No longer will our planet resemble a pale blue dot, and it will be more like Venus. Our planet is officially dead, along with all life on it. (Source)

~4.5 billion years from now, the Sun will become a red giant and possibly consume Earth. However, according to this paper, it may heat up potentially habitable bodies like Triton, to the point where they would support life. Unfortunately, the Sun won't remain in this stage for long enough — life usually takes billions of years to develop.

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    $\begingroup$ Great answer, although I'm not sure we can truthfully assert "life usually takes billions of years to develop" when we've only "witnessed" it once. $\endgroup$ – Lightness Races in Orbit Aug 11 '16 at 9:11
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    $\begingroup$ @LightnessRacesinOrbit I agree. It is also unclear what form of life is being referred to. $\endgroup$ – called2voyage Aug 11 '16 at 13:31
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    $\begingroup$ @SirCumference: How so? $\endgroup$ – Lightness Races in Orbit Aug 11 '16 at 17:16
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    $\begingroup$ It also depends on what definition of "develop" you use, as the earliest evidence for life we have here shows that life was present within 400 million years after earth's formation. $\endgroup$ – whatsisname Aug 11 '16 at 21:21
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    $\begingroup$ Alright everyone. You heard the man. We've got 1 billion years before this thing is uninhabitable. Let's get moving, people! $\endgroup$ – JS. Aug 11 '16 at 22:36
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At what point will the sun make earth uninhabitable for humans?

It nearly happened on May 23, 1967.
According to this history paper released yesterday, summarized nicely here on Space.com, at the American Geophysical Union (paper publisher), and in many other media outlets, a powerful Earth-directed solar flare on that date jammed radar of the US Air Force's Ballistic Missile Early Warning System. Radar jamming like that, especially across all three sites, was considered an act of war, with the Soviet Union as the obvious culprit. Nuke-laden planes prepared to go destroy the enemy and set off the nuclear war that likely would have rendered Earth uninhabitable for humans.

Fortunately for all of us, the US had invested resources in monitoring the sun several years earlier, and somebody figured out that this was a solar flare with just enough time to stop the nuclear deliveries and keep the Cold War cold.

Popular Mechanics points out:

If that bulletin had been delayed a few minutes, those nuclear aircraft could have launched, and the solar flares would have made it impossible to communicate in the air. If those aircraft had launched, there would have been no way to call them back.


It also nearly happened on September 26, 1983.
With the superpowers again at hair trigger tensions, a rare alignment of sunlight glinting off high altitude clouds and the highly elliptical orbits of Soviet early-warning satellites caused the detection system to report that five intercontinental ballistic missiles, likely carrying nuclear warheads, had been launched by the United States against the Soviet Union. With only a short time to act on such a report, Soviet leaders likely would have launched a large nuclear strike against the US in retaliation for the detected attack. However, the human on duty to receive this detection report was civilian-trained Stanislav Petrov, who was skeptical of the detection and correctly classified this as a false alarm. (Five nuclear missiles seemed small for the kind of attack he expected from the US.) This decision has been credited with preventing a large-scale nuclear war, which otherwise would have been triggered by misinterpreted sunlight.


So a possible answer is, "the next time some tense socio-technical system we've built misinterprets the Sun."

Thanks to user JS for the example from September 1983, and to all those who happened to be in the right place at the right time and helped keep us safe.

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    $\begingroup$ This does not answer the question of when the Sun will render Earth inhabitable. You simply mention one example of when it could have. $\endgroup$ – Sir Cumference Aug 10 '16 at 22:35
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    $\begingroup$ Would "next time people or the automated systems or extremely tense socio-technical systems they build misinterpret solar events as aggression" be a more acceptable answer, Sir? $\endgroup$ – WBT Aug 10 '16 at 22:45
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    $\begingroup$ @SirCumference Not necessarily. Maybe it's just an unexpected answer! (OP is even named "Storm.") Learning this material, I found it interesting. Others who care about astronomy and/or seek examples of why it's more generally important to care about astronomy might find it interesting too. $\endgroup$ – WBT Aug 10 '16 at 22:51
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    $\begingroup$ @SirCumference No one can say when it will render the Earth uninhabitable. Even your answer, which gives 1 billion years as a guess, whilst a good answer, is wild speculation, at best. 1 billion years is a hell of a long time in terms of human technological development. I doubt something as trivial as a 47 degrees and water at the poles would be an issue for a human race that has survived and advanced another billion years. We've only been around for about 0.02% of a billion years, and already we can live in extremely harsh conditions such as Antarctica, or the ISS. $\endgroup$ – JBentley Aug 11 '16 at 20:03
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    $\begingroup$ @JBentley "I doubt something as trivial as 47 degrees" tells me that you have a vague understanding of climatology. An average increase of one degree across Earth's entire surface means huge changes in climatic extremes. It's not the same as a single degree on human skin — this is across an entire planet. Between 4 and 5 degrees, rainforests will burn up and turn to desert. At the end of the Permian period (251m years ago), when global temperatures rose by only six degrees, 95% of species were wiped out. 47 degrees will turn the planet into a desolate rock in space. $\endgroup$ – Sir Cumference Aug 11 '16 at 20:37
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According to Wikipedia's timeline of the far future this will happen at least in 800 million years from now:

Carbon dioxide levels fall to the point at which C4 photosynthesis is no longer possible. Free oxygen and ozone disappear from the atmosphere. Multicellular life dies out.

