Climate Records from Ice Cores

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Climate Records from Ice Cores

Major Points

• Ice cores have provided the best record of climate change over the last 700K years.

• The most important climate characteristics recovered from ice cores are air temperature, atmospheric CO2 and CH4 concentrations and dust.

• One key unanswered question involves the cause of the atmospheric CO2 shifts between glacial and interglacial periods.

• Another key question, still not completely answered, is the sequence of events that occur that cause the earth to shift from glacial to interglacial periods.

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Ice Core Drilling Depths

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Dome C,Antarctica

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Tools of the Trade

L

L

L

L

L

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Ice Core Drill

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Ice Core Recovery

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Ice Cores from Greenland

Firn Ice

Compact Ice

Bedrock

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Antarctica Drilling Sites

0km 1 ,000km 2,0 00 km

80S°

70S°

60S°

Vo sto k

Do m e F

Ta ylo rDo m e

Byrd

Dro nningM a ud La nd

Sip le Do m eDo m e C

La w Do m e

Be rkne rIsla nd

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Ice Cores and Ice Sheet Flow

Age of Ice: annual layers (Greenland)- accurate but limited in age

ice flow models (Antarctica)- less accurate but extends age range

annual

“Dome”

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Air Temperature Proxies

• Two Methods– First, based on measured 18O/16O or D/H of ice

recovered from ice core.– Second, based on measured depth profile of

temperature of ice (borehole temperature)– In Greenland, the two methods do not agree and

the borehole method is considered more accurate and used to calibrate the 18O/16O method.

– In Antarctica, the 18O/16O method apparently works well.

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Effect of Condensation on the 18O (and D) of Precipitation

-the amount of water vapor contained in air depends on air temperature

-as air cools, water vapor (enriched in H2O18) condenses

-as condensation occurs, the remaining vapor becomes depleted in H2O18 [vs SMOW]

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18O of Today’s Precipitation vs

Air Temperature

18O

(

‰ vs

SMO

W)

-strong linear correlation between 18O (and D) of precip and air temp

ΔTemp/Δ18O =

~1.4ºC / 1‰

ΔTemp/ΔD =

~0.2ºC / 1‰ Air Temp (ºC)

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Borehole Temperature Record in Ice Cores

Scientists measure the temperature of ice directly by lowering a thermometer into the borehole that was drilled to retrieve the ice core. Like an insulated thermos, snow and ice preserve the temperature of each successive layer of snow, which

reflects general atmospheric temperatures when the layer accumulated (although diffusion of heat alters the depth profile, but in a predictable way).

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18O versus Borehole Paleothermometrya controversy in Greenland Ice Cores

emp/18O= 1.5 ºC / ‰) using current precipitation

emp/18O= 3 ºC / ‰using Borehole Temps

Climate scientists favor the borehole temperature changes.

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Greenland Drilling Sites

(most notable are the GISP and

GRIP sites started in 1990s)

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Greenland Ice Core 18O and Temperature Record

Using borehole temperature vs 18O calibration

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Temperature Swings between Glacial and Interglacial Conditions

ΔTemp/Δ18O about equal for borehole and precipitation in Antarctica

ΔTa

36

24

21

a Bore Hole

calibrn

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Reconstructing Atmospheric Gas Concentrations from Ice Cores

• Use trapped air bubbles as preserved samples of atmosphere.

• Measure the concentration of important (greenhouse) atmospheric gases on the trapped air bubbles (e.g., CO2, CH4, N2O)

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Trapping Air Bubbles in Ice

Snow Accumulation Rates

Greenland = 0.5 m/yr

Antarctica = 0.05 m/yr

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How does age of air bubbles compare to age of ice?

• Determine the age of the ice (annual layer or flow model).

• Determine the age of the trapped air bubble.

-bubble age doesn’t equal ice age, it’s younger.

• How long does it take for the ice to seal?

- ~50 meters divided by snow accumulation rate

- 50m / 0.5 m/yr = ~100 yrs in Greenland

- 50m / 0.05 m/yr = ~1000 yrs in Antarctica

• Why is this lag between ice and bubble ages important?

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Industrial Era Changes in

Atmospheric CH4 and CO2

Extending the record of industrial era change

Tests the accuracy of ice core gas measurements

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Methane (CH4) Gas in the Atmosphere

• A greenhouse gas and climate indicator.

• Natural (pre-anthropogenic) CH4 sources are dominated by emissions from wetlands (swamps, tundra, bogs, etc.).

• Biogenic methane is produced by microbes under anoxic (no oxygen) conditions.

