1. Climate During and Since the Last Deglaciation
Chapter 13
Climate During and Since the Last Deglaciation
David Apostalon, Ines Cobeljic, Zak Mohamoud, Jeremy Shaw, Merhawi Zerai

2. Chapter Overview History & Causes of Deglaciation
Effects of Deglaciation
Glacial lakes & floods
Milankovitch Theory
Sea level rise
Climate records
Tropical monsoons
Deglacial two-step
Changes in vegetation
The Younger Dryas
Seasonal temperature variations
Deglacial feedbacks
Current & Future Climate
Insolation predictions
Another glaciation?
Agricultural humans

3. History of Deglaciation
What is deglaciation?
2.75 million years of Ice Age
Shifts between glacial and interglacial periods
ABLATION: rate of ice loss

4. History of Deglaciation
Causes of ablation
Solar radiation
Conduction of heat by air
or rain
Calving

5. History of Deglaciation
Milankovitch Theory
Proposed by Milutin Milanković
Summer insolation controls glaciation
Snow & ice accumulate during winter
But...warm summers will melt ice
So...cool summers required to maintain ice each year
Glaciation occurs during low summer insolation at northern latitudes
Deglaciation occurs during high summer insolation at northern latitudes

6. History of Deglaciation
What is insolation?
INSOLATION: incident incoming solar radiation (W/m²)
...or...
Arriving solar radiation per unit area

7. History of Deglaciation
Insolation affects temperature

8. History of Deglaciation
Insolation levels change over time
Determined by cycles of Earth’s precession, tilt, and eccentricity

9. History of Deglaciation
Summer insolation maximum
Changes in Earth’s tilt and precession caused a summer insolatin maximum at northern latitudes ~10,000 years ago
Insolation rises... deglaciation begins

10. History of Deglaciation
Shifting climate forces
Glaciers melt
Insolation increases
CO₂ levels increase from 190 to 280 ppm
CH₄ levels double
Balance of power shifts
Melting of glaciers accelerates

11. History of Deglaciation
Coral reefs: climate records of deglaciation
We can measure the rate of deglaciation by indirect means using coral reef data
Coral reefs grow in shallow waters
As sea levels rise, corals die
We can date ancient corals to determine ancient sea levels
Sea level changes can be converted to ice volume
1 meter of sea level = 400,000 km³ of ice
Sea level rises, old coral dies

12. History of Deglaciation
Coral reefs: climate records of deglaciation
This data gives us a rate of deglaciation by measuring changes in ancient sea levels using radiocarbon and thorium/uranium dating.
2,000-3,500 year
discrepancy...?

13. History of Deglaciation
Why the discrepancy?
¹⁴C ages are younger than Th/U ages
Th/U ages are more accurate when compared to tree ring data
¹⁴C ages are too young!

14. History of Deglaciation
¹⁴Carbon Dating
¹⁴C production in atmosphere varies
Cosmic rays convert ¹⁴N into radioactive ¹⁴C
Weaker magnetic fields = more ¹⁴C
Based on half-life (decay) of ¹⁴C
more ¹⁴C = less time has elapsed
But....extra ¹⁴C causes appearance of less elapsed time

15. History of Deglaciation
THE DEGLACIAL TIME PERIOD
North American ice sheet began to retreat 15,000 14C years ago
Reached a midpoint ~10,000 14C years ago
Ended ~6,000 14C years ago.
Timeline is supported by Radiocarbon dating of material found in, under, or atop moraines deposited by the ice
Smaller Scandinavian ice sheet began retreating at the same time, but disappeared a few thousand years earlier

16. TIMING OF ICE SHEET MELTING
History of Deglaciation
TIMING OF ICE SHEET MELTING
N. American ice sheets began retreating 14,000 14C years ago and was gone by 6,000 14C years ago.
To convert area covered by retreating ice to ice volume (thickness x area = volume). The thickness of the ice is debatable because it can be effected by the conditions in its basal layer.

17. THE DEGLACIAL RISE IN SEA LEVEL
History of Deglaciation
THE DEGLACIAL RISE IN SEA LEVEL
Larger glaciers = more melt water
Generally consistent with Milankovitch theory
Rates of sea level rise changed dramatically
Based on coral reef data
Deglacial Two-Step
Rapid rise from 20K to 14K years ago
Slow from 14-12K years
More rapid rise after 12K years

18. History of Deglaciation
The Deglacial Two-Step pattern points to more complex accelerations and decelerations in melting rates.
Two major influxes of freshwater into oceans due to melting glaciers.
The rates of ice melting were at least four to five times faster during the earlier and later intervals.

19. History of Deglaciation
AN EARLY RAPID MELTING OF ICE SHEET!
A pulse of unusually negative d18O values early in deglaciation.
measured in planktic formanifera
The flux in d18O is result of early melting of nearby Barents ice sheet, north of Scandinavia.
A low- d18O pulse found in cores from the Gulf of Mexico indicates a short-term increase in meltwater down the Mississippi River from N. American ice sheet.

