Editing Climate variability and change (section)

==== Greenhouse gases ====
{{Main|Greenhouse gas}}
[[File:Carbon Dioxide 800kyr.svg|thumb|right|upright=1.35|{{CO2}} concentrations over the last 800,000 years as measured from ice cores (blue/green) and directly (black)]]
Whereas [[greenhouse gas]]es released by the biosphere is often seen as a feedback or internal climate process, greenhouse gases emitted from volcanoes are typically classified as external by climatologists.<ref>{{harvnb|Cronin|2010|p=17}}</ref> Greenhouse gases, such as {{CO2}}, methane and [[nitrous oxide]], heat the climate system by trapping infrared light. Volcanoes are also part of the extended [[carbon cycle]]. Over very long (geological) time periods, they release carbon dioxide from the Earth's crust and mantle, counteracting the uptake by sedimentary rocks and other geological [[carbon dioxide sink]]s.

Since the [[Industrial Revolution]], humanity has been adding to greenhouse gases by emitting CO<sub>2</sub> from [[fossil fuel]] combustion, changing [[land use]] through deforestation, and has further altered the climate with [[aerosols]] (particulate matter in the atmosphere),<ref>{{cite web |url=https://www.science.org.au/learning/general-audience/science-booklets-0/science-climate-change/3-are-human-activities-causing |title=3. Are human activities causing climate change? |publisher=Australian Academy of Science |website=science.org.au |access-date=12 August 2017 |archive-date=8 May 2019 |archive-url=https://web.archive.org/web/20190508094624/https://www.science.org.au/learning/general-audience/science-booklets-0/science-climate-change/3-are-human-activities-causing |url-status=live }}</ref> release of trace gases (e.g. nitrogen oxides, carbon monoxide, or methane).<ref>{{cite book
 |title        = Climate Change, Human Systems and Policy Volume I
 |chapter      = Anthropogenic Climate Influences
 |editor       = Antoaneta Yotova
 |date         = 2009
 |publisher    = Eolss Publishers
 |isbn         = 978-1-905839-02-5
 |url          = https://www.eolss.net/ebooklib/bookinfo/climate-change-human-systems-policy.aspx
 |access-date  = 16 August 2020
 |archive-date = 4 April 2023
 |archive-url  = https://web.archive.org/web/20230404081859/http://www.eolss.net/ebooklib/bookinfo/climate-change-human-systems-policy.aspx
 |url-status   = live
}}</ref> Other factors, including land use, [[ozone depletion]], animal husbandry ([[ruminant]] animals such as [[cattle]] produce [[methane]]<ref name="Steinfeld-2006">{{cite book |last=Steinfeld |first=H. |author2=P. Gerber |author3=T. Wassenaar |author4=V. Castel |author5=M. Rosales |author6=C. de Haan |title=Livestock's long shadow |year=2006 |url=http://www.fao.org/docrep/010/a0701e/a0701e00.HTM |access-date=21 July 2009 |archive-date=26 July 2008 |archive-url=https://web.archive.org/web/20080726214204/http://www.fao.org/docrep/010/a0701e/a0701e00.htm |url-status=live }}</ref>), and [[deforestation]], also play a role.<ref name="NYT-2015">{{cite news |author=The Editorial Board |title=What the Paris Climate Meeting Must Do |url=https://www.nytimes.com/2015/11/29/opinion/sunday/what-the-paris-climate-meeting-must-do.html |date=28 November 2015 |work=[[The New York Times]] |access-date=28 November 2015 |archive-date=29 November 2015 |archive-url=https://web.archive.org/web/20151129034132/http://www.nytimes.com/2015/11/29/opinion/sunday/what-the-paris-climate-meeting-must-do.html |url-status=live }}</ref>

The [[US Geological Survey]] estimates are that volcanic emissions are at a much lower level than the effects of current human activities, which generate 100–300 times the amount of carbon dioxide emitted by volcanoes.<ref>{{cite web|url=http://volcanoes.usgs.gov/Hazards/What/VolGas/volgas.html|title=Volcanic Gases and Their Effects|date=10 January 2006|publisher=U.S. Department of the Interior|access-date=21 January 2008|archive-date=1 August 2013|archive-url=https://web.archive.org/web/20130801120440/http://volcanoes.usgs.govvolcanoes.usgs.gov/|url-status=live}}</ref> The annual amount put out by human activities may be greater than the amount released by [[Supervolcano|supereruptions]], the most recent of which was the [[Toba catastrophe theory|Toba eruption]] in Indonesia 74,000 years ago.<ref name="AGU-2011">{{cite web|url=http://www.agu.org/news/press/pr_archives/2011/2011-22.shtml|title=Human Activities Emit Way More Carbon Dioxide Than Do Volcanoes|date=14 June 2011|publisher=[[American Geophysical Union]]|access-date=20 June 2011|archive-date=9 May 2013|archive-url=https://web.archive.org/web/20130509191429/http://www.agu.org/news/press/pr_archives/2011/2011-22.shtml|url-status=dead}}</ref>