Milankovitch Theory of the Ice Ages

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1 Ruddiman CHAPTER 10 Insolation Control of Ice Sheets Milankovitch Theory of the Ice Ages margin of Greenland ice sheet

2 Today s main points: 1) Review of glaciology basics. 2) Orbital changes affecting summer insolation at high latitudes are the drivers of ice sheet buildup and decay. 3) Conceptual model of ice sheet development: integrating Milankovitch with glaciology 4) Lags between orbital forcing and ice-sheet responses a) Positive feedback of thickening ice sheet on its own growth b) Isostatic depression under ice sheets as a lagged feedback

3 Today s main points: Does Milankovitch explain the timing of the ice ages?

4 Some glacier basics

5 What controls ice sheet size?

6 Glacier mass balance: at steady state, accumulation = ablation Like a river flowing into a desert.

7 equilibrium line

8 Continental ice sheets bury pre-existing topography. Ice flow follows surface gradient of ice sheet. They expand until controlled by ablation in ocean (or in continental interiors)

9 Processes of accumulation ice accumulation must > ice ablation in order to grow an ice sheet

10 previous winter s snow

11 Another way ice sheets can grow: the weird world of ice shelves Pritchard et al., 2012, Nature.

12

13 Processes of ablation melt iceberg calving

14 calving margins add novel dynamics to glacier mass balances Matusevich Glacier, East Antarctica Columbia Glacier, Washburn photo.

15 What does a calving margin do to the mass balance and equilibrium line altitude? no calving ablation now add a calving terminus like this one

16 remember hysteresis: The lowly rubber band The reverse of the climate causing glacier advance may not cause glacier retreat.

17 How does climate control glaciers? (some generalities) Kalv Glacier, Cordillera Darwin, southern Chile, viewed from Bahia Pia. Photo credit: Brenda Hall. Ventisquero Garibaldi advancing into the forest, Cordillera Darwin, southern Chile.

18 Temperature and precipitation are ying and yang of glacier systems. Warmer summers (increased melt) can be compensated by snowier winters (more accumulation). Weissmies, near Saas Grund, Switzerland. Photo J. Alean. Austre Lovénbreen, NW Spitsbergen, Svalbard. Photo M. J. Hambrey.

19 different ways to have a positive mass balance

20 Theoretical rates of accumulation and ablation differ in their responses to mean annual temperature. Oerlemans (1991)

21 in the real world:

22 In general.

23 In general, summer temperature trumps winter preciptation (snowfall) in controlling ice sheet growth Why? Glaciers tend to grow when mean annual temp. is < -10 C

24 Why summer climate is more important than winter climate in controlling ice sheets: 1) Winters are always cold 2) Solar radiation never strong in winter 3) No matter how much snow falls in winter, it will melt if next summer is warm enough. Ice edge of the Brunt Ice Shelf near Halley Research Station Chris Gilbe

25 Milankovitch proposed that ice ages are triggered by minima in summer insolation near 65 N, which enables winter snowfall to persist all year and therefore accumulate to build NH ice sheets. Why not the southern hemisphere?

26 Ideal Milankovitch conditions for ice sheet growth: 1) Low summer insolation: tilt is low and poles are pointed less directly at the sun 2) Boreal summer solstice occurring when sun is farthest from the sun (at aphelion) 3) Along with #2, when orbit is most eccentric (Sun- Earth distance at maximum)

27 eccentricity obliquity precession of equinoxes ---- interactions ,000 41,000 23,000

28 Ideal Milankovitch conditions for ice sheet growth: 1) Low summer insolation: tilt is low and poles are pointed less directly at the sun OBLIQUITY: 41 ka 1) Northern summer solstice occurring when sun is farthest from the sun (at aphelion) PRECESSION OF EQUINOXES 23 ka 1) Along with #2, when orbit is most eccentric (Sun- Earth distance at maximum) ECCENTRICITY 100 ka

29 Integrating all the astronomical drivers.

30 How much difference do these orbital variations actually make in radiative forcing? Wikipedia rem: radiative forcing..the rate of energy change per unit area of the Earth as measured at the top of the atmosphere

31 Precession ca. 22,000 year period Strongest effects at low latitudes where it amounts to +- 12% (40 W/m 2 ) of long-term, mean insolation (see Ruddiman Chapter 8)

32 Obliquity not important at low latitudes is important at high latitudes change in insolation at high latitude over a 41,000-yr obliquity cycle is about 17 W/m 2 (Cronin, 2010).

