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 (14C or U/Th dated)• Large area of continental shelves (~7% of earth)

are exposed• Ocean is more saline (by ~1 ppt)• 18O 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 14C 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

18O of CaCO3

pelagic forams

Alkenones and 18O 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)

10000 12000 14000 16000 18000 20000Age (y r BP )

-440

-420

-400

-380

D (

‰)

180

200

220

240

260

CO

2 (p

pm

)

Ta

ylo

r D

om

e

1

10

2

5

20

50n

ss

-Ca

2+ f

lux

(ng

/cm

2 /y

r)

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 (14C 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

CO2 was ~200 ppm (vs 280 ppm at interglacials)

CH4 was ~350 ppb (vs 700 ppb at interglacials)

18

Deep Ocean Circulation: 13C as a Proxy

13C and phosphate depend on respiration rates and the age of deep water. Older deep water has lower 13C and higher PO4. Today deep water in Pacific is older than in Atlantic.

19

13C of Ocean during LGM

• Measure the 13C of CaCO3 foram shells at several different locations buried during the LGM.

• Generally, the 13C 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 CO2 and CH4 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, CO2 and CH4 start increasing ~18K yrs.

• Implies changes in radiation budget (Temp), ocean circulation /biology/chemistry (CO2) and precipitation (CH4).

25

Ice Sheet Retreat

• Retreat begins ~16K and ice sheet gone by ~6K.

• (Real age = 14C age plus ~ 1700 yrs.)

26

Sea Level Rise

• Use 14C and 230Th/238U 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.

(14C 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 PulsesFrom sea level record From 18O-CaCO3 record

Why do meltwater pulses show up in 18O-CaCO3 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 (20-100 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 EventsHeinrich 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 CO2 levels increase.

• Globally, atmospheric CH4 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 13C, 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.