EARTH’S ORBITAL CHANGES

8/3/2019 EARTHS ORBITAL CHANGES

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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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EARTH’S ORBITAL CHANGES