CO2 + H2 CH4 + H2O

CH2COOH CH4 + CO2

• Methane Hydrates (?)

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Atmospheric Methane

• The primary sink for atmospheric CH4 is reaction with OH radicals in the atmosphere.

CH4 + OH• CO2 + H2O

• Currently, CH4 has a ~10 year lifetime () in atmosphere.

• Methane is a reactive gas in the atmosphere, in contrast to CO2 which is a non-reactive gas.

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Methane as Climate Indicator

• Source strength depends on extent of wet soil conditions (opposite of aridity)

• Extent of wet soils controlled primarily by precipitation rates and patterns (climate).

• In cold (tundra) regions, temperature likely has major role on CH4 emission strength.

• The ocean has small role in the CH4 cycle (in contrast to CO2).

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Atmospheric Methane from Antarctic Ice Cores

CH4 concentration doubles between glacial and

interglacial conditions

CH4 changes correlate strongly with temperature

changes

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Methane as Climate Indicator

• Higher methane levels during interglacial times, suggests that the earth was generally wetter (higher precipitation) than during glacial times, which increased the spatial extent of flooded soils and, in turn, the biogenic production rate of methane and its concentration in the atmosphere.

• Not clear whether this increase in precipitation was global or regionally specific (e.g., role of monsoons?). Where did increased methane production occur (tropics, temperate, polar latitudes)?

• Methane concentration in atmosphere contributes to overall greenhouse gas effect.

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Atmospheric Carbon Dioxide (CO2)

• Dominant greenhouse gas that has played a key role in changing the earth’s climate in the past (e.g., Snowball Earth, Cretaceous Hothouse).

• What can we learn about the role of atmospheric CO2 as a climate factor from the oscillations in CO2 that occurred over the last 700K years?

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CO2 gas concentration in the atmospheric

Atmospheric CO2 levels increase by

40% between glacial (~200 ppm)

and interglacial (~280 ppm) times.

Strong correlation between CO2 and

temperature changes in ice cores.

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Atmospheric CO2

and Ice Volume Records

- Atmospheric CO2 record from ice cores

- Ice volume record from 18O of marine CaCO3

sediments

- What is implication of strong correlation?

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What causes the Glacial-Interglacial shifts in atmospheric CO2?

• Involves a change in the earth’s carbon cycle.

• Very likely that this change involved the ocean.

• Recent evidence points the finger at changes in circulation of the Southern Ocean.

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Carbon Reservoir Changes and Exchange RatesChanges in Reservoir Sizes (Pg, %)between Interglacial and Glacial Carbon Exchange Rates (Pg/yr)

Deep Ocean accumulates the carbon lost from the atmosphere and land biota during glacial times.

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Ocean-Atmosphere CO2 System

• There is much more CO2 in the ocean (38,000 Pg C) compared to the atmosphere (600 Pg C).

• Thus the concentration of CO2 in the ocean controls the concentration of CO2 in the atmosphere.

key reaction: CO3= + CO2 + H2O 2HCO3

-

• Air-sea CO2 gas exchange is the process that links the CO2 concentrations in the atmosphere and ocean.

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13C as a Tracer of Changes in the Earth’s Carbon (CO2) Cycle

18O and 13C in CaCO3 SedimentsSize and 13C (‰) of C Reservoirs

13C (‰)= [( 13C/12C)sample/(13C/12C)standard – 1)*1000 (Standard = PDB carbonate)

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Correlation between 13C and 18O changes in CaCO3 Record

Benthic = open circle

Pelagic = filled circle

Ocean 13C is lower during Glacial versus Interglacial conditions

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Using 13C as a Carbon Cycle Tracer

• Changes in the 13C of the ocean CaCO3 record indicate that there was a significant change in the earth’s carbon cycle during Glacial vs Interglacial times.

• The 13C of CaCO3 in benthic forams decreased by ~ -0.3 to -0.4 ‰ (from Ruddiman) during glacial times.

• If this glacial ocean 13C decrease was the result of a transfer of terrestrial organic carbon to the ocean, we can calculate how much carbon was transferred using 13C. (How was it transferred?)

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Quantify the Amount of Terrestrial Carbon Transferred to Ocean

• Carbon Mass and Isotope Budget

Interglacial Ocean Carbon + Terrestrial Carbon Added = Glacial Ocean Carbon

(38,000 PgC) (0 ‰) + (Terr C added) (-25 ‰) = (38000+ Terr C added)(-0.35 ‰)

• Terrestrial Carbon added = 524 Pg C

-Terrestrial Carbon Reservoir = 2100 Pg C

• This estimate roughly agrees with estimates based on the loss of vegetation and soils during the growth of continental ice sheets.

