First a bit of background. Gamma ray bursts are rare. They come in two forms, the long bursts and the short bursts. Long bursts come from supernovae, but only 1% of them cause these bursts, so they are very rare. Short bursts may come from colliding neutron stars. For details see The biggest explosions in the Universe. The beam is tightly focused in opposite directions so has to be beamed directly at us, so the chance of us seeing one that is focused on us is low. Typically the beam is focused to within a few degrees.

f it did happen, then it is a short lived event, from seconds up to hours at most (but the very long events are very unusual).

Gamma ray bursts are very focused, with beams in opposite directions, and would need to be pointed directly at us to cause harm - which is very unlikely. We have seen many gamma ray bursts in distant galaxies, but most are over a billion light years away, which shows how rare they are.

Short bursts are caused by colliding neutron stars. The beams they produce are so narrow that we only spot 0.4% of them. The bursts are so bright we can spot them billions of light years away and most of the ones spotted are over a billion light years away.Gamma ray bursts and pencil-thin jets

Our atmosphere would shield us from most of it except some strong UV light due to depletion of the ozone layer. You'd be shielded from that by a shadow and humans could shield against it easily, just use more sunblock when out of doors until the layer heals. Other creatures of course couldn’t use sunblock and might be more affected by it.

Ozone is depleted in the upper atmosphere and the resulting UV is the main thing that affects creatures at ground level. It also leads to increased ozone at ground level through the effects of the UV light, but this is not enough to be harmful to life. See How Deadly Would a Nearby Gamma Ray Burst Be?

The oxides of nitrogen produced in the upper atmosphere are not concentrated enough to have an effect at ground level and this new research shows that ozone levels at ground level are not high enough to be hazardous even for a very close gamma ray burst. So the main effects are from the UV. So if a gamma ray burst causes extinctions then it would be due to the increased levels of UV light at ground level until the ozone hole heals. But this is something humans can protect ourselves against easily. More about its effects in this paper

Researchers reported in 2013 that Earth might have been hit by a Gamma-ray burst in 8th Century (paper: Effects of Gamma Ray Bursts in Earth’s Biosphere) but this would seem a bit unlikely considering how rare they are. Later research that same year (2013) found that the increased levels of Carbon 14 and Beryllium 10 in AD 775 could be explained by a solar flare instead, see The AD775 cosmic event revisited: the Sun is to blame

PROBABILITY OF A GAMMA RAY BURST WITHIN 50 LIGHT YEARS (SAY)

The nearest likely gamma ray burst in the last billion years is 1000 parsecs away. But could we have a really close one, as close as say 50 light years away? Gamma ray bursts happen every 10,000 to a million years in a typical galaxy. The volume of the Milky Way, our galaxy, is roughly 8 trillion cubic light years and it has has 400 billion stars approx. (going by the higher estimates here).

The volume of space within, say, 50 light years is about 500,000 light years. So you’d expect it to contain 500,000 * 400 billion / (8 trillion) or around 25,000 stars.

Or for 20 light years, 33,510 cubic light years, then you get 33,510 * 400 billion / (8 trillion) = 1675 stars. We actually have probably around 150 celestial objects including white and brown dwarfs. Stars within 20 light-years. So that’s over estimating by an order of magnitude or so as we live far out in the thinner outskirts of the galaxy.

So anyway let’s overestimate throughout for a rough back of the envelope type calculation. So 25,000 stars out of 400 billion, and assume a gamma ray burst every 10,000 years and one in 100 of those (say) is pointed towards us. So that makes it a gamma ray burst pointed towards us and within 50 light years every (400 billion / 25,000) * 10,000 years, or every 160 billion years. So such a nearby gamma ray burst seems very unlikely.

Even at 50 light years, we’d be protected from most of the damaging radiation by the thickness of our atmosphere. It’s equivalent in mass to a ten meter depth of water. It would be rather similar to a nearby supernova. It’s too unlikely to get much attention in papers on gamma ray bursts, but there are estimates of the effects for a supernova. See What’s a safe distance between us and an exploding star? And for more details, the paper here: Could a nearby supernova explosion have caused a mass extinction?

They find that a supernova within 32 light years (ten parsecs) would not heat up Earth significantly, would not be bright enough to harm the ecology through the light alone. In the year after the event so you’d get as much ionizing radiation as you get normally in between a decade and a century. So significant but it doesn’t seem to be enough to be devastating.

It seems likely to be similar for gamma ray bursts, so the main effects would be on the ozone layer and on nitric acid rain - but we don’t need to look into this any more I think as the event is so very improbable.

WHY DO MANY PEOPLE WHO ANSWER THIS QUESTION SAY THAT WE’D BE TOAST?

I think many of the stories that circulate just ignore the effect of the Earth’s atmosphere. It’s equivalent to ten meters of water which is enough to block out most radiation. Also they forget about how rare they are. Typically they will be thousands of light years away from Earth, happen only a few times in a galaxy and the galaxy is 100,000 light years in diameter.

So basically they do back of the envelope calculations rather than reading the scientific research papers on the subject. It’s understandable that they forget about our atmosphere so easily. It doesn’t feel as if it is so heavy. The pressure is equalized inside and out. A bit like the way fish swim in the sea, we breathe the air and have no idea how much weight of air there is above us because we have the same amount of pressure outwards too and are in equilibrium with it.

When you drink water with a straw what actually happens is that you create a reduced pressure at the top of the straw and the weight of the atmosphere pushes the water up the straw into your mouth. If you had a perfect vacuum then you could suck water up 10.3 meters. So the weight of the atmosphere is the same as the weight of 10.3 meters thickness of water. Every square meter of the Earth’s surface has 10.3 metric tons of atmosphere above it.

Here is a video showing how you can suck water up to several meters through a straw, six meters, but not quite 10.3 meters - because you can’t create a perfect vacuum. Anyway - at the end where it shows them trying to suck the water up to the top of a cliff - the atmosphere above us is equivalent in mass to a layer of water the height of that cliff.

That’s what they tend to forget.

So they are right, there’s no warning, but you aren’t toast. Indeed you’d not notice the event itself at all except as a very bright flash - good idea to close your eyes if that happens because the UV light could make you blind.

The effects of a nearby gamma ray burst or supernova, even if it is as close as just a few light years away would be just on the upper atmosphere on the ozone layer leading to more UV radiation - an ozone hole - and possibly nitric acid rain. The ionizing radiation effects are not significant.

SUMMARY

Perhaps Gamma ray bursts could have caused some mass extinctions in the past - but so far we don't have anything that is confirmed to have been caused by a gamma ray burst. It is a minority view hypothesis for the Ordovician–Silurian extinction events - if so the new ozone layer study suggests that they couldn’t have caused this extinction through ozone smog at ground level. That leaves the UV light but it’s hard to see that causing the extinctions to such an extent either. Paper about biological effects of gamma ray bursts here

You don’t need to worry that a gamma ray burst could make humans extinct. Though it could be a nuisance for us. Thankfully they are very very rare. Like supernovae, they can’t be predicted because they happen as a result of very distant astronomical events that we are nowhere near being able to observe with enough precision to predict such a thing.

See also:

• Gamma-ray burst
• Effects of Gamma Ray Bursts in Earth’s Biosphere
• Gamma-Ray Bursts as a Threat to Life on Earth

See also my: Debunked: A gamma ray burst could make humans extinct