60

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

James K
  • 120,702
  • 5
  • 298
  • 423
Daniel Storm
  • 711
  • 1
  • 5
  • 8

5 Answers5

68

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.

Sir Cumference
  • 8,117
  • 4
  • 42
  • 77
  • 41
    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. – Lightness Races in Orbit Aug 11 '16 at 09:11
  • 2
    @LightnessRacesinOrbit I agree. It is also unclear what form of life is being referred to. – called2voyage Aug 11 '16 at 13:31
  • @LightnessRacesinOrbit Considering it took billions of years for life to develop on Earth, we can safely rule out life appearing in the time the Sun is post-main sequence. – Sir Cumference Aug 11 '16 at 16:03
  • 4
    @SirCumference: How so? – Lightness Races in Orbit Aug 11 '16 at 17:16
  • 2
    @SirCumference based on a very representative sample of 1? – njzk2 Aug 11 '16 at 17:32
  • 5
    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. – whatsisname Aug 11 '16 at 21:21
  • 1
    Related to "life usually takes billions of years to develop". (1) development is the process by which one individual goes through the different stages of life (typically from a single cell to a grown up for multicellular lineages). The correct term is probably "originates" or "evolve" (but I am not sure what you meant exactly), no "develop" (2) Life took about 0.5 billions years to first appear on earth and much less time when you consider the existence of a range of 'putatively appropriate' conditions for life to originate. (3) Generalization from a single observation is likely misleading. – Remi.b Aug 11 '16 at 21:22
  • Off-topic: What caused that big dip in luminosity half a billion years after the sun started to shine? – Shane Aug 11 '16 at 21:31
  • @Shane I assume it's because that was when the Sun finished its contraction and became main-sequence. Before that point, the Sun was constantly losing energy as it shined. After it became main-sequence, nuclear fusion started producing much more energy. – Sir Cumference Aug 11 '16 at 22:06
  • Ahh. I thought the zero point was when it became main sequence. – Shane Aug 11 '16 at 22:07
  • 13
    Alright everyone. You heard the man. We've got 1 billion years before this thing is uninhabitable. Let's get moving, people! – JS. Aug 11 '16 at 22:36
  • 2
    While I'm nowhere close to an expert in simple life's biological development, I think Sir Cumference has a point, that even if it's a sample size of one, there's many steps between simple, first forms of life, and your basic, DNA based, photosynthetic with a nucleus. There's many more steps before you get multi celled, and after that, there's another long time before things like eyes and teeth, and a brain develop within that multi-cellular organism. It doesn't happen quickly, but once life get eyes, teeth, a brain and competition, then it things can progress fairly fast. (IMHO). – userLTK Aug 12 '16 at 10:36
  • 2
    @userLTK It's really hard to tell with the tiny sample size. For example, a lot of the "leaps" in complexity happened during (or after) a catastrophe of some kind. It might very well be that life on another planet might have been luckier and got their catastrophes earlier (if there is any causal connection in the first place, of course). We don't know how many kinds of life have existed in the past - we only have a very spotty record and a lot of indirect evidence. Heck, it's possible (though unlikely) that there was intelligent life on Earth before us and we just didn't find any record yet. – Luaan Aug 13 '16 at 23:36
  • 2
    @userLTK And we have plenty of evidence that when the evolutionary pressure is high, the "development" can get very fast indeed - while when food is abundant and there's not a lot of competition, it can stagnate for hundreds of millions of years. You also get stagnation when things get too harsh. There's so many unknowns and variables that we just can't tell - it's not like there's a physical law that prevents an advanced form of life from developing within a hundred million years; we just don't have any evidence that it did on Earth in the past. – Luaan Aug 13 '16 at 23:40
  • 2
    @Luaan I'm not an expert in such things, and I'm not sure anyone is, since we didn't see it happen and there's only fossil records, but there's an enormous variation in one celled life and many steps that need to take place just to get the first bug, or, the first plant. I remain skeptical that it can happen "quickly". There may be no physical law, but there may be a practical limit of at least a few hundred million years for first-early life to become complex, animals, fish and plants (IMHO). I do agree with you that there are significant unknowns. – userLTK Aug 14 '16 at 22:02
48

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.

