Editing Climate variability and change (section)

==== Volcanism ====
[[File:Msu 1978-2010.jpg|thumb|left|upright=1.35|In atmospheric temperature from 1979 to 2010, determined by [[Microwave sounding unit|MSU]] [[NASA]] satellites, effects appear from [[aerosols]] released by major volcanic eruptions ([[El&nbsp;Chichón]] and [[Mount Pinatubo|Pinatubo]]). [[El Niño-Southern Oscillation|El&nbsp;Niño]] is a separate event, from ocean variability.]]
The [[Volcano|volcanic eruptions]] considered to be large enough to affect the Earth's climate on a scale of more than 1 year are the ones that inject over 100,000 [[ton]]s of [[sulfur dioxide|SO<sub>2</sub>]] into the [[stratosphere]].<ref name="Miles-2004">{{cite journal
| last1 = Miles | first1 = M.G.
| last2 = Grainger | first2 = R.G.
| last3 = Highwood | first3 = E.J.
| title = The significance of volcanic eruption strength and frequency for climate
| journal = Quarterly Journal of the Royal Meteorological Society
| date = 2004
| volume = 130 | pages = 2361–76
| issue = 602
| doi = 10.1256/qj.03.60
| bibcode = 2004QJRMS.130.2361M
| s2cid = 53005926
}}</ref> This is due to the optical properties of SO<sub>2</sub> and sulfate aerosols, which strongly absorb or scatter solar radiation, creating a global layer of [[sulfuric acid]] haze.<ref>{{cite web
| title = Volcanic Gases and Climate Change Overview
| url = http://volcanoes.usgs.gov/hazards/gas/climate.php
| website = usgs.gov
| publisher = USGS
| access-date = 31 July 2014
| archive-date = 29 July 2014
| archive-url = https://web.archive.org/web/20140729142333/http://volcanoes.usgs.gov/hazards/gas/climate.php
| url-status = live
}}</ref> On average, such eruptions occur several times per century, and cause cooling (by partially blocking the transmission of solar radiation to the Earth's surface) for a period of several years. Although volcanoes are technically part of the lithosphere, which itself is part of the climate system, the IPCC explicitly defines volcanism as an external forcing agent.<ref>[http://archive.ipcc.ch/publications_and_data/ar4/syr/en/annexes.html Annexes] {{Webarchive|url=https://web.archive.org/web/20190706041420/https://archive.ipcc.ch/publications_and_data/ar4/syr/en/annexes.html |date=6 July 2019 }}, in {{Harvnb|IPCC AR4 SYR|2008|p=58}}.</ref>

Notable eruptions in the historical records are the [[1991 eruption of Mount Pinatubo]] which lowered global temperatures by about 0.5&nbsp;°C (0.9&nbsp;°F) for up to three years,<ref>{{cite web
|url=http://pubs.usgs.gov/fs/1997/fs113-97/
|title=The Cataclysmic 1991 Eruption of Mount Pinatubo, Philippines
|last=Diggles
|first=Michael
|date=28 February 2005
|work=U.S. Geological Survey Fact Sheet 113-97
|publisher=[[United States Geological Survey]]
|access-date=8 October 2009
|archive-date=25 August 2013
|archive-url=https://web.archive.org/web/20130825233934/http://pubs.usgs.gov/fs/1997/fs113-97/
|url-status=live
}}</ref><ref>{{cite web
| last1 = Diggles
| first1 = Michael
| title = The Cataclysmic 1991 Eruption of Mount Pinatubo, Philippines
| url = http://pubs.usgs.gov/fs/1997/fs113-97/
| website = usgs.gov
| access-date = 31 July 2014
| archive-date = 25 August 2013
| archive-url = https://web.archive.org/web/20130825233934/http://pubs.usgs.gov/fs/1997/fs113-97/
| url-status = live
}}</ref> and the [[1815 eruption of Mount Tambora]] causing the [[Year Without a Summer]].<ref>{{cite journal
|doi=10.1191/0309133303pp379ra
|title=Climatic, environmental and human consequences of the largest known historic eruption: Tambora volcano (Indonesia) 1815
|year=2003
|last1=Oppenheimer|first1=Clive
|journal=Progress in Physical Geography
|volume=27
|pages=230–59
|issue=2
|bibcode=2003PrPG...27..230O
|s2cid=131663534
}}</ref>

At a larger scale—a few times every 50 million to 100 million years—the eruption of [[large igneous province]]s brings large quantities of [[igneous rock]] from the [[mantle (geology)|mantle]] and [[lithosphere]] to the Earth's surface. Carbon dioxide in the rock is then released into the atmosphere.<ref>{{Cite journal|title=Deep Carbon and the Life Cycle of Large Igneous Provinces|last1=Black|first1=Benjamin A.|last2=Gibson|first2=Sally A.|date=2019|journal=Elements|doi=10.2138/gselements.15.5.319|volume=15|issue=5|pages=319–324|doi-access=free|bibcode=2019Eleme..15..319B }}</ref>
<ref>{{cite journal
|doi=10.1016/S0012-8252(00)00037-4
|title=Large igneous provinces and mass extinctions
|year=2001
|last1=Wignall|first1=P
|journal=Earth-Science Reviews
|volume=53
|issue=1
|pages=1–33
|bibcode=2001ESRv...53....1W
}}</ref> Small eruptions, with injections of less than 0.1&nbsp;Mt of sulfur dioxide into the stratosphere, affect the atmosphere only subtly, as temperature changes are comparable with natural variability. However, because smaller eruptions occur at a much higher frequency, they too significantly affect Earth's atmosphere.<ref name="Miles-2004" /><ref name="Graf-1997">{{cite journal
| last1 = Graf | first1 = H.-F.
| last2 = Feichter | first2 = J.
| last3 = Langmann | first3 = B.
| title = Volcanic sulphur emissions: Estimates of source strength and its contribution to the global sulphate distribution
| journal = Journal of Geophysical Research: Atmospheres
| date = 1997
| volume = 102 | issue = D9
| pages = 10727–38
| doi = 10.1029/96JD03265
| bibcode=1997JGR...10210727G
| hdl = 21.11116/0000-0003-2CBB-A
| hdl-access = free
}}</ref>