33 For comparison, Total, net anthropogenic radiative forcing since AD 1750: W/m 2 (Forster et al., IPCC, 2007)

34 combined effect of all GHGs = W/m 2 (note the negative effects) IPCC 2007

35 But when comparing orbital forcing to GHGforcings, Remember: insolation changes caused by precession and obliquity occur seasonally and are localized at particular latitudes.

36 But when comparing orbital forcing to GHG-forcings, Remember: insolation changes caused by precession and obliquity occur seasonally and are localized at particular latitudes. In contrast, GHG forcing is perennial and global

37 radiative forcing from orbital changes is fundamentally different from GHG forcing in time and space So. expect quite different effects on climate systems

38 Today s main points: 1) Review of glaciology basics. 2) Orbital changes affecting summer insolation at high latitudes are the drivers of ice sheet buildup and decay. 3) Conceptual model of ice sheet development: integrating Milankovitch with glaciology 4) Lags between orbital forcing and ice-sheet responses a) Positive feedback of thickening ice sheet on its own growth b) Isostatic depression under ice sheets as a lagged feedback

39 How do changes in summer insolation trigger development of ice sheets? Today, glaciers cover 11% of Earth s land surface. During LGM, glaciers covered 25%. Oerlemans (1991)

40 Ruddiman s imaginary north south transect

41 summers too warm for previous winter snow to survive Ice sheet forms insolation minimum in summer shifts ELA south, and ice sheet starts to accumulate now cold enough for equilibrium line to move onto land

42 Ice accumulates only if ELA intersects land

43 As ice sheet thickens, it creates strong + feedback on its own continued growth

44 Positive feedback of thickening ice sheet: the paleo-surface of the glacier filling this Norwegian fjord probably intersected the ELA of that time.? ELA today? The Pulpit, Lysefjorden, Norway

45 Today s main points: 1) Review of glaciology basics. 2) Orbital changes affecting summer insolation at high latitudes are the drivers of ice sheet buildup and decay. 3) Conceptual model of ice sheet development: integrating Milankovitch with glaciology 4) Lags between orbital forcing and ice-sheet responses a) Positive feedback of thickening ice sheet on its own growth b) Isostatic depression under ice sheets as a lagged feedback

46 lags between forcing and response

47 Ice sheet growth and wastage will lag solar insolation. But by how much?

48 Part of lag arises from ice sheet s ability to grow itself Summers may get warmer, but ice sheet can continue to grow.

49 Laurentide ice sheet at the last glacial maximum, 20,000 years ago A. McIntyre, CLIMAP Project, Lamont-Doherty Earth Observatory

50 Part of lag arises from ice sheet s ability to grow itself Other causes of lag: 1) length of time required to melt a huge mass of ice 2) Ice sheet s effects on its own weather and climate 3) Isostatic depression under ice sheet

51 The isostatic lag : crustal depression under ice sheet auto-lowers ELA. slowly rem: 1 km of elev change = 6.5 d C temp change at isostatic equilibrium

52 Which is the density of granite? Of ice? 2.75 g/cm g/cm 3

53 Isostatic depression of Earth s crust Isostatic rebound

54 Raised beach ridges are ubiquitous in regions formerly occupied (or adjacent to) ice sheets

55 14,000 years after the LGM: land rebound in mm/yr Gulf of Bothnia west Loch Tarbert, Scotland

56 Isostatic sea level changes common in glaciated areas Muir Inlet Glacier Bay LIA ice limit Gustavus Gulf Of Alaska Chichagof Island

57

58 2-part isostatic adjustment: immediate (elastic) + viscous half-life of isostatic adjustment due to viscous response of mantle approximately 3000 years

59 How isostatic depression makes an ice sheet lag behind insolation forcing:

60 Isostatic depression keeps sea level higher, which enhances glacier ablation, even after climate becomes glacier-favorable again Result = lag behind climate forcing

61 calving margins!

62 Putting (most) of it together. LAG LAG LAG

63 lags between ice-sheet response and Milankovitch insolation changes are thought to be on the order of 6-10,000 yr Longer lag because of longer wavelength of forcing

64 Today s main points: 1) Review of glaciology basics. 2) Orbital changes affecting summer insolation at high latitudes are the drivers of ice sheet buildup and decay. 3) Conceptual model of ice sheet development: integrating Milankovitch with glaciology 4) Lags between orbital forcing and ice-sheet responses a) Positive feedback of thickening ice sheet on its own growth b) Isostatic depression under ice sheets as a lagged feedback

65 Does Milankovitch explain the timing of the ice ages?

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