Chapter 13

Chapter 13
Climate During and Since the Last Deglaciation
David Apostalon, Ines Cobeljic, Zak Mohamoud, Jeremy Shaw, Merhawi Zerai
Chapter Overview
• History & Causes of
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
• Current & Future Climate
– Insolation predictions
– Another glaciation?
– Agricultural humans
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 Deglaciation
• Causes of ablation
– Solar radiation
– Conduction of heat by air
or rain
– Calving
History of Deglaciation
• Milankovitch Theory
– Proposed by Milutin Milanković
– Summer insolation controls glaciation
• Snow & ice accumulate during winter
• But...warm summers will melt ice
• 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 Deglaciation
• What is insolation?
– INSOLATION: incident incoming solar radiation (W/m²)
Arriving solar radiation per unit area
History of Deglaciation
• Insolation affects
History of Deglaciation
• Insolation levels change over time
• Determined by cycles of Earth’s precession, tilt, and
History of Deglaciation
• Summer insolation
– 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 Deglaciation
• Shifting climate forces
– Glaciers melt
– Insolation increases
– CO₂ levels increase
from 190 to 280
– CH₄ levels double
• Balance of power
– Melting of glaciers
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 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
2,000-3,500 year
History of Deglaciation
• Why the discrepancy?
– ¹⁴C ages are younger than Th/U
– Th/U ages are more accurate
when compared to tree ring data
• ¹⁴C ages are too young!
History of Deglaciation
• ¹⁴Carbon Dating
– ¹⁴C production in atmosphere
• Cosmic rays convert ¹⁴N into
radioactive ¹⁴C
– Weaker magnetic fields =
more ¹⁴C
– Based on half-life (decay) of
• more ¹⁴C = less time has
• But....extra ¹⁴C causes
appearance of less elapsed
History of Deglaciation
• 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
History of Deglaciation
• N. American ice sheets began
retreating 14,000 14C years ago
and was gone by 6,000 14C years
• 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.
History of Deglaciation
• Larger glaciers = more melt
– Generally consistent with
Milankovitch theory
• Rates of sea level rise changed
– Based on coral reef data
• Deglacial Two-Step
– Rapid rise from 20K to 14K years
– Slow from 14-12K years
– More rapid rise after 12K years
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
• The rates of ice melting were at
least four to five times faster
during the earlier and later
History of Deglaciation
• 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
• A low- d18O pulse found in cores from
the Gulf of Mexico indicates a shortterm increase in meltwater down the
Mississippi River from N. American ice
History of Deglaciation
• Climate records of Younger Dryas event
– All three graphs have correlating time
Younger Dryas
History of Deglaciation
The Younger Dryas
• Northern Hemisphere
unexpectedly returned to nearglacial conditions.
• Interruption in general deglacial
• An Artic plant called Dryas
reached Europe
• Reversal toward Artic vegetation
• Evidence comes from pollen
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
• moderate climate allowed trees to advance northward from prior full-glacial
spot in far-southern Europe
History of Deglaciation
• 13,000 years ago - 11,700
years ago (Younger Dryas
• 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
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
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
Effects of Deglaciation
Proglacial Lakes
proglacial lakes develop in bedrock
depressions left by melting ice
over time lakes move north behind the
ice sheets,while the land farther
south rebounds toward its
undepressed elevation.
Effects of Deglaciation
Lake Agassiz
Largest pro-glacial lake in
north America.
covered more than 200,000
km2, at depths of 100 m or
Effects of Deglaciation
A) Missoula flood deposits.
B) ripples in the land, too large to be
seen on the ground.
Effects of Deglaciation
flooding of
Large-scale change on
the earths surface by
deglacial rise of sea
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.
Kutzbach Orbital Monsoon
model simulations supported by geological observations.
Lake levels higher in:
- Arabia
-North Africa
- Southeastern Asia
Effects of Deglaciation
dates for lake deposits in N.
Match the 10K insolation
When corrected for greater 14C
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
in Arabian
years ago
Effects of Deglaciation
Climate evidence
• Evidence for wet climate range
– Large dry river valleys in deserts
– fossil evidence includes:
• Grass pollen in lake deposits
• Variety of water loving animals
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
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
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.
Effects of Deglaciation
insolation reduced monsoons
decreased summer
insolation expected
to weaken summer
Lake levels in N. Africa
match well expected
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
• displacement of high-albedo tundra by lowalbedo spruce caused positive feedback.
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
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 seaice 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)
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
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:

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