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Effect on Atmospheric CO2

• What effect will this ocean inorganic carbon increase have on atmospheric CO2 concentrations?

-increases CO2 in the atmosphere (~ 15 ppm)

• (Remember: ocean CO2 controls atmospheric CO2)

• This is opposite to the trend observed in ice cores

Interglacial CO2 = 280 ppm

Glacial CO2 = 190 ppm

• Some other change in Earth’s carbon cycle caused lower CO2 levels during Glacial times.

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Why was the atmospheric CO2 concentration lower by ~90 ppm during glacial compared to interglacial times?

• It’s very likely that the mechanism lies in the ocean since the ocean has the biggest carbon reservoir active on relevant time scales and surface ocean CO2 controls atmospheric CO2.

• It is likely a combination of physical, biological and chemical changes to the ocean that cause the CO2 level in the ocean (and thus atmosphere) to change.

• Recent evidence (2009) indicates that the Southern Ocean is the key region.

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Mechanism: Change CO2 Solubility in Seawater

• CO2 gas solubility depends inversely on temperature– Increases by ~4% per 1ºC cooling– Cool surface ocean by 2.5 ºC lowers pCO2 by –22 ppm

• CO2 gas solubility depends inversely on salinity– Increase salinity by ~ 1 ppt increases pCO2 by ~11 ppm(Why does ocean salinity increase during Glacial times?

Net Effect: – 11 ppm

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Mechanism: Make the Surface Ocean More Alkaline during Glacial Times

• Key Reaction: CO2 + H2O + CO3= 2 HCO3

-

-an increase in CO3= concentration will decrease CO2

• Change CO3= by changing the ratio of biological organic

carbon (CH2O) to CaCO3 production and sedimentation

-if diatoms were favored over forams during glacial times there would be less CaCO3(s) production and an increase in CO3

= concentration (iron supply favors diatoms)

• Change CO3= by increasing supply of CO3

= ion to the surface of Southern Ocean by a change in ocean circulation rates and/or pathways

Mechanism: Ocean Circulation

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• Increasing the circulation (exchange) rate between deep ocean and surface ocean affects surface ocean CO2 levels by two processes.

• Increased mixing brings up deep water with high CO2 concentrations to the surface ocean. This increases CO2 levels in surface ocean.

• However, at the same time increased mixing brings deep water with high nutrients which stimulates photosynthesis. This decreases CO2 levels in surface ocean.

• Where does most of this surface-deep ocean exchange occur? Which effect wins out?

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Mechanism: Increase the Ocean’s Photosynthesis Rate during Glacial Times

• Photosynthesis consumes CO2

CO2 + H2O CH2O (sugar) + O2

• Currently there are a lot of nutrients in the surface waters of the Southern Ocean that could be utilized

• Hypothesis: Increase supply rate of iron to the ocean

-iron is a trace nutrient that plankton need and is thought to limit photosynthesis rates in the Southern Ocean

“Give me half a tanker of iron, and I’ll give you the next Ice Age” (John Martin, ~1990)

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Falkowski Behrenfeld depth integrated model calculates total euphotic zone productivity to 1% surface irradiance. Primary inputs are PAR, SST, Chlor_a_3. Units gm Carbon/m2/yr.

OPP P1 December 10, 2000Current Distribution of Photosynthesis in the

Ocean estimated from Satellite Data

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Current Distribution of

Nitrate in Surface Pacific

Ocean

Purple = high nitrate

Green = low nitrate

Unused nutrients in Southern Ocean

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Increase in Dust in Ice Cores Prior to Glacial to Interglacial Transition

Link: Dust contains iron.

20K yrs 130K yrs

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Possible Ocean Photosynthesis

effects on atmospheric CO2

Current CO2 Level

- photosynthesis has the potential to cause

ocean and, thus, atmospheric CO2

changes

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What effect would these ocean changes have on atmospheric

pCO2?

pCO2 (Glacial) = 190ppm

pCO2 (Interglacial) = 280 ppm

-

(Qualitative)

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Ice Core Records over last 750K years

• Critical climate record:– air temperature– atmospheric gas concentrations (CO2, CH4, N2O, O2)– Dust (iron supply?)– Marine aerosols

• What do ice core records tell us about links between temperature change and forcing?

• What do ice core records tell us about sequence of climate events during transition from glacial to interglacial conditions?

