1 Last Glacial Maximum (~20K yrs ago) and afterwards What was climate like during LGM? What happened to end LGM? How has climate varied since LGM? What were the likely mechanisms of climate change?

2 Repeating Series of Glacial Conditions Punctuated with Interglacial Events

3 A Glacial Threshold on Earth

4 Solar Insolation Solar insolation during the LGM was about the same as today.

5 Sea Level Sea Level lower than today by ~125m –based on depth of submerged corals that lived ~20K yrs ( 14 C or U/Th dated) Large area of continental shelves (~7% of earth) are exposed Ocean is more saline (by ~1 ppt)  18 O of seawater enriched (higher) by ~1.1 ‰ –as a result of Ice Sheet growth Continental Ice Sheets were about about double the size of today.

6 Ice Sheet Extent Laurentide, Cordilleran and Scandinavian Ice Sheets Areal extent at LGM reconstructed by 14 C dated end moraines (25% of land covered at LGM vs 10% today). Thickness of ice sheets is harder to estimate than area.

7 Ice Sheet Volume

8 Sea Surface Temperature (today)

9 Ocean Temperatures at LGM CLIMAP (1970s)- use temperature sensitivity of foram distribution in today’s surface ocean, coupled with paleodistribution of forams at LGM measured in sediment cores to estimate sea surface temperature at LGM.

10 Sea Surface Temperature Change at LGM (CLIMAP)

11 Other Estimates of SST Change at LGM Alkenone content of pelagic Plankton  18 O of CaCO 3 pelagic forams Alkenones and  18 O indicate tropical SST decreased by ~2-4ºC.

12 Increased Global Aridity at LGM Ice Core Record of Dust - increased dust at LGM due either to increased strength of winds or dust source (aridity)

13 Aridity Soil cores indicate more extensive desert areas (sand dunes) during LGM Also increased deposition of wind blown dust (loess) during LGM

14 Aridity Exceptions Southwest US was wetter during LGM as indicated by paleo-lake shore elevations. Jet Stream position? (El Nino analog?)

15 Pollen Record in Lakes Pollen records from lake cores ( 14 C age dated) indicate larger extent of tundra and arctic steppe type vegetation during LGM. Can use to qualitatively estimate temperature and precipitation changes.

16 Vegetation Changes Use pollen records from several lakes to reconstruct regional vegetation distribution during LGM.

17 Atmospheric Gases during LGM CO 2 was ~200 ppm (vs 280 ppm at interglacials) CH 4 was ~350 ppb (vs 700 ppb at interglacials)

18 Deep Ocean Circulation:  13 C as a Proxy  13 C and phosphate depend on respiration rates and the age of deep water. Older deep water has lower  13 C and higher PO 4. Today deep water in Pacific is older than in Atlantic.

19  13 C of Ocean during LGM Measure the  13 C of CaCO 3 foram shells at several different locations buried during the LGM. Generally, the  13 C of the deep Atlantic was lower than today which implies water is older and/or respiration rates were higher. Deep ocean circulation rates were likely slower at LGM.

20 Summary: Climate Conditions during LGM Insolation rates about the same as today. Colder (~ -4 º C globally and ~ -10 ºC near the poles and ~-2 to -3 ºC in tropics). Ice Sheet volume was ~ twice today. Sea Level lower by ~ 125m. Drier and dustier (globally). Reduced atmospheric CO 2 and CH 4 levels Vegetation more arctic like (tundra, steppe). Deep Ocean circulation more sluggish.

21 Climate Change after the LGM What triggers the change? Was it a smooth transition from glacial to interglacial conditions? What mechanisms could have caused episodes of rapid climate change?

22 Solar Insolation Changes Summer insolation starts to increase at ~20K and reaches a maximum at ~10K.

23 A Glacial Threshold on Earth

24 Rise in Temperature and Atmospheric Gases Temperature, CO 2 and CH 4 start increasing ~18K yrs. Implies changes in radiation budget (Temp), ocean circulation /biology/chemistry (CO 2 ) and precipitation (CH 4 ).

25 Ice Sheet Retreat Retreat begins ~16K and ice sheet gone by ~6K. (Real age = 14 C age plus ~ 1700 yrs.)

26 Sea Level Rise Use 14 C and 230 Th/ 238 U to date the age of a sequence of submerged corals that lived close to the sea surface. The rate of sea level rise has pulses. ( 14 C ages are too young by up to ~3K yrs.)

