Editing Gamma-ray burst (section)

=== Effects on Earth ===
Earth's atmosphere is very effective at absorbing high energy electromagnetic radiation such as x-rays and gamma rays, so these types of radiation would not reach any dangerous levels at the surface during the burst event itself. The immediate effect on life on Earth from a GRB within a few kilo[[parsec]]s would only be a short increase in ultraviolet radiation at ground level, lasting from less than a second to tens of seconds. This ultraviolet radiation could potentially reach dangerous levels depending on the exact nature and distance of the burst, but it seems unlikely to be able to cause a global catastrophe for life on Earth.<ref name=Threat>{{Cite journal |arxiv = 0903.4710|doi = 10.1017/S1473550409004509|title = Gamma-ray bursts as a threat to life on Earth|journal = International Journal of Astrobiology|volume = 8|issue = 3|pages = 183–186|year = 2009|last1 = Thomas|first1 = B.C.|s2cid = 118579150|bibcode = 2009IJAsB...8..183T}}</ref><ref name="Effects">{{Cite journal | doi=10.1007/s10509-009-0211-7| title=Effects of gamma ray bursts in Earth's biosphere| journal=Astrophysics and Space Science| volume=326| issue=1| pages=61–67| year=2010| last1=Martin| first1=Osmel| last2=Cardenas| first2=Rolando| last3=Guimarais| first3=Mayrene| last4=Peñate| first4=Liuba| last5=Horvath| first5=Jorge| last6=Galante| first6=Douglas| s2cid=15141366| bibcode=2010Ap&SS.326...61M| arxiv=0911.2196}}</ref>

The long-term effects from a nearby burst are more dangerous. Gamma rays cause chemical reactions in the atmosphere involving [[oxygen]] and [[nitrogen]] [[molecules]], creating first [[nitrogen oxide]] then [[nitrogen dioxide]] gas. The nitrogen oxides cause dangerous effects on three levels. First, they deplete [[ozone layer|ozone]], with models showing a possible global reduction of 25–35%, with as much as 75% in certain locations, an effect that would last for years. This reduction is enough to cause a dangerously elevated [[UV index]] at the surface. Secondly, the nitrogen oxides cause [[photochemical smog]], which darkens the sky and blocks out parts of the [[sunlight]] spectrum. This would affect [[photosynthesis]], but models show only about a 1% reduction of the total sunlight spectrum, lasting a few years. However, the smog could potentially cause a cooling effect on Earth's climate, producing a "cosmic winter" (similar to an [[impact winter]], but without an impact), but only if it occurs simultaneously with a global climate instability. Thirdly, the elevated nitrogen dioxide levels in the atmosphere would wash out and produce [[acid rain]]. [[Nitric acid]] is toxic to a variety of organisms, including amphibian life, but models predict that it would not reach levels that would cause a serious global effect. The [[nitrates]] might in fact be of benefit to some plants.<ref name=Threat/><ref name="Effects"/>

All in all, a GRB within a few kiloparsecs, with its energy directed towards Earth, will mostly damage life by raising the UV levels during the burst itself and for a few years thereafter. Models show that the destructive effects of this increase can cause up to 16 times the normal levels of DNA damage. It has proved difficult to assess a reliable evaluation of the consequences of this on the terrestrial ecosystem, because of the uncertainty in biological field and laboratory data.<ref name=Threat/><ref name="Effects"/>

==== Hypothetical effects on Earth in the past ====
There is a very good chance (but no certainty) that at least one lethal GRB took place during the past 5 billion years close enough to Earth as to significantly damage life. There is a 50% chance that such a lethal GRB took place within two kiloparsecs of Earth during the last 500 million years, causing one of the major mass extinction events.<ref>{{Cite journal |last1=Piran |first1=Tsvi |last2=Jimenez |first2=Raul |date=2014-12-05 |title=Possible Role of Gamma Ray Bursts on Life Extinction in the Universe |url=https://link.aps.org/doi/10.1103/PhysRevLett.113.231102 |journal=Physical Review Letters |volume=113 |issue=23 |pages=231102 |doi=10.1103/PhysRevLett.113.231102|pmid=25526110 |arxiv=1409.2506 |bibcode=2014PhRvL.113w1102P |hdl=2445/133018 |s2cid=43491624 }}</ref><ref name="TerrestrialOzoneDepletion">{{cite journal |last1=Thomas |first1=Brian C. |last2=Jackman |first2=Charles H. |last3=Melott |first3=Adrian L. |last4=Laird |first4=Claude M. |last5=Stolarski |first5=Richard S. |last6=Gehrels |first6=Neil |last7=Cannizzo |first7=John K. |last8=Hogan |first8=Daniel P. |date=28 February 2005 |title=Terrestrial Ozone Depletion due to a Milky Way Gamma-Ray Burst |url=https://iopscience.iop.org/article/10.1086/429799/meta |journal=The Astrophysical Journal |volume=622 |issue=2 |pages=L153–L156 |doi=10.1086/429799 |arxiv=astro-ph/0411284 |bibcode=2005ApJ...622L.153T |hdl=2060/20050179464 |s2cid=11199820 |access-date=22 October 2022}}</ref>

