Ruddiman CHAPTER 13. Earth during the LGM ca. 20 ka BP

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1 Ruddiman CHAPTER 13 Earth during the LGM ca. 20 ka BP

2 The Last Glacial Maximum When? How much more ice than today? How much colder was it than today (global average)?

3 How much lower were snowlines? Did the atmospheric circulation system change in fundamental ways? Did the ocean circulation system change in fundamental ways?

4 TODAY: oceans and background to millennial-scale climate changes (remember: we are interested in non-milankovitch climate drivers) 1) Ice-age changes in ocean temperatures 2) Oceanic trigger points in global climate change 3) Western boundary currents, thermohaline circulation 4) Atlantic Meridional Overturning Circulation (AMOC) 5) Why the Atlantic Ocean is key in global climate changes

5 Millennial-Scale Climate Changes 6) The Greenland Ice Sheet as a climate archive 7) Dansgaard-Oeschger Events: quick, radical, and mysterious

6 CLIMAP: the mean annual temperature of the LGM world was 4-5 degrees C colder than today CLIMAP Project Members, 1994, CLIMAP 18K Database. IGBP PAGES/ World Data Center-A for Paleoclimatology Data Contribution Series # NOAA/NGDC Paleoclimatology Program, Boulder CO, USA.

7 CLIMAP reconstruction of SSTs at the LGM

8 warmer than today much colder warmer than today

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10 but tropical ELAs fell 900 m (approx. 4.7 degrees C cooling)

11 Proxy-temperature data from deep sea and alpine glaciers do not agree in tropics.

12 Proxy-temperature data from deep sea and alpine glaciers do not agree in tropics. Glaciers say it was about 5 degrees C cooler in mountains, but SST proxies say some places actually warmer than today (S.C. Porter (2001) Quat Sci Rev)

13 non-linearity..thresholds..key regional responses to forcing that can trigger global responses Where would you pick to be a trigger point in the oceanatmosphere system?

14 warmer than today much colder warmer than today

15 western boundary current.wind-driven Gulf Stream

16 North Atlantic Drift / Norway Current.thermohaline currents

17 Gulf Stream & its northern extension warm NW Eurasia

18 During LGM, currents shunted east to Portugal Large numbers of icebergs melted off French coast (CLIMAP 1988)

19 The Gulf Stream s Gate During ice ages, cold, freshwater episodically covered N Atlantic triangles are ice bergs circles are locations of core records TODAY episodically, 40,000 to 14,000 yr BP Zahn, R., J. Schonfeld, H.-R. Kudrass, M.-H. Park, H. Erlenkeuser, and P. Grootes, 1997: Thermohaline instability in the North Atlantic during meltwater events: Stable isotope and ice-rafted detritus records from core S075-26KL, Portugese margin. Paleoceanography, 12,

20 North Atlantic transports more heat than North Pacific

21 Atlantic Meridional Overturning Circulation (AMOC) = thermohaline circulation in N Atlantic Wikipedia

22 evaporation NORWAY

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25 Atlantic Meridional Overturning Circulation.an example of a thermohaline current system Gulf Stream: wind-driven N Atlantic Drift: thermohaline-driven

26 Where is the other region of deep-water formation in the Atlantic? hint

27 Antarctic bottom water: dense because it is cold and because..

28 it is salty.why? brinicle Brine rejection during the formation of sea ice

29 2 sources of deep water formation that trade-off dominance

30 Why is so much heat transported poleward in the N Atlantic? note difference!

31 1) N Atlantic gets closer to pole than the N Pacific does

32 Why is getting further poleward going to cause a positive feedback on poleward heat transport?

33 2) net transfer of water vapor from the Atlantic to the Pacific over the Isthmus of Panama, equivalent to approximately 0.35 Sverdrup (10 6 m 3 per second). Broecker, W. S. et al. The magnitude of global fresh-water transports of importance to ocean circulation. Climate Dynamics 4, (1990a). It rains a lot in Panama.

34 salinity of the North Pacific lower because of relatively low evaporation, little exchange with salty tropical waters, and an influx of fresh water from precipitation and river runoff Deep Atlantic Circulation During the Last Glacial Maximum and Deglaciation By: Delia W. Oppo (Department of Geology and Geophysics, Woods Hole Oceanographic Institution) & William B. Curry (Department of Geology and Geophysics, Woods Hole Oceanographic Institution) 2012 Nature Education

35 So. Atlantic is set up to have a strong, high-latitude current that is sensitive to changes in salinity and temperature.

36 Why is the Atlantic Ocean so important in regard to climate change caused by shifts in ocean currents? And it is only ocean with transformation of surface-todeep water near both poles.

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38 Bottom line: If you change the circulation in the N Atlantic, you change the global conveyor belt

39 Was the Atlantic Meridional Overturning Circulation the trigger point for rapid climate changes during the ice ages?

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41 Millennial Oscillations and Climate Our understanding of this topic profited greatly from Cold War interest in Geeenland.

42 Camp Century US Army an englacial habitat in northern Greenland with portable nuclear power station Try turning that knob, sarge.

43 The first Greenland ice cores from Camp Century revealed short-lived fluctuations in del Although the time scale was uncertain back then, these fluctuations appeared to be ca years in duration.

44 Dansgaard-Oeschger events (D-O Oscillations) Willi Dansgaard, Danish glaciologist Hans Oeschger, Swiss physicist

45 Could all these short-lived fluctuations be some sort of Lots of different ice cores mistake? Wolff et al (2010) QSR very unlikely at this point: all the Greenland ice cores show them

46 4 D-O Events are a hallmark of ice age climate NGRIP del 18O using GICC05 time scale HOLOCENE 1 YD LGM composite Greenland methane on GICC05 time scale CH (ppbv) YD 1 LGM Calibrated years before present x 1000.

47 Much of this discussion based on review by:

48 timing of D-O Oscillations: every years the Bolling-Allerod GI = Greenland interstadial = D-O event

49 structure of a D-O event: sudden onset of warmth then slow drift back to colder conditions del 18O Greenland Interstadial 8 TEMP

50 amplitudes of D-O Events: 8 to 16 degrees C change in mean annual temperature Warming estimated using the changes in the stable isotopes of enclosed air to range from 9C up to 16C (comparable to glacial-interglacial changes)

51 D-O Events: changes of 8 to 16 degrees C

52 Rate of temperature changes during D-O Events? Rapid warming at onset within several decades Slow cooling over centuries

53 How far back in time did D-O events occur? HOLOCENE ice record smeared out here Last interglacial

54 .to at least 1 mya * geology/dsdp/area/areas.htm Temporal resolution poor, answer = probably

55 What caused D-O Events? Their effects were centered in North Atlantic region

56 but their effects show up throughout northern hemisphere Genevieve Woillard ( ) Grande Pile (peat bog)

57 Santa Barbara Basin

58 Greenland Peruvian cave 3800 m in Peruvian Andes SCIENCE 2012

59 Dansgaard-Oeschger events occurred throughout N hemisphere and into tropics

60 What was happening in the S Hemisphere when D-O Oscillations were happening in N hemisphere?

61 TODAY: background for exploring millennial-scale climate changes (remember: we are interested in non-milankovitch climate drivers) 1) Ice-age changes in ocean temperatures 2) Oceanic trigger points in global climate change 3) Western boundary currents, thermohaline circulation 4) Atlantic Meridional Overturning Circulation (AMOC) 5) Why the Atlantic Ocean may key in global climate changes

62 Millennial-Scale Climate Change 6) The Greenland Ice Sheet as a climate archive 7) Dansgaard-Oeschger Events: quick, radical, and mysterious