WBT
  • 597
  • 1
  • 4
  • 8
  • 6
    This does not answer the question of when the Sun will render Earth inhabitable. You simply mention one example of when it could have. – Sir Cumference Aug 10 '16 at 22:35
  • 2
    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? – WBT Aug 10 '16 at 22:45
  • 3
    @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. – WBT Aug 10 '16 at 22:51
  • 3
    @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. – JBentley Aug 11 '16 at 20:03
  • 1
    And that's not considering terraforming technologies that could delay such conditions on Earth from occuring in the first place. – JBentley Aug 11 '16 at 20:08
  • 5
    @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. – Sir Cumference Aug 11 '16 at 20:37
  • @SirCumference a desolate rock in space Do you mean like Mars, that desolate rock we're getting ready to colonize with humans, in addition to the robots already there? – WBT Aug 11 '16 at 20:40
  • 1
    @WBT Sigh...no one is getting ready to colonize Mars. We don't nearly have the resources or even proper concepts of how we can do that. – Sir Cumference Aug 11 '16 at 20:42
  • 2
    @SirCumference no one is getting ready to colonize Mars. I don't consider Elon Musk to be a nobody. Mars One is also getting ready. There are some seeds pointing to further discussion in the Wikipedia article about the topic. We're a ways off (not on a billion-year time scale), but still getting ready. – WBT Aug 11 '16 at 20:51
  • 2
    @WBT You don't understand the effect that increases in temperature have on planets. 7° increase on Earth would make supervolcanoes much more prominent and powerful. They would put nuclear weapons to shame; supervolcanoes like the Yellowstone one can cause continental destruction. Cold temperatures on Mars are incomparable to constant volcanic activity and weather storms. – Sir Cumference Aug 11 '16 at 20:55
  • @SirCumference Did you mean to direct that last comment at JBentley? I was just pointing out that some humans are now figuring out how to take what you might describe as "a desolate rock in space" and make it habitable for themselves and their descendants. – WBT Aug 11 '16 at 20:57
  • 2
    @WBT: To yours I'll add this and this. Why is it every time someone declines to start a nuclear war, it turns out to have been the right choice? – JS. Aug 11 '16 at 22:43
  • 2
    Those events woudn't have rendered the earth uninhabitable. Cold War becoming hot could cause billions of deaths and could cause a collapse of the world as we know it, but exploding all the nuclear weapons we have, even accounting for the climate aftereffects, would still leave more people alive than there were a few thousand or a few hundred thousand years ago. It wouldn't even render the world totally uninhibited, much less uninhabitable, which is a much stricter criteria. – Peteris Aug 12 '16 at 14:40
13

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.

Sebastian
  • 231
  • 1
  • 4
  • 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. – Chris H Aug 11 '16 at 11:22
  • "Free oxygen and ozone disappear from the atmosphere" so if you consider the moon to be uninhabitable... – Sebastian Aug 11 '16 at 11:34
  • 1
    I assumed that the lack of free oxygen was caused by the lack of photosynthesis and would be averted if we could supply CO2 – Chris H Aug 11 '16 at 11:41
  • True, if you imagine such a level of human influence at that time. – Sebastian Aug 11 '16 at 11:47
  • I took that assumption from the question, though it didn't rule out humanity having regressed to pre-industrial society before that. – Chris H Aug 11 '16 at 11:54
  • 2
    The OP's question was when the Sun would render Earth uninhabitable to humans, not when atmospheric changes would. – Sir Cumference Aug 11 '16 at 12:09
  • 5
    @Sir Cumference: The atmospheric changes are due to the sun's increasing luminocity. I extended the corresponding quote in my answer. – Sebastian Aug 11 '16 at 13:38
  • 2
    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. – Luaan Aug 13 '16 at 23:48
4

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.

Sir Cumference
  • 8,117
  • 4
  • 42
  • 77
Mikey Mouse
  • 141
  • 3
  • 1
    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. – userLTK Aug 12 '16 at 10:43
  • 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. – Jack R. Woods Jul 30 '18 at 23:23
3

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.

Loren Pechtel
  • 301
  • 1
  • 5