Editing Causes of climate change (section)

== Factors affecting Earth's climate ==
[[File:Schematic_presentation_on_the_Earth_heat_inventory_for_the_current_anthropogenically_driven_positive_Earth_energy_imbalance_(EEI)_at_the_top_of_the_atmosphere_(TOA).png|thumb|A diagram which shows where the extra heat retained on Earth due to the energy imbalance is going.|alt=Infographic]]
A [[climate forcing|forcing]] is something that is imposed externally on the [[climate system]]. External forcings include natural phenomena such as volcanic eruptions and variations in the sun's output.<ref>Le Treut ''et al.'', [http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch1.html Chapter 1: Historical Overview of Climate Change Science] {{Webarchive|url=https://web.archive.org/web/20111221183427/http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch1.html|date=21 December 2011}}, [http://www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-1-1.html FAQ 1.1, What Factors Determine Earth's Climate?] {{Webarchive|url=https://web.archive.org/web/20110626004451/http://www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-1-1.html|date=26 June 2011}}, in {{Harvnb|IPCC AR4 WG1|2007}}.</ref> Human activities can also impose forcings, for example, through changing the composition of [[Earth's atmosphere]]. [[Radiative forcing]] is a measure of how various factors alter the [[Earth's energy balance|energy balance]] of planet Earth.<ref>Forster ''et al.'', [http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2.html Chapter 2: Changes in Atmospheric Constituents and Radiative Forcing] {{Webarchive|url=https://web.archive.org/web/20111221183528/http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2.html|date=21 December 2011}}, [http://www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-2-1.html FAQ 2.1, How do Human Activities Contribute to Climate Change and How do They Compare with Natural Influences?] {{Webarchive|url=https://web.archive.org/web/20110706022116/http://www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-2-1.html|date=6 July 2011}} in {{Harvnb|IPCC AR4 WG1|2007}}.</ref> A positive radiative forcing will lead towards a warming of the surface and, over time, the climate system. Between the start of the [[Industrial Revolution]] in 1750, and the year 2005, the increase in the atmospheric concentration of [[Carbon dioxide in Earth's atmosphere|carbon dioxide]] ([[chemical formula]]: {{CO2}}) led to a positive radiative forcing, averaged over the Earth's [[surface area]], of about 1.66 watts per square metre (abbreviated W&nbsp;m<sup>−2</sup>).<ref name="ipcc radiative forcing">IPCC, [http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spm.html Summary for Policymakers] {{Webarchive|url=https://web.archive.org/web/20181102182309/http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spm.html|date=2 November 2018}}, [http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-human-and.html Human and Natural Drivers of Climate Change] {{Webarchive|url=https://web.archive.org/web/20181102212113/http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-human-and.html|date=2 November 2018}}, Figure SPM.2, in {{Harvnb|IPCC AR4 WG1|2007}}.</ref>

Climate feedbacks can either amplify or dampen the response of the climate to a given forcing.<ref name="nrc2008" />{{Rp|7|date=November 2012}}
There are many feedback mechanisms in the climate system that can either amplify (a [[positive feedback]]) or diminish (a [[negative feedback]]) the effects of a change in climate forcing.

The climate system will vary in response to changes in forcings.<ref>{{cite book |last=Committee on the Science of Climate Change, US National Research Council |title=Climate Change Science: An Analysis of Some Key Questions |publisher=[[National Academies Press]] |year=2001 |isbn=0-309-07574-2 |location=Washington, D.C., US |page=8 |chapter=2. Natural Climatic Variations |doi=10.17226/10139 |access-date=20 May 2011 |chapter-url=http://www.nap.edu/openbook.php?record_id=10139&page=8 |archive-url=https://web.archive.org/web/20110927120212/http://www.nap.edu/openbook.php?record_id=10139&page=8 |archive-date=27 September 2011 |url-status=live}}</ref>
The climate system will show internal variability both in the presence and absence of forcings imposed on it. This internal variability is a result of complex interactions between components of the climate system, such as the [[coupling (physics)|coupling]] between the atmosphere and ocean.<ref>Albritton ''et al.'', [http://www.grida.no/climate/ipcc_tar/wg1/010.htm Technical Summary] {{Webarchive|url=https://web.archive.org/web/20111224074756/http://www.grida.no/climate/ipcc_tar/wg1/010.htm|date=24 December 2011}}, [http://www.grida.no/climate/ipcc_tar/wg1/011.htm#box1 Box 1: What drives changes in climate?] {{Webarchive|url=https://web.archive.org/web/20170119085908/http://www.grida.no/climate/ipcc_tar/wg1/011.htm#box1|date=19 January 2017}}, in {{Harvnb|IPCC TAR WG1|2001}}.</ref> An example of internal variability is the [[El Niño–Southern Oscillation]].
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