ENIGMA: something that is mysterious, puzzling, or difficult to understand.

Transcription

1 Lecture 12. Attempts to solve the Eccentricity Enigma ENIGMA: something that is mysterious, puzzling, or difficult to understand.

2 Milankovitch forcing glacier responses pre-900,000 yr BP glacier responses post-900,000 yr BP

3 41,000-yr Earth 100,000-yr Earth

4 Eccentricity Doesn t show up Q1. Why is 41-ka period dominant and not 23 ka?

5 Eccentricity Doesn t show up Q1. Why is 41 dominant and not 23 ka? Q2. Why this 100,000 yr dominance? (the Eccentricity Enigma)

6 4 hypotheses explaining the dominance of the 41-ka (obliquity) cycle and the 41-ka to 100-ka transition 1) Ruddiman (2003 and in our textbook) 2) Raymo et al (2006) 3) Jung-Eun Lee at al (2017) 4) Marshall and Clark (2002)

7 Ruddiman, W.F. (2006). Ice-driven CO 2 feedback on ice volume. Climates of the Past 2, I have made millions off your textbook. Thanks!

8 precession is the stronger forcing, but obliquity s persists longer

9 Ruddiman: We can dispense with the question of why the 41-ka (obliquity) cycle dominates over the stronger 21-ka precession cycle because. Please recommend my text book to your family and friends.

10 Ruddiman: We can dispense with the question of why the 41-ka (obliquity) cycle dominates over the stronger 21-ka precession cycle because. if you take into account the longerlasting forcing from the 41-ka cycle, it has a 50% greater effect on ice sheets.

11 Analogy: given enough time, even a small stream can create a large valley.

12 on the warmer, pre-900,000-yr Earth: colder warmer precession + tilt + CO 2 positive feedback on icesheet growth.ice sheets can grow to a 41-ka size

13 Four hypotheses explaining the dominance of the 41-ka (obliquity) cycle and the 41-ka to 100-ka transition 1) Ruddiman (2003 and in our textbook) 2) Raymo et al (2006) 3) Lee at al (2017) 4) Marshall and Clark (2002) Maureen Raymo, Lamont Doherty Earth Observatory

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15 Increases in obliquity (tilt) accentuate seasonal differences in radiation received at both poles.

16 In contrast, the precessional cycle causes changes in incident solar radiation that are opposite in each hemisphere over the course of a year.

17 If the Arctic s summer occurs at aphelion (less radiation), then the Antarctic summer occurs at perihelion (more radiation), and vice versa. perihelion in Arctic summer 23 ka aphelion in Antarctic summer Precession of the Equinoxes

18 the precessional cycle causes changes in incident solar radiation that are opposite in each hemisphere over the course of a year.

19 perihelion in Arctic summer 23 ka aphelion in Antarctic summer

20 Arctic cooling Antarctic warming net result: global changes in sea level and del 18 O precession cancel out. and vice versa

21 Milankovitch forcing.which leaves obliquity (tilt) the dominant Milankovitch glacier responses pre-900,000 yr BP forcing frequency glacier responses post-900,000 yr BP

22 .and a higher tilt means more / less radiation simultaneously at both poles.

23 When obliquity high, summers in both hemispheres receive more insolation, and winters less.

24 back to Raymo et al (2006):

25 Raymo et al (2006): the 900,000-yr transition was caused by Antarctic ice sheets switching from land-terminating glaciers sensitive to summer warmth to marine-based (calving) ice sheets insensitive to summer climate and sensitive instead only to global sea level.

26 and what controls global eustatic sea level?

27 .continental ice sheets

28 .but only those that lose and gain a lot of ice mass not Antarctica perched on its frozen continent. Arctic Basin can only accumulate sea ice and ice shelves Antarctica: 1000s meters of ice safely stored on land Norwegian Polar Institute

29 Today (in midst of an interglacial) ablation in Antarctica mainly through iceberg calving

30 most calving occurs at termini of ice shelves

31 calving rates sensitive to changes in global sea level Ice edge of the Brunt Ice Shelf near Halley Research Station Chris Gilbe

32 Rising sea level floats an ice shelf, which triggers calving retreat.

33 So.after 900,000 yr, Antarctica so cold even in summer that most ablation occurs via iceberg calving (like today).

34 So.Antarctica now so cold even in summer that all ablation occurs via ice berg calving (like today). Antarctic stops being sensitive to summer temperature.

