April 13 - 27, 2005April 13 - 27, 2005

NOTE: Reviews will be handed back Thurs/Fri toNOTE: Reviews will be handed back Thurs/Fri tomailboxes, fax, etc.mailboxes, fax, etc.Midterms handed back next week.Midterms handed back next week.

Searching for periods and patterns in climate records:Searching for periods and patterns in climate records:• Geometry of Earth’s orbitGeometry of Earth’s orbit• Astronomical control of solar radiationAstronomical control of solar radiation• Periods and patterns as chronometersPeriods and patterns as chronometers• Astronomical control of monsoons, ice sheets, Astronomical control of monsoons, ice sheets, El Niño, GHGsEl Niño, GHGs• Ice cores as climate recordersIce cores as climate recorders

In addition to long-term trends, Zachos found regularly-spaced warm-cold alternations

Ultimately named “Milankovitch cycles” after Milutin Milankovitch,Serbian mathematician (1879 - 1958).

Developed mathematical theory of Earth’s ice ages based onamount of sunlight Earth receives (published in 1920).

After Raymo, M.E., Ann. Rev. Earth Plan. Sci. 22, 353-383, 1994

View of Sun and Earth from above the North Pole

day nightN

Eccentricity and the shape of Earth’s Orbit

Earth’s currently 0.017 --> nearly circularMaximum = 0.06

2a = major axis

2b = minor axisempty focus

E

Ruddiman, W.F., “Earth’s Climate, Past and Future,” W.H. Freeman, 2001, Fig. 8-6

Eccentricity and the shape of Earth’s Orbit

empty focus

rr*

r + r* = 2a

EPerihelion

Aphelion

E

E

What happens to the solar flux at perihelion and aphelion?Ruddiman, W.F., “Earth’s Climate, Past and Future,” W.H. Freeman, 2001, Fig. 8-6

Where are Northern Hemisphere Summer and Winter located?

Shape of Earth’s Orbit Today

Equinox: length ofnight and day are equal.

Solstice: longest andshortest days of the yearRuddiman, W.F., “Earth’s Climate, Past and Future,” W.H. Freeman, 2001, Fig. 8-2

Milutin Milankovitch

Calculated the changes in Earth’s orbital parameters during the past.

= 0.005

Presently = 0.0167

Ruddiman, W.F., “Earth’s Climate, Past and Future,” W.H. Freeman, 2001, Fig. 8-7

All of the Planets in the Solar System are pulling on Earth

E V

M

J

S

EV

M

J

S

All of the Planets in the Solar System are pulling on Earth

Sun’s equator

Mercury VenusEarth Mars

Jupiter

Ecliptic

Earth is not “straight up and down” with respect to the ecliptic.

Obliquity and the Eclipitc

Obliquity- Earth’s tilted rotation axis

No tilt --> No seasons

Ruddiman, W.F., “Earth’s Climate, Past and Future,” W.H. Freeman, 2001, Fig. 8-3a

Obliquity- Earth’s tilted rotation axis

Ruddiman, W.F., “Earth’s Climate, Past and Future,” W.H. Freeman, 2001, Fig. 8-1

Obliquity- Earth’s tilted rotation axis

Northern Hemisphere Winter:• occurs at perihelion• tilted AWAY from Sun• winter radiation higher than for circular orbit

Southern Hemisphere Summer:• occurs at perihelion• tilted TOWARDS Sun• summer radiation higher than for circular orbit

At present

Ruddiman, W.F., “Earth’s Climate, Past and Future,” W.H. Freeman, 2001, Fig. 8-5

Obliquity- Earth’s tilted rotation axis

Northern Hemisphere Summer:• occurs at aphelion• tilted towards Sun• summer radiation lower than for circular orbit

Southern Hemisphere Winter:• occurs at aphelion• tilted away from Sun• winter radiation lower than for circular orbit

Ruddiman, W.F., “Earth’s Climate, Past and Future,” W.H. Freeman, 2001, Fig. 8-5

Obliquity- Earth’s tilted rotation axis

Northern Hemisphere Summer:• occurs at aphelion• tilted towards Sun• summer radiation lower than for circular orbit

Northern Hemisphere Winter:• occurs at perihelion• tilted AWAY from Sun• winter radiation higher than for circular orbit

At present, northern summers and winters are both milderthan they could be. Low seasonal contrast.

