Transcript Climate Records from Ice Cores

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 is 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
2
Dome C,
Antarctica
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Tools of the
Trade
L
L
L
L
L
4
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
°
60
S
°
70
Dronning
Maud Land
S
Berkner
Island
Dom e F
°S
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Byrd
Vostok
Law Dom e
Dom e C
Siple Dom e
Taylor
Dom e
0km
1,000km
2,000km
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Ice Cores and Ice Sheet Flow
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Age of Ice: annual layers (Greenland) and ice flow models (Antarctica)
ΔTemp/Δd18O =
~1.4ºC / 1‰
d18O (‰)
d18O of Today’s
Precipitation vs
Air Temperature
ΔTemp/ΔdD =
~0.2ºC / 1‰
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Effect of Condensation on the d18O (and dD) of
Precipitation
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Borehole Temperature Record in Ice Cores
Scientists measure the
temperature of an ice sheet
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.
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d18O versus Borehole Paleothermometry
a controversy in Greenland Ice Cores
(DTemp/Dd18O= 1.5 ºC / ‰)
Current Precipitation
DTemp/Dd18O= 3 ºC / ‰
Borehole Temps
Climate scientists favor the borehole temperature changes.
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Greenland
Drilling Sites
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Greenland Ice Core d18O and Temperature
Record
Using borehole temperature vs
d18O
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calibration
Temperature Swings between Glacial and
Interglacial Conditions
ΔT*
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*BH
corr’n
ΔTemp/Δd18O about equal for borehole and precipitation in Antarctica
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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
Test of accuracy of ice
core gas measurements
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Atmospheric Methane (CH4)
• 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
• During interglacial times, 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.
• Currently unclear 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
greenhouse gas warming 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 our future climate
change, in a world of high atmospheric CO2
levels, from climate changes over the last
700K years when atmospheric CO2 levels
oscillated by ~40%?
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Atmospheric CO2 and Temperature from
an Antarctic Ice Core
Atmospheric CO2
levels increase by
40% between
glacial and
interglacial times.
Strong correlation
between CO2 and
temperature
changes.
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Atmospheric CO2
and Ice Volume
Records
- CO2 from Ice Cores
- Ice Volume from d18O
of marine CaCO3
- What is implication of
strong correlation?
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What causes the Glacial-Interglacial shifts
in atmospheric CO2?
• No clear answer yet. (one of the Holy Grails)
• Involves a change in the earth’s carbon cycle.
• Very likely that changes in the ocean are
involved.
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Carbon Reservoir Changes and Exchange Rates
Changes in Reservoir Sizes (Pg, %)
Interglacial to 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. (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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d13C as a Tracer of Changes in the Earth’s
Carbon Cycle
Size and d13C of C Reservoirs
d18O and d13C in CaCO3 Sediments
d13C (‰)= [(13C/12C)sample/(13C/12C)standard – 1)*1000
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(Standard = PDB carbonate)
Correlation between d13C and d18O changes
in CaCO3 Record
Benthic =
open circle
Pelagic =
filled circle
d13C is lower during Glacial versus Interglacial conditions
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Using d13C as a Carbon Cycle Tracer
• Changes in the d13C of the ocean CaCO3 record
indicate that there was a significant change in the
earth’s carbon cycle during Glacial vs Interglacial
times.
• The d13C of CaCO3 in benthic forams decreased
by ~ -0.3 to -0.4 ‰ (avg. Ruddiman) during
glacial times.
• If this glacial ocean d13C decrease was the result
of a transfer of terrestrial organic carbon to the
ocean, we can calculate how much carbon was
transferred using d13C. (How was it transferred?) 34
Quantify the Amount of Terrestrial Carbon
Transferred to Ocean
• Carbon Mass and Isotope Budget
Interglacial Ocean Carbon + Terr 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
• 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 (~ 2 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 did the atmospheric CO2 decrease by
90 ppm during glacial times?
• Don’t know yet…. but a lot of smart people are
trying to figure it out.
• It’s very likely that the mechanism lies in the ocean.
• 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.
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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: 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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Current Distribution of Photosynthesis in the
OPPestimated
P1 December
10,Data
2000
Ocean
from Satellite
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.
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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
Dust contains iron
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Current CO2 Level
Possible Ocean
Photosynthesis
effects on
Atmospheric CO2
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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
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Possible Ocean Mechanisms to Reduce
Atmospheric CO2
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d13C as a tracer of Ocean Photosynthesis
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Record of d13C
depth gradient
in the Ocean
d13C of pelagic
CaCO3 minus d13C
of benthic CaCO3
Some evidence for
increased ocean
productivity during
glacial times.
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What effect would
these ocean changes
have on atmospheric
pCO2?
pCO2 (Glacial) = 190ppm
(Qualitative)
pCO2 (Interglacial) = 280 ppm
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Where do we stand?
• Currently, model calculations that attempt to
simulate the biological, chemical and physical
changes in the ocean during the LGM cannot
reproduce the glacial concentrations of atmospheric
CO2 found in ice cores and independent evidence of
ocean change.
• Thus our current understanding of the processes
controlling the earth’s carbon (CO2) cycle on glacial
to interglacial time scales is incomplete.
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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
Repeating ‘sawtooth’
patterns. Why?
Consistent limits for
temp and gases.
Why?
(Petit et al., 1999)
Petit et al., 1999
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Termination II at 120K yrs
Glacial
Terminations
What was sequence of
climate events that
ended glacial eras?
What about gas age vs
ice age offset?
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Higher
Resolution
Record during
Termination
Does temperature
rise in Antarctica
precedes global
CO2 and CH4 rise?
Monnin EPICA Dome C (Science 2001)
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Timing during Termination I
20
10
5
2
1
-380
260
-400
240
-420
220
-440
dD (‰)
CO 2 (ppm)
Taylor Dome
nss-Ca 2+ flux
(ng/cm2/yr)
50
200
180
10000
12000
14000
16000
Age (yr BP)
18000
20000
Does temperature change precede CO2 change?
How important is dust?
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Röthlisberger et al., GRL, 2004
Sequence of Events during Termination
• Insolation increase at high latitudes
• Dust increases, 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)
-360
EPICA
Dome C
dD / ‰
-400
-440
Vostok
-480
0
200
400
600
800
Age / kyr BP
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Reduced Temperature Cycles >400K yrs
dD / ‰
-370
-390
WARM
-410
9 C
-430
COLD
-450
0
200
400
Age / kyr before present
600
Interglacials were less warm at > 400K yrs
800
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Weak Interglacials have lower CO2
Vostok
Siegenthaler et al., Science 2005 (EPICA gas consortium)
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Weak Interglacials have lower CH4
Spahni et al., Science 2005, EPICA gas consortium
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CO2 predicted (Mudelsee)
Temperature and CO2 are tightly coupled
300
240
CO2 predicted (Mudelsee)
180
300 0
200
400
600
200
400
600
240
180
0
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?
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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 temp 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?
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Termination V (450K yrs BP)
• CO2 increase
precedes temperature
and CH4 increase and
dust decrease.
• Different from 20K
termination sequence.
• Errors in ice age and
bubble age?
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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/CH4 and, finally, Ice Volume (except
Termination V at 450K).
• Earth’s climate feedback system has keep range in
temperatures very consistent over the last 750K yrs.
• Increasing evidence that Southern Ocean may be an
important feedback factor in controlling global CO2 and
temperatures.
• What is the implication for future climate change?
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