Transcript Insolation Control of Ice Sheets

What Controls the
Size of Ice Sheets?
The Last Glacial Maximum
• 20,000 years ago
– Ice sheets surrounded much of the arctic ocean
– Ice covered North America, Europe, and Asia
• NY was completely covered by ice
Positive Glacial Budget
• Accumulation > Ablation
– Cooling trend over the last 55 My
– Summer melting < winter accumulation
• Ice can accumulate
– Annual mean temperature is 10o C (50o F)
• Rate of growth
– Typically 0.5 meters/year accumulates
Negative Glacial Budget
• Accumulation < Ablation
– Summer melting < Winter accumulation
• Summer temperatures above 0o C (32o F)
• Glaciers recede
– Much faster than growth rate
– Ablation can be as much as 3 meters/year
Temperature and Ice Mass Balance
• Temperature is the main
factor that determine
whether ice sheets are in
a regime of:
– Net ablation
• Negative mass balance
– Net accumulation
• Positive mass balance
• Ablation increases
sharply at higher
temperatures
Summer Insolation
• Primary Control an ice sheet’s:
– Size
– Extent
• Determines the rate of ablation
Ablation:
Caused by Three Factors
Amount of Insolation
Warm Air Masses or Rain
Calving of Icebergs
Milankovitch Theory N. Hemisphere Ice Growth
• Earth is aphelion in summer
• Tilt of axis is low
• Results in lower insolation
Milankovitch Theory N. Hemisphere Ice Decay
• Earth is perihelion in summer
• Tilt of axis is high
• Results in greater insolation
Milankovitch Theory
• High summer insolation
heats the land
– Results in greater ablation
• Low summer insolation
allows the land to cool
– Snow accumulates and
glaciers grow
Insolation Control of Ice Sheet Size
Values are
thickness of ice
gained (+) or
lost (-) in
meters
• The Equilibrium Line
– The boundary between areas of net accumulation and net ablation
– Dependent on latitude and elevation
• The climate point is where the equilibrium line intersects Earth’s
surface.
Insolation Changes Displace the
Equilibrium Line
• Net Ablation
– Maximum summer
insolation moves the
equilibrium north
– No ice sheet
Climate Point (P)
Where the equilibrium line intersects
Earth’s surface
• Net Accumulation
– Summer insolation minima
– Ice sheets grow on
northern landmasses
Ice Elevation Feedback
• As Ice Sheets
Increase in Elevation
– Prevailing
temperatures are
colder
• At 2 to 2 km
temperatures can be 12
to 19 C cooler than at
sea level
– Increases
accumulation
• Ice mass balance is
more positive
With increased elevation more of the
ice surface is above the equilibrium line
– Positive Feedback
Phase Lag
Delay in Ice Volume Response to
• Summer Insolation
• Axial Tilt and Precession
Ice Volume Lags Insolation:
The Bunsen Burner Analogy
• Same lag between heating
and cooling of water as
with the variation Bunsen
burner’s flame
• Lag between summer
insolation
– Much longer time scale
• Thousands of year
– Maximum size of ice sheet is
not reached until
• Insolation is just reaching
values that will cause the
next ablation
Ice Volume Lags Tilt and
Precession
Bedrock Response to the
Weight of the Ice Sheet
Isostacy
• Balance or equilibrium of adjacent rocks of
brittle crust that float on the plastic mantle.
Wood blocks float in
water with most of
their mass submerged
Crustal blocks “float” on
mantle in a similar way.
• The thicker the block
the deeper it extends
into the mantle.
Isostatic Adjustment
•
•
•
Areas that lose mass rise.
Areas that gain mass sink.
Isostatic Adjustment
– Vertical movement to reach
equilibrium:
•
Depth of Equal Pressure
– Depth where each column of rock
is in balance with others.
Huge Mass of Ice
in a Glacial Ice Sheet
• Even though the density of ice is lower
than the underlying bedrock
– Ice: A little less than 1 g/cm3
– Continental bedrock: Averages 3.3 g/cm3
• The huge thickness of glacial ice of 3,000
meters or more:
– Equivalent to the weight of 1,000 m of solid
rock
– This load can cause underling bedrock to be
depressed
Bedrock Sinking
• A 3.3 km thick ice
sheet
– Eventually would
reach equilibrium by
depressing the
bedrock 1.0 km.
– This would lower the
ice sheet’s surface
elevation 1.0 km
• Resulting 6.5o C
change in temperature
• Large effects on mass
balance of the ice
sheet.
Bedrock Sinking
• Two phases of
response to heavy ice
load
– Elastic Response
• Immediate sinking
action
• 30% of total response
– Viscous Response
• Slower adjustment due
to slow flow of rock in
the plastic
asthenosphere of the
upper mantle
• 70% of total response
Bedrock Feedback to Ice Growth
• Positive Feedback
– Delayed sinking due to elastic response results in the ice
remaining at higher elevations for a longer time.
– Cooler temperatures promote ice growth.
Crustal Rebound
•
•
Upward movements
of the crust
Loss of huge mass
of ice (glaciers) at
the end of the
Pleistocene Epoch
Crustal Rebound in Canada and the
northern United States
Red contours show
amount of uplift in
meters since the ice
disappeared 7,000
years ago.
Bedrock Feedback to Melting
• Negative Feedback
– Quick elastic rebound is followed by a much slower viscous rebound.
– The ice sheet remains a lower, warmer elevation for a longer time.