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Ice Core Records from

Vostok,Antarctica

Petit et al., 1999(Petit et al., 1999)

Repeating ‘sawtooth’ patterns. Why?

Consistent limits for temp and gases. Why?

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Glacial Terminations

What was sequence of climate events that ended glacial eras?

What about gas age vs ice age offset?

Termination II at 120K yrs

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Higher Resolution

Record during Termination

Monnin EPICA Dome C (Science 2001)

Does temperature rise in Antarctica precedes global

CO2 and CH4 rise?

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Timing during Termination I

Röthlisberger et al., GRL, 2004

10000 12000 14000 16000 18000 20000Age (y r BP )

-440

-420

-400

-380

D (‰

)

180

200

220

240

260

CO2 (

ppm

)Ta

ylor

Dom

e

1

10

2

5

20

50

nss-

Ca2+

flux

(ng/

cm2 /y

r)

Does temperature change precede CO2 change?

How important is dust?

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Sequence of Events during Termination

• Insolation increase at high latitudes

• Dust decreases, then Temperature, CO2, CH4 increases

• Ice Volume decreases

• No single change (e.g., insolation, greenhouse gases, albedo) can account for the observed temperature change.

• Several processes must act together to amplify initial climate change trigger.

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EPICA Antarctic Ice Core (going back to 750K yrs)

-480

-440

-400

-360

0 200 400 600 800

EPICADome C

Vostok

Age / kyr BP

D /

‰

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Reduced Temperature Cycles >400K yrs

-450

-430

-410

-390

-370

0 200 400 600 800

9C

COLD

WARM

Age / kyr before present

D /

‰

Interglacials were less warm at > 400K yrs

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Weak Interglacials have lower CO2

Siegenthaler et al., Science 2005 (EPICA gas consortium)

Vostok

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Weak Interglacials have lower CH4

Spahni et al., Science 2005, EPICA gas consortium

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180

240

300

0 200 400 600

CO2 p

redi

cted

(Mud

else

e)

180

240

300

0 200 400 600

CO2 pr

edict

ed (M

udels

ee)

Mudelsee (based only on Vostok data): pCO2 = 922 + 1.646 * δDt-2000

What does ability to accurately predict CO2 from Antarctic temperatures tell us about the role of CO2 in temperature change?

Atmospheric temperature and CO2 changes are very tightly coupled

CO2 predicted from EPICA Temp record using CO2 vs Temp from Vostok

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Extremely Tight Coupling between Temperatures in Antarctica and Global CO2 levels

• Why are temperature and CO2 so tightly (linearly) coupled when other feedbacks (albedo, ocean heat transfer, etc.) are needed to explain the earth’s temperature change?

• Global CO2 levels controlled by ocean.• Unused surface nutrients present in Southern Ocean.• Air temperatures in Antarctica impacted by heat

released in Southern Ocean.• Does a change in circulation and productivity in

Southern Ocean provide the link between earth’s radiation budget and CO2 cycle?

Hypothesis: Southern Ocean controls CO2

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• Deep water is upwelled to the surface ocean in the S. Ocean.• Upwelling rate depends on strength and position of Westerlies.• Upwelled deep water has high nutrient and CO2 concentrations and is warmer than surface layer.• Sequence: Strengthen Westerlies, increase upwelling, increase CO2 concentrations and temperatures in surface waters of S. Ocean. (In this scenario, increased delivery rate of CO2 from below exceeds increased photosynthetic uptake rate of CO2.)

Anderson et al in Science (March 2009)

Evidence: sedimentary opal record in S. Ocean

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• Major plankton species in S. Ocean are siliceous diatoms.

• Use changes in silica (opal) content to identify periods of increased upwelling in S. Ocean.

• Strong correlation between changes in temperature and CO2 from ice cores and opal changes in marine sediments in S. Ocean.

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Where do we stand?• Glacial/Interglacial changes in temperature and atmospheric CO2 and CH4 levels show an extremely tight correlation.

• Change sequence looks like Solar Insolation, Dust, Temperature, CO2 (and CH4) and, finally, Ice Sheet Volume.

• Earth’s climate feedback system has keep range in temperatures and CO2 very consistent over the last 750K yrs.

• Increasing evidence that Southern Ocean may be the major factor controlling global CO2 and temperatures.

• What is the implication for future climate change?

- strength of Westerlies in S. Hemisphere has increased over last few decades (thought to be a result of increased atmospheric CO2 levels). Impact on atmospheric CO2?