The major [[Ordovician–Silurian extinction event]] 450 million years ago may have been caused by a GRB.<ref name="GeographicPatterns" /><ref name="GammaRayBurstsAndTheEarth">{{cite journal |last1=Thomas |first1=Brian C. |last2=Melott |first2=Adrian Lewis |last3=Jackman |first3=Charles H. |last4=Laird |first4=Claude M. |last5=Medvedev |first5=Mikhail V. |last6=Stolarski |first6=Richard S. |last7=Gehrels |first7=Neil |last8=Cannizzo |first8=John K. |last9=Hogan |first9=Daniel P. |last10=Ejzak |first10=Larissa M. |date=20 November 2005 |title=Gamma-Ray Bursts and the Earth: Exploration of Atmospheric, Biological, Climatic, and Biogeochemical Effects |url=https://iopscience.iop.org/article/10.1086/496914/meta |journal=[[The Astrophysical Journal]] |volume=634 |issue=1 |pages=509–533 |doi=10.1086/496914 |arxiv=astro-ph/0505472 |bibcode=2005ApJ...634..509T |s2cid=2046052 |access-date=22 October 2022}}</ref> Estimates suggest that approximately 20–60% of the total phytoplankton biomass in the Ordovician oceans would have perished in a GRB, because the oceans were mostly oligotrophic and clear.<ref name="renamed_from_2021_on_20231204051223">{{cite journal |last1=Rodríguez-López |first1=Lien |last2=Cardenas |first2=Rolando |last3=González-Rodríguez |first3=Lisdelys |last4=Guimarais |first4=Mayrene |last5=Horvath |first5=Jorge |date=24 January 2021 |title=Influence of a galactic gamma ray burst on ocean plankton |url=https://onlinelibrary.wiley.com/doi/abs/10.1002/asna.202113878 |journal=Astronomical Notes |volume=342 |issue=1–2 |pages=45–48 |doi=10.1002/asna.202113878 |arxiv=2011.08433 |bibcode=2021AN....342...45R |s2cid=226975864 |access-date=21 October 2022}}</ref> The [[late Ordovician]] species of [[trilobite]]s that spent portions of their lives in the [[plankton]] layer near the ocean surface were much harder hit than deep-water dwellers, which tended to remain within quite restricted areas. This is in contrast to the usual pattern of extinction events, wherein species with more widely spread populations typically fare better. A possible explanation is that trilobites remaining in deep water would be more shielded from the increased UV radiation associated with a GRB. Also supportive of this hypothesis is the fact that during the late Ordovician, burrowing [[bivalve]] species were less likely to go extinct than bivalves that lived on the surface.<ref name="Melott2004" />

A case has been made that the [[774–775 carbon-14 spike]] was the result of a short GRB,<ref name="pavlov">{{cite journal | last1=Pavlov | first1=A.K. | last2=Blinov | first2=A.V. | last3=Konstantinov | first3=A.N. | s2cid=118638711 | display-authors=etal | title=AD 775 pulse of cosmogenic radionuclides production as imprint of a Galactic gamma-ray burst | journal=Mon. Not. R. Astron. Soc. | date=2013 | volume=435 | issue=4 | pages=2878–2884 | doi=10.1093/mnras/stt1468 | doi-access=free | arxiv=1308.1272 | bibcode=2013MNRAS.435.2878P}}</ref><ref name="hamb">{{cite journal | last1=Hambaryan | first1=V.V. | last2=Neuhauser | first2=R. | s2cid=765056 | date=2013 | title=A Galactic short gamma-ray burst as cause for the <sup>14</sup>C peak in AD 774/5 | journal=[[Monthly Notices of the Royal Astronomical Society]] | volume=430 | issue=1 | pages=32–36 | arxiv=1211.2584 | bibcode=2013MNRAS.430...32H | doi=10.1093/mnras/sts378| doi-access=free }}</ref> though a very strong [[solar flare]] is another possibility.<ref name="mek15">{{cite journal | date=2015 | author=Mekhaldi | display-authors=etal | journal =Nature Communications | volume=6 | pages=8611 | doi=10.1038/ncomms9611 | title=Multiradionuclide evidence for the solar origin of the cosmic-ray events of ᴀᴅ 774/5 and 993/4 |bibcode = 2015NatCo...6.8611M | pmid=26497389 | pmc=4639793}}</ref>