Orbital Control of Climate

The last 600,000 years

Objective

To understand orbital control of natural climate change during the Quaternary

(late Tertiary),

the last 2.5My

2.5My-0.65My (weak ice ages, every 50Ky), 0.65My-present (strong ice ages, every 100Ky)

Broecker, p.167

The last 600ky…LGMLast GlacialMaximum18Ky

Orbital Cycles

Orbital Cycle Changes

1• Shape of Earth’s yearly path of revolution around the SunECCENTRICITY

2• Tilt of Earth’s axisTILT

3• Changing positions of the seasons along that pathPRECESSION

Various records of climate (ice cores, deep ocean circulation,greenhouse gas concentrations, cyclic stratigraphy in sediments)have these same orbital cycles.

1. Eccentricity• 3% range ofvariation of E-Sdistance due to ellipticalorbit around Sun,Pull of large planets

• Now, perihelion occurs during northernWinter (3 January)

• 100ky and 413kyperiods

• Now, e =.0167(relatively circular)

2. Tilt - Reason for Seasons

• Now tilt is 23.5°, andis decreasing.

• Higher the tilt, higherthe seasonal differences

• Has gone from 24.5°-22.2°, varied by pullof large planets

• 41 ky period

3. Precession - WobbleVaries E-S distance with season

• Variation of the direction in which the E axis leans.

• Caused by the pull of Sun and moon on Earth

• Now Earth’s axis points to Polaris

• 23 ky stronger period - combined wobble and slow turning of elliptical orbit.

Now

11Kya

On the long term…the precessional index

Eccentricity variations at 100Ky and 413Kymodulate the amplitude of the precession cycle.This may be why we have the 100Ky ice age cycle

Variation of insolation with time - Big changes at high northern latitudes during deglaciations (arrows, June solstice at perihelion)

How orbital theory, climate records (18O)and northern hemisphere insolation (i.e. melting in N. Atlantic) compare over the past 600ky.

Cold warm

nowLGM18ky

Orbital Cycles

Isotopic and spectral evidence of transition to 100ky ice age periodicity…

Raymo, 1994, Ann Rev Ea Planet Sci 22, 353

Figs 10-15 and 10-17

Milankovitch Theory of ice ages

What triggers an ice age? Low insolation:• when tilt is small (~22°)(seasonality differences are smallest)• Eccentricity is large (very elliptical orbit)• Perihelion occurs during the northern hemisphere’s winter

Deglaciation can be triggered when:• perihelion occurs in July (e.g. 11K ya)• the Earth’s tilt is near its maximum

What are the mechanisms that amplify orbital variations in insolation?(1) Ice-albedo feedback(2) ocean circulation/CO2 feedback (Shackleton 2000) – ice lags CO2 and orbital changes

Relative effects of orbital cycles on

insolation at 65°N

20-30W

25-90W

25W

Crucial fordeglaciation melting

Broecker, p.159

Cycle of ice sheet growth - FACTS:

Ice accumulation rates (0-.5m/y) << Ablation rates (0-4m/y)

Summer insolation is most important for ice sheet growth and decay

Fig. 10-12

Cycle of ice sheet growth - What makes ice sheets grow and shrink?

ablationaccumulation

P = Climate Point, where equilibrium line intersects surface

Equilibrium line =Boundary betweenareas of net ice ablationand accumulation.

Cooling at higher altitudes,6.5°C cooling per km of alti-tude. = Ice Elevation Feedback

D.* Ice growth continues becauseInsolation levels are still relativelyLow and most of ice is at highelevation

Cycle of ice sheet growth -

ablationaccumulation

3 Factors control ice sheets:

1. Insolation control of IS size

2. Initial lag of vol behindInsolation

3. Subsequent lag of bedrock depression and rebound behindice loading and unloading.

Milankovitch