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EARTHS ORBITALEARTHS ORBITALCHANGESCHANGES
ANDAND
ITS EFFECT ONITS EFFECT ONCLIMATECLIMATE
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SOME BASIC CONCEPTSSOME BASIC CONCEPTS
Earths climate engine is theEarths climate engine is theSUNSUN..
Small variations in solar radiation can haveSmall variations in solar radiation can have
large climatic effects.large climatic effects. Earths orbital movementsEarths orbital movements -- REVOLUTIONREVOLUTION,,
ROTATIONROTATION andandWOBBLINGWOBBLING..
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OLDER THEORIESOLDER THEORIES
Joseph Alphonse AdhemarJoseph Alphonse Adhemar was the first to suggest thatwas the first to suggest that iceice
agesages were controlled bywere controlled by astronomical forces. Adhemarastronomical forces. Adhemar
reasoned that because the southern hemisphere had more hoursreasoned that because the southern hemisphere had more hours
of darkness in winter, it must be cooling, and attributedof darkness in winter, it must be cooling, and attributed
thethe AntarcticAntarctic ice sheetice sheet to this. Adhemar knew of the 22,000to this. Adhemar knew of the 22,000year cycle ofyear cycle of precession of the equinoxes, and theorised thatprecession of the equinoxes, and theorised that
the ice ages occurred in this cycle.the ice ages occurred in this cycle.
JamesJames CrollCroll, suggested in 1875 that long term changes in the, suggested in 1875 that long term changes in the
amount of solar radiation reaching the earth were controlled byamount of solar radiation reaching the earth were controlled byrhythmic variations in the earths orbit around the sun and thatrhythmic variations in the earths orbit around the sun and that
the solar radiation variations produced the periodic climatethe solar radiation variations produced the periodic climate
changes which were being discovered by contemporarychanges which were being discovered by contemporary
geologists.geologists.
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Milankovitch VariationsMilankovitch Variations
Milutin Milankovitch,Serbian mathematician.
1924--He suggested variations in earths orbitvariations in earths orbit
around the Sun affect the amount of solararound the Sun affect the amount of solar
radiation received on Earths surface.radiation received on Earths surface.
Orbital scale changes occur over tens toOrbital scale changes occur over tens to
hundreds of thousands of years.hundreds of thousands of years.
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Milankovitch CycleMilankovitch Cycle
ECCENTRICITYECCENTRICITY-- EarthsElliptical Orbit.EarthsElliptical Orbit.
OBLIQUITYOBLIQUITY-- Earths Tilt.Earths Tilt.
PRECESSIONPRECESSION -- Earths Wobble.Earths Wobble.
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EccentricityEccentricity Eccentricity is a measure of how elliptical an orbit is.Eccentricity is a measure of how elliptical an orbit is.
A planets closest approach to the sun is called theA planets closest approach to the sun is called the perihelionperihelion
and the furthest distance is theand the furthest distance is the aphelionaphelion..
When e=0 , the orbit is circular. As e gets close to 1 , the orbitWhen e=0 , the orbit is circular. As e gets close to 1 , the orbit
becomes more elongated.becomes more elongated.
When e>0 , note that the sun is not in the centre of the ellipse.When e>0 , note that the sun is not in the centre of the ellipse.
Eccentricity0 Eccentricity0.5
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The larger the eccentricity ,the greater the difference in solarThe larger the eccentricity ,the greater the difference in solar
radiation that reaches the earth at perihelion versus aphelion.radiation that reaches the earth at perihelion versus aphelion.
At its current value of e=0.017 , this difference is 6.7%.At its current value of e=0.017 , this difference is 6.7%.
When the orbit is at its most elliptical, the amount of solarWhen the orbit is at its most elliptical, the amount of solar
radiation at perihelion will be about 23% more than at aphelion.radiation at perihelion will be about 23% more than at aphelion.
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Eccentricity VariationsEccentricity Variations
Current value:Current value: 0.0170.017
Range:Range: 0.00050.0005 --0.06070.0607
Period(s):Period(s): ~100,000 yrs~100,000 yrs
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What makesWhat makes ECCENTRICITYECCENTRICITY vary?vary?
TheThe gravitationalgravitational pull of the otherplanets.pull of the otherplanets.
The pull of another planet is strongest when the planets
are close together.
The Earth's eccentricity varies primarily du
eto interactions with the gravitational fields of
Jupiter and Saturn.
y y
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ObliquityObliquity --Axial TiltAxial Tilt
The changes in the axial tilt (22The changes in the axial tilt (22oo toto
24.524.5oo) of the Earth.) of the Earth.
The greater the tilt the greater theThe greater the tilt the greater thecontrast between summer and wintercontrast between summer and winter
temperatures.temperatures.
Cycle is about 41,000 years.Cycle is about 41,000 years.
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OBLIQUITY VARIATIONSOBLIQUITY VARIATIONS
Current value:Current value: 23.523.5oo
Range:Range: 2222oo--24.524.5oo
Period:Period: 41,000 yrs.41,000 yrs.
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PrecessionPrecession The wobble of the Earth as it spins on its axisThe wobble of the Earth as it spins on its axis
This wobble follows a 26,000 year cycle.This wobble follows a 26,000 year cycle.
