Chapter 10— Insolation control of ice sheets

Ruitang Soong

Outline

What controls the size of ice sheet? Modeling the behavior of ice sheets North hemisphere ice sheet history

What controls the sizeof ice sheet? Elevation & Latitude Temperate Insolation Orbit of Solar-Earth

Figure 10-1

Orbital-scale control of ice sheet:The Milankovitch theory Key point Summ

er insolation control of ice sheets

Previous study

J.A. Adhemar (1842)

James Croll (1864, 1875)

Milutin Milankovitch (1915~ 1940) combined precession, eccentricity,

and tilt. Chose summer in the northern high latitudes as the important season; predicted warm periods at 125, 105, and 82 ka.

Diagram depicting variation in precession. Image by Robert Simmon, NASA GSFC.Taken from http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/milankovitch.html. 

Figure 10-3

Figure 10-2

Modeling the behavior of ice sheets

weak insolation in summer

Ice growth, ice & bedrock depression strong insolation in summer

Ice melting, ice slipping & calving, bedrock swelling

Two feedback mechanism

Insolation Control of ice sheet size

Climate point Equilibrium line

Figure 10-5

Figure 10-6

Figure 10-7

Ice sheet lags behind summer insolation forcing

Ice volume response to lnsolation

)(1

)(

)(lS

Ttd

ld

where l is ice volumed(I)/d(t) is the rate of change of ice

volume per unit of time (t)

T is the response time of the ice sheet

S is the curve of changing summer insolation

Figure 10-8

Figure 10-9

Delayed bedrock response beneath ice sheet Elastic response Viscous response

Tack from: Richard E. Goodman, Introduction to Rock Mechanics, 2/e, 1989,Wiley

Figure 10-10

Figure 10-11 Bedrock feedback to ice growth and meltingInsolation control of ice sheet size the initial lag of ice volume behind insolation the subsequent lag of bedrock depression and rebound behind ice loading and unloading

Full cycle of ice growth and decay

Figure 10-12

Ice slipping and calving

Basal slipping (terrain, stress) Calving Ice sheet models can be coupled to 3D GCM

models.

North hemisphereice sheet history

Conceptual Model: Evolution of ice sheet cycles

Changes in summer insolation (shorter-term change) Gradual global cooling (longer-term change)

Note:

cycle of tilt is 41,000 year

cycle of precession is 23,000 year

cycle of eccentricity is 100,000 and 413,000 year

Figure 10-13

Figure 10-14

Evidence from δ18O:How ice sheets actually evolved

Ocean sediments contain two key indicator of past glaciations

Ice-rafted debris

δ18O

N. ShackletonFigure 10-15

Figure 10-16Figure 10-17

Confirming ice volume change:Coral reefs and sea level δ18O (ice volume) Radiocarbon (234U→230Th) Ice-rafted debris Coral reefs (sea level)

Box 10-3

Using astronomical and δ18O signals as a chronometer Orbital tuning

Figure 10-21