But it might already end 200 million years earlier:

The Sun's increasing luminosity begins to disrupt the carbonate–silicate cycle; higher luminosity increases weathering of surface rocks, which traps carbon dioxide in the ground as carbonate. As water evaporates from the Earth's surface, rocks harden, causing plate tectonics to slow and eventually stop. Without volcanoes to recycle carbon into the Earth's atmosphere, carbon dioxide levels begin to fall. By this time, carbon dioxide levels will fall to the point at which C3 photosynthesis is no longer possible. All plants that utilize C3 photosynthesis (~99 percent of present-day species) will die.

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  • $\begingroup$ There are already attempts to genetically engineer C4 photosynthesis into crops - 600My should be long enough to crack that. And it wouldn't be surprising if we could boost the geological sources of CO₂ by then. $\endgroup$ – Chris H Aug 11 '16 at 11:22
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    $\begingroup$ I assumed that the lack of free oxygen was caused by the lack of photosynthesis and would be averted if we could supply CO2 $\endgroup$ – Chris H Aug 11 '16 at 11:41
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    $\begingroup$ The OP's question was when the Sun would render Earth uninhabitable to humans, not when atmospheric changes would. $\endgroup$ – Sir Cumference Aug 11 '16 at 12:09
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    $\begingroup$ @Sir Cumference: The atmospheric changes are due to the sun's increasing luminocity. I extended the corresponding quote in my answer. $\endgroup$ – Sebastian Aug 11 '16 at 13:38
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    $\begingroup$ Of course, this is the typical kind of prediction that assumes that everything but the monitored quantity stays the same. For example, as carbon dioxide levels drop off slowly, C4 photosynthesising (or whatever better approach appears in the meantime) will be heavily favoured - by the point C3 plants would begin to die off, there would be scarcely any left in the first place. And with plate tectonics stopped, a lot of the carbon wouldn't go back into the mantle to need recycling either - all you need is life that recycles the carbonates, not impossible especially with the increased sun energy. $\endgroup$ – Luaan Aug 13 '16 at 23:48
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I read a good book that covered the sun expanding scenario. I don't remember the time line but at some point in its lifetime the sun will grow larger and swallow Mercury and the earth will become too hot to sustain life.

I think this is the scenario you're asking about and others have answered, but in the book he points out that over the vast time scales involved, it's not a very big deal for a space faring civilization to move the Earth out to a safe distance then back in when the sun shrinks.

I would have expected some kind of Star Trek level technology, but he describes how to use a simple Gravity tractor (not to be confused with a Tractor Beam) to adjust comets flight paths to speed up the earth (the reverse of the gravity slingshot used to speed up our current space probes) moving it further away from the sun.

So to answer your original question assuming the human life on earth is still advanced and spacefaring, human life on earth could last until the sun burns out and maybe a bit beyond that.

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    $\begingroup$ The sun will go red giant and swallow Mercury and probably Venus in about 5 billion years, but it will make the Earth too hot for most life long before then. The increased luminosity of the sun could make life unsustainable for us humans in as little as 600 million years. A gravity tractor would require a lot of work and energy, but with 600 million years to accomplish it, it's certainly possible. Gravity assists are also possible. $\endgroup$ – userLTK Aug 12 '16 at 10:43
  • $\begingroup$ The answer above is reminder not to count out human resourcefulness (if we don't kill ourselves first). I think we will be able to and should move the Earth farther from the Sun over a 600 million year period (or so). There will never be a better place for Earth-life (including humans) than the place we co-evolved with for 4 billion years. I think the question assumes that we don't move the Earth. $\endgroup$ – Jack R. Woods Jul 30 '18 at 23:23
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I would be very surprised if the sun ever ends all life on Earth.

1) If we care about the Earth we will move it. If we are still around by then it certainly will be within our technology.

2) I would say the most likely outcome (assuming we survive) is that the Earth gets taken apart for building materials. It's an awful lot of mass simply being used as a gravity source--incredibly inefficient.

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    $\begingroup$ I think an orbital mirror in L1 would solve the problem much easier. $\endgroup$ – peterh Aug 14 '16 at 11:22

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