– Results in faster melting of the ice sheet
Full Cycle of Ice Growth and Decay (1)
A
0
1000
2000
0
1000
2000
0
1000
2000
B
C
Full Cycle of Ice Growth and Decay (2)
D
0
1000
2000
E
0
F
1000
2000
Evolution of Ice Sheets
• Long-term evolution of ice sheets results from the interaction of:
– Slow global cooling over the last 3 Myr
• Slowly changing equilibrium line threshold
– More rapidly changing curve of summer insolation
• Ice sheets grow when summer insolation falls below a critical
threshold
Four Intervals in the
Development of Northern
Hemisphere Glaciation
The Preglaciation Phase
• No ice can accumulate
– The Equilibrium-line threshold is near the conditions necessary
for glaciation to develop.
– Even the deepest summer insolation fails to reach critical
threshold
– High latitudes remain too warm for ice sheets to form.
The Small Glaciation Phase
• Global cooling allows the equilibrium-line threshold to
interact with summer insolation
– Insolation minima at 41,000 year cycle last about twice as long
as those at the 23,000 year
– Ice sheets have more time to grow at the tilt cycle.
– Ice accumulates during individual summer insolation minima but
melts entirely during the next insolation maximum
The Large Glaciation Phase
• Eventually some of the weaker insolation maxima remain in the
regime of ice accumulation
• Ice sheets don’t disappear and last until a stronger insolation
maximum occurs.
– They last longer than the 23,000 year and 41,000 year cycles of
insolation
The Permanent Glaciation Phase
• The equilibrium line is completely above the range of the summer
insolation curve.
• All points on the insolation curve are in the regime of positive ice
mass.
• Even strongest insolation maxima fail to reach ablation.
• Permanent ice sheets remain on the continents.
– Ice sheets never disappear
Best Records of Glaciation
• From the ocean
– Deposition of sediments is generally
uninterrupted
– Two key indicators of past glaciation
Oceanic Indicator 1
δ18O Records
Positive δ18O Records From Shells
• Foraminifera shells
• 2.74 Myr glacial history of N. Hemisphere
– Numerous cyclic oscillations from positive to negative
values
– Gradual shift towards positive values
– Positive values indicate colder ocean temperatures
and likely more ice on land
Before 2.75 Myr Ago
• δ 18O values were relatively negative (less
than 3.5 o/oo)
• Either
– Ice sheets didn’t exist or
– They didn’t attain the size needed for icebergs
to reach the central North Atlantic
• Preglacial phase for the northern
Hemisphere
Oceanic Indicator 2
Ice Rafted Debris
Ice-Rafted Debris
• Mixture of coarse and fine sediments
• Delivered to the ocean by melting icebergs
– Calve off from margins of ice sheets
Beginning 2.75 Myr Ago
• Significant amount of ice-rafted debris appear in the
record
• Accumulates during intervals of positive δ18O values
• Suggests that ice sheets were forming as some snow
and ice survived during intervals of low summer
insolation
Evidence of Ice Sheet Evolution: δ 18O
• North Atlantic Sediment
Core containing 3 Myr
record of
– Ice volume
– Deep water temperature
• Diagonal white line
– Shows a gradual longterm δ18O trend toward
colder temperature and
more ice
Evidence of Ice Sheet Evolution: δ 18O
• No major ice sheets
before 2.75 Myr ago
• Until 0.9 Myr ago
– Small ice sheets grew and
melted at 41,000 yr and
23,000 yr cycles
• After a transition period
– Large ice sheets grew and
melted at a 100,000 yr
cycle
Coral Reefs and Sea Level
• Coral reefs grow near sea level
• Acropora palmata
– Species most useful to climate scientists
– Grow only at sea level or a few meters below
Coral Reefs Follow Changes in
Sea Level
• Coral reefs migrate upslope and downslope as
sea level rises and falls
– Ancient corals can be considered “dipsticks” that
measure past sea level.
• Fluctuations in sea level
– Result from changes in the amount of water extracted
from the ocean and stored in ice sheets on land
• Sea level history recorded by coral reefs is a
direct record of ice volume
Fossil Reefs are
Radiometrically Dated
• The absolute age of the fossil coral must
be determined to compare with δ18O
• Small amounts of 234U which decays to
230Th is incorporated into the coral’s
skeleton.
– Best suited for dating rocks only several
hundred years old.
Bermuda
• Stable island (no uplift)
• Fossil coral reefs dated to
about 125,000 years ago
• Are about 6 meters above
sea level
Supports the Use of δ18O as an
Indicator of Ice Volume
• Bermuda’s Limestone
reefs are near S.L.
• Age indicates high
sea level
– As little ice as today,
perhaps less
– Correlates with low
δ18O within the last
150,000 years
• If all present-day ice on
Greenland or 10% of
Antarctic ice melted
Present
Diagrams not to scale
(Adapted from Ruddiman)
– Sea level would rise 6m
Problems using coral reefs . . .
• No other coral reefs younger than 150,000
years are exposed on tectonically stable
islands for comparison with δ18O.
• More ice existed at all other times during
the last 150,000 years
– Other coral reefs that formed during this time
are below modern-day sea level.
Tectonically Unstable Islands
• Gradual uplift of coral
reefs
– As time passes, uplift
steadily raised the island
and the fossil reef to higher
elevations.
• Sea level moves up and down
against the island due to
changes in ice volume
– Old fossil reefs may have
been uplifted well above sea
level.
Two Well-Studied Islands
Barbados
New Guinea
Terraces formed by erosion-resistant
coral reefs lie well above sea level
Reconstructing Sea Level at the
Time of Reef Formation
• Effects of uplift must be factored out
– Assume constant rate of uplift over the
interval of time studied
– Two reefs on New Guinea
• 82,000 years old
• 104,000 years old
– Formed when sea level was 15 to 20 meters
below its modern position
• Significant ice on land during these intervals
Reconstructing Sea Level From
Ancient Reefs