At present Ruddiman, W.F., “Earth’s Climate, Past and Future,” W.H. Freeman, 2001, Fig. 8-13

Obliquity- Earth’s tilted rotation axis

At present, southern summers and winters are both extreme.High seasonal contrast.

Southern Hemisphere Summer:• occurs at perihelion• tilted TOWARDS Sun• summer radiation higher than for circular orbit

Southern Hemisphere Winter:• occurs at aphelion• tilted away from Sun• winter radiation lower than for circular orbit

Ruddiman, W.F., “Earth’s Climate, Past and Future,” W.H. Freeman, 2001, Fig. 8-13

Obliquity- Earth’s tilted rotation axis

Earth’s obliquity varies from ~22 to 24.5˚with a period of 41,000 years.

Ruddiman, W.F., “Earth’s Climate, Past and Future,” W.H. Freeman, 2001, Fig. 8-4

Wobble- Precession of Earth’s rotation axis

Wobble = DIRECTION in which spin axis leans

Ruddiman, W.F., “Earth’s Climate, Past and Future,” W.H. Freeman, 2001, Fig. 8-8

Wobble- Precession of Earth’s rotation axisEarth’s rotation axis sweeps out a circle every 27,500 years.• Right now, axis points at Polaris (North Star)• ~2000 B.C.. Alphas Draconis was pole star• ~ 14,000 A.D. Vega will be pole star

en.wikipedia.org

Appearsclockwisewhen viewedfrom abovenorth pole.Appears counter-clockwisewhen startsviewedfrom Earth.

Ruddiman, W.F., “Earth’s Climate, Past and Future,” W.H. Freeman, 2001, Fig. 8-9

Precession of Earth’s orbit

Elliptical shape of Earth’s orbit is rotating,

Precession of Earth’s orbit

Elliptical shape of Earth’s orbit is rotating

Precession of Earth’s orbit

Elliptical shape of Earth’s orbit is rotating

Appears counter-clockwise when viewed from above Earth’sNorth pole

Precession of Earth’s orbit

Elliptical shape of Earth’s orbit is rotating

Wobble and precession of ellipse IN OPPOSITE DIRECTIONS.Combined effect: 23,000 years to make a full cycle.

Appearsclockwisewhen viewedfrom aboveNorth pole

Ruddiman, W.F., “Earth’s Climate, Past and Future,” W.H. Freeman, 2001, Fig. 8-9

Equinoxes and solstices move around the ellipse

Ruddiman, W.F., “Earth’s Climate, Past and Future,” W.H. Freeman, 2001, Fig. 8-2

Effect of Precession on sunlight received

0.5 Myr0.5 Myr

Ruddiman, W.F., “Earth’s Climate, Past and Future,” W.H. Freeman, 2001, Fig. 8-14

Today

Half a precessional cycle in the past

• Summer at perihelion • LOTS of solar radiation

• Winter at aphelion• Minimal solar radiation

Ruddiman, W.F., “Earth’s Climate, Past and Future,” W.H. Freeman, 2001, Fig. 8-13

Insolation changes with latitude AND season through time

Ruddiman, W.F., “Earth’s Climate, Past and Future,” W.H. Freeman, 2001, Fig. 8-16

Inco

min

g F

lux

W/m

2

Calendar Year AD

1.3 W/m2

0.1 %

Redrawn from Frolich and Lean, 1998

Does all this affect Earth’s climate?

Critics said: effect exceeds the cause.

Data to support Milankovitch was a long time coming

Oscillations every100,000 years

Oscillations every23,000 and 41,000 years

After Raymo, M.E., Ann. Rev. Earth Plan. Sci. 22, 353-383, 1994

150

200

250

300

350

400

0 100000 200000 300000 400000

Age (yr BP)

1960 - 2000 AD

450,000 year record of CO2 from Vostok Ice Core, Antarctica

Data generously made available online by the Vostok Ice Core group,

N

R

R

N

Normal Reverse

Orbital Tuning-- what do you think?

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20

10

0

Meters below seafloor (

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mbsf

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2

4

6

8

12

14

16

18

1 0 -123

δ18

O

1st reversal = 780,000 yr B.P.

top = 0 yr

] = ~ 90 - 115,000 years apart

Orbital Tuning-- what do you think?

Orbital tuning:1. Apply magnetostratigraphy2. Assign wiggles to be spaced 100,000 years3. Match peaks and troughs to insolation curve

Orbital Tuning in the Vostok ice core

Generously made available online at http://www.usgcrp.gov/usgcrp/images/Vostok.jpg