27 Transition from Glacial to Interglacial Conditions Greenland Ice Core Rapid Bolling-Allerod warming event at ~14.5 Kyrs. Younger-Dryas is a period of cooling at ~12 Kyrs that lasted for ~ 1000 yrs. Transition from the LGM climate to present interglacial climate was not smooth.

28 Younger-Dryas Event: atmospheric, marine and continental proxies. Y-D climate signal is strongest in N. Atlantic region.

29 Rapid Temperature Change during the Y-D At end of Y-D, temperature in Greenland increased by 7ºC in 50 yrs. Sawtooth Pattern of Y-D

30 Role of Proglacial Lakes in Climate Change

31 Possible Pathways of Meltwater Flow

32 Appearance of Meltwater Pulses From sea level record From  18 O-CaCO 3 record Why do meltwater pulses show up in  18 O-CaCO 3 record?

33 Meltwater Pulses as a Climate Change Trigger Pulse of freshwater discharge into the N. Atlantic would reduce the formation rate of deep water (N. Atlantic Deep Water) by reducing salinity (density) of surface water. This would reduce the ocean’s transport rate of heat via Gulf Stream to N. Atlantic region and cause cooling in the region.

34 Deep Water Formation: Present vs LGM

35 Possible Impact of Reduced NADW Formation Rates on Air Temperatures

36 Millenial Scale Temperature Oscillations During last glacial period ( K yrs BP) there were lots of large and fast temperature swings recorded in the Greenland ice cores and deep sea sediments from N. Atlantic.

37 Heinrich Events Heinrich Events: Ice Rafted Debris

38 Dansgaard-Oeschger and Heinrich Events recorded in the N. Atlantic Region Fairly consistent ~1500 yr period between warm periods (D-O events). Several of the coldest periods are associated with Heinrich events (e.g., Younger-Dryas). Sawtooth pattern of fast warming and slow cooling.

39 Possible Mechanism: Salt Oscillator Hypothesis: Changes in the salinity of the N. Atlantic, resulting from ice melting or ice formation, determines the strength of NADW formation and, as a result, the rate of heat transport to the N. Atlantic region.

40 Salt Oscillator: Ocean circulation has two modes

41 Salt Oscillator: Explaining the time sequence of D-O and Heinrich events Hypothesis: The sawtooth pattern of temperature change is caused by a slow decrease in rate of NADW formation, until it eventually stops, which is then followed by rapid return of NADW formation after a critical value of salinity is reached.

42 Model Simulations Does S. Hemisphere warm when the N. Hemisphere cools?

43 Antarctica temperature increases during Y-D cooling. Globally, atmospheric CO 2 levels increase. Globally, atmospheric CH 4 levels decrease. Antarctic warmed during Younger Dryas

44 Interhemispheric Seesaw Model simulations of Heinrich events indicate that reduced heat transport into the N. Atlantic yields less heat loss from S. Hemisphere and thus warming. Models indicate that when NADW formation is reduced, then Antarctic Bottom Water formation rate increases, which in turn means higher ocean to atmosphere heat transfer and warmer temperatures in Antarctica. Temperature records during Y-D from Antarctic ice cores indicate warming while Greenland cooled.

45 Were D-O events and Younger-Dryas global? Figure 1. Map showing locations where abrupt climate changes (i.e., Dansgaard-Oeschger events) have been documented in records kept in marine sediments or polar ice (red and blue dots). Yellow dots show those locations where the last of these events (i.e., Younger Dryas) is recorded by major advances of mountain glaciers. While for most of the globe, these events are in phase, in parts of the Southern Ocean and of the Antarctic ice cap, they are clearly antiphased.

46 Rapid Climate change evidence in Santa Barbara Basin Warming events associated with negative  13 C, which author interprets is a result of methane hydrate release. Did ocean circulation rates change between warming and cooling events?

47 Summary: Rapid Climate Change During the last 100K yrs there have been repeated oscillation between warm (D-O) and cold (Heinrich) conditions, with very fast temperature changes (7ºC in 50 yrs in Greenland). The most recent (strong) cold event occurred about 12K yrs ago (Younger-Dryas) during the transition from LGM to current interglacial period. Current hypothesis is that variations in deep water formation rates in N. Atlantic driven by salinity, and thus poleward heat transport, is a likely mechanism. These rapid climate change events are strongest in the N. Atlantic, but some evidence that they occurred globally. Generally, there is an antiphasing of temperature fluctuations between N. and S. Hemispheres.