35 and N hemisphere ice sheets control global sea level. Arctic Basin can only accumulate sea ice and ice shelves Antarctica: 1000s meters of ice safely stored on land Norwegian Polar Institute

36 Antarctica relinquishes control of ice age climate to the northern hemisphere

37 Eustatic sea level brings ice sheets in both polar hemispheres into phase perihelion in Arctic summer 23 ka aphelion in Antarctic summer

38 Ice sheet dynamics at both poles now in phase. Result = Climatic changes greater, Ice sheet fluctuations larger

39 But what about the Eccentricity Enigma? 41,000-yr Earth 100,000-yr Earth

40 Raymo et al (2006): Once the two poles are in phase, larger ice sheets can develop, and global climate can cool further.

41 To the point where N hemisphere ice sheets are large enough to survive most summer-insolation maxima.

42 This leads the way to the development of the 100,000-yr cycle.

43 Three hypotheses explaining the dominance of the 41- ka (obliquity) cycle and the 41-ka to 100-ka transition 1) Ruddiman (2003 and in our textbook) 2) Raymo et al (2006) 3) Lee at al (2017) 4) Marshall and Clark (2002)

44 Jung-Eng Lee

45 Today: Larger seasonal changes in sea ice cover in Southern Ocean than in northern hemisphere

46 Lee et al s hypothesis: 1) eccentricity (100 ka) modulates strength of precession (21 ka) on incoming solar radiation 2) high eccentricity amplifies the effect of summer aphelion on sea ice extent in southern hemisphere eccentricity sea ice maxima 21 ka 21 ka 21 ka 400, , , ,000 today

47 Why is this effect not symmetric? (if it were symmetrical, sea ice changes in the N hemisphere would cancel out sea ice changes in the S hemisphere)

48 Why is this effect not symmetric? (if it were symmetrical, sea ice changes in the N hemisphere would cancel out sea ice changes in the S hemisphere) (not enough additional space for ice to grow in N hemisphere)

49 Lee et al (2017) methods: Model does not include ocean circulation, glacial dynamics, or carbon cycle

50 How much difference does it make? Increase in sea ice between S hemisphere summer perihelion and S hemisphere summer aphelion

51 Lee et al s hypothesis: 1) eccentricity (100 ka) modulates strength of precession (21 ka) on incoming solar radiation 2) high eccentricity amplifies the effect of summer aphelion on sea ice extent in southern hemisphere eccentricity sea ice maxima 21 ka 21 ka 21 ka 400, , , ,000 today

52 Two other attempts at explaining the Eccentricity Enigma Peter Clark: switch from 41 ka to 100 ka resulted from change at bed of ice sheets basal melting: the more there is, the faster the glacier slides

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54 Rate of glacier sliding (hence rate of mass transfer) depends on temperature and pressure at bed swisseduc.ch Canadian Rockies glacier movement = sliding at bed + internal deformation

55 Decaying radioactive isotopes in Earth generate heat flow = 1/10 W per m 2 on average over continental crust swisseduc.ch East Antarctic: heated from below and piling up

56 The thicker the ice, the greater its basal pressure, and the more heat it retains from geothermal heat flux swisseduc.ch regelation ice at base of Swiss glacier

57 Basal melt fraction Marshall and Clark (2002) Geophys Res Lett: Basal temperature evolution of North American ice sheets and implications for the 100-kyr cycle Ice sheet thickening

58 While we cannot make firm conclusions without a rigorous model of basal flow processes*, these results clearly suggest the potential for increased dynamical activity of the North American ice sheets in response to this thermal evolution.

59 100,000-yr period is determined by an autogenic, glaciological time keeper ice sheet evolves towards faster and faster sliding velocity: termination approaches! Marshall and Clark (2002) course of glaciation

60 And there are more Peter Huybers (2009) Pleistocene glacial variability as a chaotic response to obliquity forcing. Climates of the Past 5,

61 Lecture 12. the Eccentricity Enigma: Still unresolved Answer would help us understand how the Earth s climate system works