1. 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.

2. Ice Core Drilling Depths

3. Dome C,
Antarctica

4. Tools of the Trade L

5. Ice Core Drill

6. Ice Core Recovery

7. Ice Cores from Greenland
Firn Ice
Compact Ice
Bedrock

8. Antarctica Drilling Sites

9. Ice Cores and Ice Sheet Flow
“Dome”
Age of Ice: annual layers (Greenland)- accurate but limited in age
ice flow models (Antarctica)- less accurate but extends age range
annual

10. 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.

11. Effect of Condensation on the d18O (and dD) 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]

12. d18O of Today’s Precipitation vs Air Temperature
d18O (‰ vs SMOW)
-strong linear correlation between d18O (and dD) of precip and air temp
ΔTemp/Δd18O =
~1.4ºC / 1‰
ΔTemp/ΔdD =
~0.2ºC / 1‰
Air Temp (ºC)

13. 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).

14. d18O versus Borehole Paleothermometry a controversy in Greenland Ice Cores
(DTemp/Dd18O= 1.5 ºC / ‰) using current precipitation
DTemp/Dd18O= 3 ºC / ‰
using Borehole Temps
Climate scientists favor the borehole temperature changes.

15. Greenland Drilling Sites
(most notable are the GISP and GRIP sites started in 1990s)

16. Greenland Ice Core d18O and Temperature Record
Using borehole temperature vs d18O calibration

17. Temperature Swings between Glacial and Interglacial Conditions
ΔTa 36 24 21
a Bore Hole calibrn
ΔTemp/Δd18O about equal for borehole and precipitation in Antarctica

18. 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)

19. Trapping Air Bubbles in Ice
Snow Accumulation Rates
Greenland = 0.5 m/yr
Antarctica = 0.05 m/yr

20. 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?

21. Industrial Era Changes in Atmospheric CH4 and CO2
Extending the record of industrial era change
Tests the accuracy of ice core gas measurements

22. 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 (?)

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

24. 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).

25. Atmospheric Methane from Antarctic Ice Cores
CH4 concentration doubles between glacial and interglacial conditions
CH4 changes correlate strongly with temperature changes

26. 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.

27. 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?

28. 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.

29. Atmospheric CO2 and Ice Volume Records
- Atmospheric CO2 record from ice cores
Ice volume record from d18O of marine CaCO3 sediments
What is implication of strong correlation?

30. 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.

31. Carbon Reservoir Changes and Exchange Rates
Changes 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.

32. 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.

33. d13C as a Tracer of Changes in the Earth’s Carbon (CO2) Cycle
Size and d13C (‰) of C Reservoirs
d18O and d13C in CaCO3 Sediments
d13C (‰)= [(13C/12C)sample/(13C/12C)standard – 1)* (Standard = PDB carbonate)

34. Correlation between d13C and d18O changes in CaCO3 Record
Benthic = open circle
Pelagic = filled circle
Ocean d13C is lower during Glacial versus Interglacial conditions

35. 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 ‰ (from 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?)

36. 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 ‰) = ( 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.

37. 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 CO = 190 ppm
• Some other change in Earth’s carbon cycle caused lower CO2 levels during Glacial times.

38. 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.

39. 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

40. 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

41. Mechanism: Ocean Circulation
• 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?

42. 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)

43. Current Distribution of Photosynthesis in the Ocean estimated from Satellite Data

44. Current Distribution of Nitrate in Surface Pacific Ocean
Purple = high nitrate
Green = low nitrate
Unused nutrients in Southern Ocean

45. Link: Dust contains iron.
Increase in Dust in Ice Cores Prior to Glacial to Interglacial Transition
20K yrs
130K yrs
Link: Dust contains iron.

46. Possible Ocean Photosynthesis effects on atmospheric CO2
Current CO2 Level
- photosynthesis has the potential to cause ocean and, thus, atmospheric CO2 changes

47. What effect would these ocean changes have on atmospheric pCO2?-
pCO2 (Glacial) = 190ppm
pCO2 (Interglacial) = 280 ppm
(Qualitative)

48. 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?

49. 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)

50. 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?

51. Higher Resolution Record during Termination
Does temperature rise in Antarctica precedes global CO2 and CH4 rise?
Monnin EPICA Dome C (Science 2001)

52. Timing during Termination I
Does temperature change precede CO2 change?
How important is dust?
Röthlisberger et al., GRL, 2004

53. 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.

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

55. Reduced Temperature Cycles >400K yrs
Interglacials were less warm at > 400K yrs

56. Weak Interglacials have lower CO2
Vostok
Siegenthaler et al., Science 2005 (EPICA gas consortium)

57. Weak Interglacials have lower CH4
Spahni et al., Science 2005, EPICA gas consortium

58. Atmospheric temperature and CO2 changes are very tightly coupled
CO2 predicted from EPICA Temp record using CO2 vs Temp from Vostok
Mudelsee (based only on Vostok data): pCO2 = * δDt-2000
What does ability to accurately predict CO2 from Antarctic temperatures tell us about the role of CO2 in temperature change?

59. 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?

60. Hypothesis: Southern Ocean controls CO2
• 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)

61. Evidence: sedimentary opal record in S. Ocean
• 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.

62. 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?