Lecture 10: Orbital Control of Ice Sheets (Chapter 9)

Ice sheet cover at 20,000 years ago The Last Glacial Maximum (LGM)

Ice-sheet balance Slow Accumulation vs. fast Ablation Implication: Annual mean temperature Implication: which season is most important for ice sheet? Winter vs. summer

Milankovitch theory Small tilt +Aphelion =>low summer insolation favor glaciation Large tilt + Perihelion => High summer insolation favor deglaciation

Milankovitch theory High summer insolation => net melting => deglaciation Low summer insolation => net accumulation => glaciation

The Last Glacial Maximum (LGM): 21,000 yrs ago Why the greatest ice sheet not over Siberia?

How does Insolation Control Ice Sheet Equilibrium line, Climate point and ice-sheet formation Cold Warm

High insolation northern/high equilibrium line northern climate point less ice-sheet formation Low insolation southern/low equilibrium line southern climate point more ice-sheet formation

Two positive feedbacks Ice-albedo feedback Ice-elevation feedback

Thermal inertial and Phase lag Ice volume lags insolation mdT/dt = SeiΩt-bT

Phase lag Ice volume lags Insolation by about ¼ cycle (ocean effect on seasonal cycle!)

Bedrock sinking Bedrock sinking and Bedrock rebound

Bedrock feedbacks Bedrock feedbacks: Initial elastic response is negative feedback; Later viscous response is positive feedback relative to the elastic sinking response by producing a delayed sinking response

The cycle of ice-sheet growth and decay

Conceptual model for ice-sheet cycle: Repeat constant insolation Threshold temperature: 0oC summer, -10oC annual mean

Deep sea 18O and ice volume history 18O ~ O18/O16 O16 light, easy to evaporate Deep sea higher 18O  more ice volume over land (evaporated to be accumulated)

Quantify δ18O changes Δ (18Oland*land ice volume)+ Δ( 18Oocean*ocean volume)=0 18Oland* Δ land ice volume + Δ 18Oocean* ocean volume=0 1/(-40) = 18Oocean/ 18Oland = -Δ land ice volume / ocean volume = -100m sl/4000m sl = -1/40

Deep sea 18O and ice volume history ~1o/oo Small glaciation phase Large glaciation phase

Revival of Milankovith Theory 1976 but, relative magnitude ?

Insolation Forcing and Ice Volume Response Small glaciation phase Large glaciation phase The remaining question: 100 kyr cycle?

Ice sheet 18O in the last 150,000 years Dominant 100 kyr cycle, why? 0 ka: Interglacial 21 ka: Last Glacial Maximum 125 ka: Last interglacial

Ice Core 18O in the last 100,000 years is it ice volume effect?

Confirming sea level change Coral reefs lives in coastal water – a proxy of sea level

Uplifted coral reef terraces: New Guinea (western Pacific)

Uplift of coral reefs: Quantifying the ice volume from the sea level change -110m (115ka) to +6m (125ka)

Sea Level Reconstruction of the last 30 kyr LGM IPCC/AR4 2007

References for Further reading and analysis Hays et al., 1976, Variations in the Earth’s Orbit: pacemaker of the ice ages.” Science, 194, 1121- P. Huybers, C. Wunsch, Nature 434, 491 (2005). P. Huybers, 2006, Early Pleistocene Glacial Cycles and the Integrated Summer Insolation Forcing. Science, 313,508-511

End of Lecture 10