The earths axis now pointsThe earths axis now points
at Polaris ,our north star.at Polaris ,our north star.
In 13,000 years it willIn 13,000 years it will
point towards the star Vegapoint towards the star Vega..
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11,000 yrs ago*TODAY
y
N
S
y
N
S
POLARIS VEGA
*Actually, Vega would have been the North Star morelike 13,000 years ago.
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PRECESSION VARIATIONSPRECESSION VARIATIONS
Range:Range: 00--360360oo
Current value:Current value: PerihelionPerihelion
presently occurs aroundpresently occurs aroundJanuary 3, whileJanuary 3, whileaphelion is around Julyaphelion is around July4.4.
North poleNorth pole is pointedis pointed
almost directlyalmost directly awayaway fromfromthe Sun at perihelion.the Sun at perihelion.
Periods:Periods: ~26,000 yrs~26,000 yrs
y
N
S
Today
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What makes theWhat makes the OBLIQUITYOBLIQUITY andand PRECESSIONPRECESSION
vary?vary?
gEquator
N
S
g
The Moons torque on
the Earth is about twice
as strong as the Suns.
ii) The pull of the Sun and the Moon on Earths) The pull of the Sun and the Moon on Earthsequatorial bulge.equatorial bulge.
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ii) Also, the tilting of Earths orbital plane.ii) Also, the tilting of Earths orbital plane.
N
y
S
y
S
N
Tilting of the orbital plane is like
a dinner plate rolling on a tableIf the earth was perfectly spherical,
its spin axis would always point in
the same direction but it would make
a different angle with its orbital plane
as the plane moved around.
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Optimal Conditions for Glaciation:
1. Low obliquity (low seasonal contrast)
2. High eccentricity and NH summers during aphelion
(cold summers in the north).
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Optimal Conditions forDeglaciation:
1. High obliquity (high seasonal contrast)
2. High eccentricity and NH summers during perihelion
(hot summers in the north)
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INSOLATIONINSOLATION
Insolation is described as the intensity of SolarInsolation is described as the intensity of Solar
radiation that is received on Earth.radiation that is received on Earth.
The intensity of the solar radiation and itsThe intensity of the solar radiation and its
correlation between eccentricity and obliquitycorrelation between eccentricity and obliquity
help define seasons and play a major role inhelp define seasons and play a major role in
glacial cycles.glacial cycles.
Milankovitchs key insight:
I
ce and snow are not completely melted during very cold summers.
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Orbital variations in received insolation:
Each orbital influence has its own distinctive
pattern of influence through time. They combine to
vary sunlight on Earth.
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INSOLATION AT 65INSOLATION AT 65NN
High latitude summerHigh latitude summerinsolation (june, 65insolation (june, 65n) hasn) has
been regarded as an index ofbeen regarded as an index oforbital forcing of glaciation.orbital forcing of glaciation.
(This is the original(This is the originalMilankovitch hypothesis:Milankovitch hypothesis:cool summers are beneficialcool summers are beneficialto ice growth.)to ice growth.)
For low summer insolation:For low summer insolation:
aphelion in summer (esp.aphelion in summer (esp.With high eccentricity), lowWith high eccentricity), lowobliquity.obliquity.
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http://www.ngdc.noaa.gov/paleo/slides/images/base/iceage13.gif
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This diagram shows the combined effect of Eccentricity ,This diagram shows the combined effect of Eccentricity ,
Obliquility and Precession on solar radiation.Obliquility and Precession on solar radiation.
The blue strips marked on the graph shows the periods ofThe blue strips marked on the graph shows the periods of
glaciation.glaciation.
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HH1818OO andandGlobal Ice VolumeGlobal Ice Volume
Asicesheetsgrow, thewaterremovedfromthe oceanAsicesheetsgrow, thewaterremovedfromthe ocean
haslowerhaslowerHH1818O thanthewaterthatremains.O thanthewaterthatremains.
ThustheThustheHH1818O value ofseawaterintheglobal oceanisO value ofseawaterintheglobal oceanis
linearlycorrelatedwithice volume (largerlinearlycorrelatedwithice volume (largerHH1818OO largericesheets).largericesheets).
Regions ofthe oceaninwhichevaporationexceedsRegions ofthe oceaninwhichevaporationexceeds
precipitationareenrichedinprecipitationareenrichedinHH1818O, and vice versa.O, and vice versa.
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This image (Below) is a graphical represtentation of
the Milankovitch
Climate Theory over a 15,000 yearperiod. It
illustrates changesin insolation due the Milankovitch Cycles. Note the
uniform insolation
levels at -19000 yr, this would denote the end of a
glacialperiod.Looking at this model we can see a visualrepresentation of our current interglacialperiod. At -
11000 we were
experiencing a maxima in solar radiation in thenorthern hemisphere.
Difference in northern hemisphere insolation at summer solstice
relative to today.
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Orbital forcing of Earths climateOrbital forcing of Earths climate
Changes in Earths orbital geometry
(eccentricity, tilt, precession).
Changes in the seasonal distribution of
Insolation (heat) as a function of latitude.
Glacial-interglacial climate change.
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