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Radiative forcing
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=== Variations in total solar irradiance (TSI) === The intensity of [[solar irradiance]] including all wavelengths is the [[Total Solar Irradiance]] (TSI) and on average is the [[solar constant]]. It is equal to about 1361 W m<sup>β2</sup> at the distance of Earth's annual-mean orbital radius of one [[astronomical unit]] and as measured at the top of the atmosphere.<ref>{{cite journal |title=A new, lower value of total solar irradiance: Evidence and climate significance |journal=Geophysical Research Letters |author1=Gregg Kopp | author2=Judith L. Lean |author2-link=Judith Lean |volume=38 |issue=1 |date=2011-01-14 |pages=n/a |doi=10.1029/2010GL045777 |bibcode=2011GeoRL..38.1706K |s2cid=8190208 |doi-access=free }}</ref> Earth TSI varies with both solar activity and planetary orbital dynamics. Multiple satellite-based instruments including [[Nimbus 7|ERB]], [[ACRIMSAT|ACRIM 1-3]], [[Solar and Heliospheric Observatory|VIRGO]], and [[Solar Radiation and Climate Experiment|TIM]]<ref>{{cite web |url=https://lasp.colorado.edu/home/sorce/ |title=Solar Radiation and Climate Experiment |publisher=University of Colorado, Laboratory for Atmospheric and Space Physics |accessdate=2021-05-15 |archive-date=2021-05-19 |archive-url=https://web.archive.org/web/20210519020104/https://lasp.colorado.edu/home/sorce/ |url-status=live }}</ref><ref>{{cite web |url=https://www.nasa.gov/content/goddard/tsis-1-mission-overview |title=TSIS-1 Mission Overview |date=28 November 2017 |publisher=NASA |accessdate=2021-05-20 |archive-date=2021-07-18 |archive-url=https://web.archive.org/web/20210718123934/https://www.nasa.gov/content/goddard/tsis-1-mission-overview/ |url-status=live }}</ref> have continuously measured TSI with improving [[accuracy and precision]] since 1978.<ref name="kopp">{{cite journal |url=https://www.swsc-journal.org/articles/swsc/full_html/2014/01/swsc130036/swsc130036.html |title=Solar variability, solar forcing, and coupling mechanisms in the terrestrial atmosphere |author=Gregg Kopp |journal=Journal of Space Weather and Space Climate |date=2014-04-24 |doi=10.1051/swsc/2014012 |volume=4 |number=A14 |pages=1β9 |bibcode=2014JSWSC...4A..14K |access-date=2021-05-24 |archive-date=2021-05-06 |archive-url=https://web.archive.org/web/20210506134317/https://www.swsc-journal.org/articles/swsc/full_html/2014/01/swsc130036/swsc130036.html |url-status=live |doi-access=free }}</ref> Approximating Earth as a [[sphere]], the cross-sectional area exposed to the Sun (<math display="inline">\pi r^2</math>) is equal to one quarter the area of the planet's surface (<math display="inline">4\pi r^2</math>). The globally and annually averaged amount of solar irradiance per square meter of Earth's atmospheric surface (<math display="inline">I_0</math>) is therefore equal to one quarter of TSI, and has a nearly constant value of <math display="inline">I_0=340~~\mathrm{W}~\mathrm{m}^{-2}</math>. Earth follows an [[elliptical orbit]] around the Sun, so that the TSI received at any instant fluctuates between about 1321 W m<sup>β2</sup> (at [[aphelion]] in early July) and 1412 W m<sup>β2</sup> (at perihelion in early January), and thus by about Β±3.4% over each year.<ref name="apperi">{{cite web |url=https://www.msn.com/en-us/weather/topstories/earth-reaches-perihelion-closer-to-the-sun-than-any-other-day/ar-BB1cpVCF |title=Earth reaches perihelion, closer to the sun than any other day |date=2021-01-02 |author=Sophie Lewis |publisher=CBS News |access-date=2021-05-24 |archive-date=2021-05-24 |archive-url=https://web.archive.org/web/20210524165821/https://www.msn.com/en-us/weather/topstories/earth-reaches-perihelion-closer-to-the-sun-than-any-other-day/ar-BB1cpVCF |url-status=live }}</ref> This change in irradiance has minor influences on Earth's seasonal weather patterns and its [[climate zone]]s, which primarily result from the annual cycling in Earth's relative tilt direction.<ref>{{cite web |url=https://www.weather.gov/cle/seasons |title=The Seasons, the Equinox, and the Solstices |publisher=National Weather Service |accessdate=2021-05-20 |archive-date=2021-05-24 |archive-url=https://web.archive.org/web/20210524165823/https://www.weather.gov/cle/seasons |url-status=live }}</ref> Such repeating cycles contribute a net-zero forcing (by definition) in the context of decades-long climate changes.
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