What Controls the What Controls the Size of Ice Sheets?Size of Ice Sheets?

The Last Glacial MaximumThe 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 BudgetPositive 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 BudgetNegative 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 BalanceTemperature 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 InsolationSummer Insolation

• Primary Control an ice sheet’s:– Size – Extent

• Determines the rate of ablation

Ablation:Ablation:

Caused by Three FactorsCaused by Three Factors

Amount of InsolationAmount of Insolation

Warm Air Masses or RainWarm Air Masses or Rain

Calving of IcebergsCalving of Icebergs

Milankovitch Theory -Milankovitch Theory -N. Hemisphere Ice GrowthN. Hemisphere Ice Growth

• Earth is aphelion in summer• Tilt of axis is low• Results in lower insolation

Milankovitch Theory -Milankovitch Theory -N. Hemisphere Ice DecayN. Hemisphere Ice Decay

• Earth is perihelion in summer• Tilt of axis is high• Results in greater insolation

Milankovitch TheoryMilankovitch 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 SizeInsolation Control of Ice Sheet Size

• 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.

Values are thickness of ice

gained (+) or lost (-) in meters

Insolation Changes Displace the Insolation Changes Displace the Equilibrium LineEquilibrium Line

• Net Ablation– Maximum summer

insolation moves the equilibrium north

– No ice sheet

• Net Accumulation– Summer insolation minima– Ice sheets grow on

northern landmasses

Climate Point (P) Where the equilibrium line intersects Earth’s surface

Ice Elevation FeedbackIce 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

– Positive FeedbackWith increased elevation more of theice surface is above the equilibrium line

Phase LagPhase LagDelay in Ice Volume Response toDelay in Ice Volume Response to

• Summer InsolationSummer Insolation

• Axial Tilt and PrecessionAxial Tilt and Precession

Ice Volume Lags Insolation:Ice Volume Lags Insolation:The Bunsen Burner AnalogyThe 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 Ice Volume Lags Tilt and PrecessionPrecession

Bedrock Response to the Bedrock Response to the Weight of the Ice SheetWeight of the Ice Sheet

IsostacyIsostacy• Balance or equilibrium of adjacent rocks of

brittle crust that float on the plastic mantle.

Wood blocks float inwater with most of their mass submerged

Crustal blocks “float” onmantle in a similar way.• The thicker the block the deeper it extends into the mantle.

Isostatic AdjustmentIsostatic 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 Huge Mass of Ice in a Glacial Ice Sheetin 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 SinkingBedrock 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 SinkingBedrock 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 GrowthBedrock 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 ReboundCrustal 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 showamount of uplift in meters since the icedisappeared 7,000years ago.

Bedrock Feedback to MeltingBedrock 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)Full Cycle of Ice Growth and Decay (1)

A

B

C

0 1000 2000

0 1000 2000

0 1000 2000

Full Cycle of Ice Growth and Decay (2)Full Cycle of Ice Growth and Decay (2)

D

E

F

0 1000 2000

0 1000 2000

Evolution of Ice SheetsEvolution 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 Four Intervals in the Development of Northern Development of Northern

Hemisphere GlaciationHemisphere Glaciation

The Preglaciation PhaseThe 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 PhaseThe 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 PhaseThe 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 PhaseThe 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 GlaciationBest Records of Glaciation

• From the ocean– Deposition of sediments is generally

uninterrupted– Two key indicators of past glaciation

Oceanic Indicator 1Oceanic Indicator 1

δδ1818O RecordsO Records

Positive δPositive δ1818O Records From ShellsO 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 AgoBefore 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 2Oceanic Indicator 2

Ice Rafted DebrisIce Rafted Debris

Ice-Rafted DebrisIce-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 AgoBeginning 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 Evidence of Ice Sheet Evolution: δ 1818OO

• North Atlantic Sediment Core containing 3 Myr record of– Ice volume– Deep water temperature

• Diagonal white line– Shows a gradual long-

term δ18O trend toward colder temperature and more ice

Evidence of Ice Sheet Evolution: δ Evidence of Ice Sheet Evolution: δ 1818OO

• 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 LevelCoral 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 Coral Reefs Follow Changes in Sea LevelSea 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 areFossil Reefs are Radiometrically Dated 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.

BermudaBermuda• 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 δSupports the Use of δ1818O as an O as an Indicator of Ice VolumeIndicator 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– Sea level would rise 6m

Diagrams not to scale(Adapted from Ruddiman)

Present

Problems using coral reefs . . .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 IslandsTectonically 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 IslandsTwo Well-Studied Islands

BarbadosBarbados

New GuineaNew Guinea

Terraces formed by erosion-resistantcoral reefs lie well above sea level

Reconstructing Sea Level at the Reconstructing Sea Level at the Time of Reef FormationTime 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 Reconstructing Sea Level From Ancient ReefsAncient Reefs