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So the situation, for those who aren’t familiar, is that Earth is in trouble. Again. A gamma ray burst from a dying star somewhere close by (a couple hundred to a couple thousand ly) has grazed or even struck our planet head on, dumping its payload of high-energy radiation directly into our atmosphere. At this distance, the burst fries half the planet outright, scorching or even glassing whichever hemisphere is facing the burst. Half the global population is dead, the global ecosystem can safely be assumed to be crippled, and so much of the burst’s radiation has attenuated in the atmosphere that our ozone layer has lost all its protective benefits. Anything that stands in direct sunlight after this event is going to be badly burned, meaning the other half of the population and whatever’s left of Earth’s biosphere is living on borrowed time.

Can humans build underground (or underwater) shelters with enough food and water to sustain life beneath the surface of the Earth indefinitely before what little remains of our planet and our civilization collapses, or would these subterranean cities have to either be built prior to the burst or dug out after the fact by the survivors who’d already taken shelter underground before the burst hit? Could humans reasonably be expected to survive this ultimate global apocalypse with our technological know-how and scientific expertise, or is a continued existence underground post-gamma ray burst a fantasy?

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    $\begingroup$ Not unless they have several months (year or more) of prior notice. Otherwise, only a handful of individuals will survive the initial event (submariners, people already in bunkers for various reasons), and even they will likely die in only a few short years. Also, mind the language. $\endgroup$ – AndreiROM Nov 29 '17 at 19:00
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    $\begingroup$ I think you are vastly overestimating the lethality of GRB in the short term: en.wikipedia.org/wiki/Gamma-ray_burst#Effects_on_Earth $\endgroup$ – Pablo Oliva Nov 29 '17 at 19:00
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    $\begingroup$ 100 to 1000 light-years is much too far to have an impact on Earth; you'll need to reduce that by one or two orders of magnitude. $\endgroup$ – HDE 226868 Nov 29 '17 at 19:05
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    $\begingroup$ The energy needed to fry half a planet would kill everyone nearly instantly. $\endgroup$ – A. C. A. C. Nov 29 '17 at 19:29
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    $\begingroup$ Are you just asking if underground cities could be built in the 12 hours before the opposite side of the planet is exposed to this burst? $\endgroup$ – Samuel Nov 29 '17 at 22:31
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There are no known stars that could go nova close enough for it to have effects you describe. This means that cosmic body and mechanism of such burst are unknown to us. Without knowledge, we will know it's coming when it's already here, as it moves with the speed of light.*

Now, there are two ways of looking at this. More realistic is to expect ozone depletion and nitrogen mono and dioxide formation. This is bad, but life would survive and humanity will be intact, if hungry and sunburned. Decimated, maybe,but most of us would live. Natural ecosystems would be in trouble, most crops would probably manage.

Other is to assume what you wrote about half the Earth on fire is true. Then, we will have a lot of ash in the air. Oxygen burnt, trapped in CO2, nitrogen compounds etc. Atmosphere will no longer be suitable for breathing. Way too hot, even if we'll ignore anything else. In few days (at best, I'd rather bet few hours) everybody except submarine sailors will be dead. No way your people will be able to plan and prepare. Following cosmic winter will not be different from impact winter or nuclear one.


* As Snyder005 pointed out in comment, you might have warning few minutes before gamma burst strikes, if you'll catch neutrino spike. It does not really matter when we talk about shelter construction, but it's worth adding for the sake of precision.

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    $\begingroup$ Depending on your GRB mechanism you could have some advance knowledge (order minutes) from neutrino detections. $\endgroup$ – Snyder005 Nov 29 '17 at 22:04
  • $\begingroup$ @Snyder005 good point, didn't think about that. Not that it would change much :) $\endgroup$ – Mołot Nov 30 '17 at 9:01
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    $\begingroup$ Yeah not enough time to build something, but if you had some pre-built precautions, e.g. underground GRB safe bunker, GRB failsafes, you could get some warning within scientific plausibility. $\endgroup$ – Snyder005 Nov 30 '17 at 21:33
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Humans don't really need to dig underground.

Gamma-ray burst will have two major destructive factors - UV radiation and atmospheric pollution.

UV radiation will spike during the initial burst event, and later due to ozone layer depletion. Ozone is expected to go back to normal within several years. On Earth, our existing buildings will provide sufficient shelter from UV radiation, and after putting screens or shutters on windows, people indoors will have nothing to worry about. When going outside, bundle up and wear sunglasses, that's all. High level of UV, though, will affect plants and animals, so we should expect food shortages.

Atmospheric pollution problem seems to be more serious. After GRB, significant amounts of nitrogen oxides will end up in the atmosphere, which will cause respiratory problems, acid rains and reduction of sunlight amount. Again, the most serious problem will be the food shortage.

Overall, humanity will suffer, but there's nothing that we can not deal with.

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I believe not only would we see it coming, but we would see it coming decades or centuries in advance.

*just to say it, you are badly over-estimating the impact of a GRB here, short of Jupiter somehow collapsing and causing one (which it cant naturally), the effects you have here are beyond the realms of reality.

*We are not 100% sure if this could even happen. We have yet to see a GRB originate from the milky way...it tends to be an event from much older and metal-poor galaxies that have this occur. It could still happen, but our current knowledge does suggest this won't happen within the milky way or similar galaxy.

A GRB travels at the speed of light, so we actually have no chance of detecting the actual wave coming at us. That being said, we know the scenario's that tend to result in GRB's and we could see this scenario happening long before the wave hits.

Short bursts are generally associated to massive events such as a neutron star being swallowed by a black hole (I'd think short burst wouldn't achieve the effect you are going for anyway) and we would be aware of a black hole interacting with a neutron star a few thousand light years away by now.

Long Gamma Ray bursts are associated with the deaths of massive stars in galaxies showing rapid star formation. The milky way doesn't fit the profile very well, but we've actually identified a possible candidate some 7500 light years away in WR 104 https://en.wikipedia.org/wiki/WR_104 We are able to predict this event will happen in a few hundred thousand years.

As a summary - yes, we would have tons of time to start underground...possibly to the tune of "we see what our great great great great great grandchildren are going to have to contend with".

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Probably not necessary.

Even ultralong GRBs last only 10,000 seconds -- about 3 hours. Most are a few to a few 10's seconds. Half the planet gets off scot free.

But half the planet has a lot of NO and NO2 formed in the upper atmosphere. This disrupts the ozone layer, resulting is several times as much UV at the surface. UV index up to 30. Big increase in skin cancer, Lot higher mutation rate in plants. This effect has a half life of something like 1-2 years. If you burn in spring in 15 minutes, now you will burn in 5. Those people who have to work hard at getting a sunburn, won't have to work nearly as hard. You'll get your daily dose of Vitamin D in 1/3 the time.

Some crops may be adversely affected.

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