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Chapter 13. Climate During and Since the Last Deglaciation. David Apostalon, Ines Cobeljic, Zak Mohamoud, Jeremy Shaw, Merhawi Zerai. Chapter Overview. History & Causes of Deglaciation Milankovitch Theory Climate records Deglacial two-step The Younger Dryas Deglacial feedbacks

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chapter 13

Chapter 13

Climate During and Since the Last Deglaciation

David Apostalon, Ines Cobeljic, Zak Mohamoud, Jeremy Shaw, Merhawi Zerai

chapter overview
Chapter Overview
  • History & Causes of Deglaciation
    • Milankovitch Theory
    • Climate records
    • Deglacial two-step
    • The Younger Dryas
    • Deglacial feedbacks
  • Effects of Deglaciation
    • Glacial lakes & floods
    • Sea level rise
    • Tropical monsoons
    • Changes in vegetation
    • Seasonal temperature variations
  • Current & Future Climate
    • Insolation predictions
    • Another glaciation?
    • Agricultural humans
history of deglaciation
History of Deglaciation
  • What is deglaciation?
    • 2.75 million years of Ice Age
    • Shifts between glacial and interglacial periods
      • ABLATION: rate of ice loss
history of deglaciation1
History of Deglaciation
  • Causes of ablation
    • Solar radiation
    • Conduction of heat by air

or rain

    • Calving
history of deglaciation2
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
history of deglaciation3
History of Deglaciation
  • What is insolation?
    • INSOLATION: incident incoming solar radiation(W/m²)

...or...

Arriving solar radiation per unit area

history of deglaciation4
History of Deglaciation
  • Insolation affects temperature
history of deglaciation5
History of Deglaciation
  • Insolation levels change over time
  • Determined by cycles of Earth’s precession, tilt, and eccentricity
history of deglaciation6
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
history of deglaciation7
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
history of deglaciation8
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

history of deglaciation9
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...?

history of deglaciation10
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!
slide14

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
slide15

History of Deglaciation

  • THE DEGLACIAL TIME PERIOD
  • North American ice sheet began to retreat 15,000 14C years ago
  • Reached a midpoint ~10,00014C years ago
  • Ended ~6,00014C 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
timing of ice sheet melting
TIMING OF ICE SHEET MELTING

History of Deglaciation

  • 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.
the deglacial rise in sea level
THE DEGLACIAL RISE IN SEA LEVEL

History of Deglaciation

  • 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
slide18

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.
slide19

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.
slide20

History of Deglaciation

  • Climate records of Younger Dryas event
    • All three graphs have correlating time

Younger Dryas

the younger dryas
The Younger Dryas

History of Deglaciation

  • 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
slide22

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
slide23

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
why the younger dryas oscillation
Why the Younger Dryas oscillation?

History of Deglaciation

  • 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
feedbacks
Feedbacks

History of Deglaciation

  • 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
slide26

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.

slide28

Effects of Deglaciation

A) Missoula flood deposits.

B) ripples in the land, too large to be seen on the ground.

increased insolation produced monsoons
Increased insolation produced Monsoons

Effects of Deglaciation

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

timing
Timing

Effects of Deglaciation

14C dates for lake deposits in N. Africa

Match the 10K insolation maximum

When corrected for greater 14C production

slide32

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

climate evidence
Climate evidence

Effects of Deglaciation

  • 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
shifts in vegetation
Shifts in Vegetation

Effects of Deglaciation

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

slide35

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
vegetation moisture feedback
Vegetation-moisture feedback

Effects of Deglaciation

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.

insolation reduced monsoons
insolation reduced monsoons

Effects of Deglaciation

decreased summer insolation expected to weaken summer monsoons

Lake levels in N. Africa match well expected patterns.

peak warmth
Peak Warmth

Effects of Deglaciation

  • 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.
cooling down
Cooling Down

Effects of Deglaciation

6,000 Years earth tilt and precession motion move Northern Hemisphere toward Aphelion position

5% decrease overall

Tundra move southward replacing forest

evidence for cooling summer insolation
Evidence for cooling summer Insolation

Effects of Deglaciation

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)

slide41

Current and Future Climate

  • 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
slide42

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:
  • http://www.youtube.com/watch?v=oRdyNn1tB-E&list=PLmen0eQI4-Lof_GS8Amc9sysdRMrRYzps