20. History of Deglaciation
Climate records of Younger Dryas event
All three graphs have correlating time
Younger Dryas

21. History of Deglaciation
The Younger Dryas
Northern Hemisphere unexpectedly returned to near-glacial conditions.
Interruption in general deglacial warming
An Artic plant called Dryas reached Europe
Reversal toward Artic vegetation
Evidence comes from pollen records

22. History of Deglaciation
Last Glacial Maximum
Polar water reached southward across the North Atlantic (45 N°)
southern margin defined by polar front, area of fast transition to more temperate waters to south
15,000 years ago (early deglaciation)
polar front shifted northwest to point close to eastern Canada
warm water began to flow northward along European coast to moderate climate
moderate climate allowed trees to advance northward from prior full-glacial spot in far-southern Europe

23. History of Deglaciation
13,000 years ago - 11,700 years ago (Younger Dryas period)
polar front suddenly shifts back to the south almost reaches glacial position
North Atlantic Ocean cools, and Artic vegetation (Dryas) return to northern Europe
11,700 years ago polar front quickly retreats north, and forest begin final push into north-central Europe

24. Why the Younger Dryas oscillation?
History of Deglaciation
Why the Younger Dryas oscillation?
Wally Broecker (geochemist)
lower N. Atlantic surface density prevented formation of deep water
criticized because the factor of global melting rate was slowing during Younger Dryas
The cause of the Younger Dryas remains a mystery

25. History of Deglaciation
Feedbacks
Positive feedbacks accelerated loss of ice
Between 17,000 and 14,000 years ago spikes in sea level rise caused by ice melting
Increases in concentrations of greenhouse gases
Ice sheets melted and CO₂ and methane levels increased at nearly the same time
Increases in these greenhouse gases caused warming and more melting of ice

26. Effects of Deglaciation
Proglacial Lakes
proglacial lakes develop in bedrock depressions left by melting ice sheets.
over time lakes move north behind the ice sheets,while the land farther south rebounds toward its undepressed elevation.

27. Effects of Deglaciation
Lake Agassiz
Largest pro-glacial lake in
north America.
covered more than 200,000
km2, at depths of 100 m or
more.

28. Effects of Deglaciation
A) Missoula flood deposits.
B) ripples in the land, too large to be seen on the ground.

29. Effects of Deglaciation
flooding of coastlines
Large-scale change on
the earths surface by deglacial rise of sea level.

30. Increased insolation produced Monsoons
Effects of Deglaciation
Increased insolation produced Monsoons
Stronger summer monsoons near 10,000 years ago due to earth's configuration.
summer insolation were 8% higher than today in N. Hemisphere
Kutzbach Orbital Monsoon
model simulations supported by geological observations.
Lake levels higher in:
- Arabia
-North Africa
- Southeastern Asia

31. Effects of Deglaciation
Timing
14C dates for lake deposits in N. Africa
Match the 10K insolation maximum
When corrected for greater 14C production

32. Effects of Deglaciation
Upwelling in the Arabian Sea
Strong Monsoon winds blowing across Somalia and eastern Arabia
Enhanced Coastal upwelling
altering the planktic foraminifera Species
Upwelling in Arabian Sea happened 9,000 years ago

33. Effects of Deglaciation
Climate evidence
Evidence for wet climate range from:
Large dry river valleys in deserts
fossil evidence includes:
Grass pollen in lake deposits
Variety of water loving animals

34. Effects of Deglaciation
Shifts in Vegetation
Strong summer insolation led to a northward shift from glaciers.
Oaks (warm vegetation) transition from southeast US to Mid Atlantic State
Spruce (Cold Vegetation) transition from Central US to Northeastern Canada
Models versus Observed data, Spruce and Oak

35. Effects of Deglaciation
No Analog Vegetation
No record of such vegetation in modern times
Caused by difference change in temperature and environmental variables
Specific spruce tree can’t be found at Midwest US
From Individual pollen record

36. Vegetation-moisture feedback
Effects of Deglaciation
Vegetation-moisture feedback
High summer insolation peaked at about 9,000 years ago. Wetter soils and increased vegetation provided positive feedback, bringing more moisture farther into the continent of Africa.

37. insolation reduced monsoons
Effects of Deglaciation
insolation reduced monsoons
decreased summer insolation expected to weaken summer monsoons
Lake levels in N. Africa match well expected patterns.

38. Effects of Deglaciation
Peak Warmth
with atmospheric CO2 levels steady and high, glacial ice largely melted.
summer insolation and vegetation changed
changes affected temperatures.
insolation 5 percent higher warmed higher latitudes.
displacement of high-albedo tundra by low-albedo spruce caused positive feedback.

39. Effects of Deglaciation
Cooling Down
6,000 Years earth tilt and precession motion move Northern Hemisphere toward Aphelion position
5% decrease overall
Tundra move southward replacing forest

40. Evidence for cooling summer Insolation
Effects of Deglaciation
Evidence for cooling summer Insolation
Ice cap record from ice cores shows summer melting before 5,000 years ago
Greenland diatoms from sea-ice were less before 5,000 years ago
Glacier margin reappear 3,500 despite glacial margin significantly melted before 3,500 years
Greatly affect diatom species that’s sensitive to temperature (great change in Norway’s southwest coast)

41. Current and Future Climate
Where will we be 10,000 years from now, climatically speaking? Let’s begin by looking exclusively at astronomical indications (via Milankovic Theory):
The future according to axial tilt
Dominant effect on higher latitudes
Diminishing tilt with time => reduced seasonality => reduced insolation at the higher latitudes
The future according to general precession
Dominant effect on low latitudes (and the global average)
Northern summer solstice will take place at perihelion 10,000 yrs from now; implies greater insolation at the northern tropics and stronger monsoons

42. Current and Future Climate
The current interglacial period: is global cooling long overdue?
All records of the past indicate that glaciation should be well underway by now
Future models indicate only further heating over the next 50,000 yrs
Agricultural humans: the turning point?
Our video presentation of this last leg –
Placing these projections in the context of changing axial tilt, precession, eccentricity, and so on! Please see: