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

Caused by Three Factors 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 Net Accumulation Summer insolation minima Ice sheets grow on northern landmasses Climate Point (P) Where the equilibrium line intersects Earth’s surface

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 Positive Feedback With increased elevation more of the ice surface is above the equilibrium line

Delay in Ice Volume Response to Axial Tilt and Precession 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) 0 1000 2000 B 0 1000 2000 C 0 1000 2000

Full Cycle of Ice Growth and Decay (2) 0 1000 2000 E 0 1000 2000 F

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 long-term 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 Sea level would rise 6m Present Diagrams not to scale (Adapted from Ruddiman)

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