OC 450: Orbital Controls on Climate (Chaps 8 and 10)

1. Main Points: • Small cyclic variations in the earth’s orbital characteristics affect the distribution of solar radiation on earth that, in turn, initiated the advance and retreat of ice sheets over the last 1M years. • Evidence for these cyclic variations in climate is clearly present in the deep sea carbonate d18O record. • Reconstructions of sea level change from coral reefs and the d18O-CaCO3 record indicate the extent and timing of ice sheet growth and retreat in the past. OC 450: Orbital Controls on Climate(Chaps 8 and 10)

2. Orbital Effects • Variations in the tilt of the earth’s axis. • Variations in the shape of the earth’s elliptical orbit around the sun. • Variations in the position of the earth’s tilt in its elliptical orbit. • 4. All three of these orbital variations can be accurately reconstructed back through time because they depend on the position of the earth relative to the positions of the sun and other planets (gravitational attraction).

3. Variations in Tilt Angle of tilt varies from 22.2º to 24.5º(23.5º today) Higher tilt causes stronger seasonality. Variations in tilt angle yield variations in seasonality. No tilt…..little or no seasonality.

4. Periodicity of Tilt ~41K yr periodicity

5. Orbital Shape or Eccentricity Shape of earth’s elliptical orbit oscillates from more circular to less circular (called eccentricity). Variations in eccentricity affect the seasonality.

6. Periodicity of Eccentricity • - 100K yr periodicity • 413K yr periodicity • If orbit was circular, then e=0. Today e=0.017

7. Axial Precession(Axis Wobble) - The position of the earth’s axis of rotation relative to a fixed point (e.g., North Star) - Mainly a result of gravitational effects by sun and moon. ~25.7K yr periodicity

8. Orbital Precession - The earth’s elliptical orbit rotates around the sun. - ~21-26K yr periodicity (similar to axial precession)

9. Effect of Axial and Orbital Precession on Seasonality The location along the orbit when the earth is at its winter and summer solstice affect the insolation received.

10. Variations in Precession -combined effects of axial and orbital precession affects the position of the equinoxes in the earth’s orbit -period is 23K yrs

11. Modulation of Precession by Eccentricity

12. Precessional Changes over the last 1.5 Myrs

13. Effect of Orbital Changes of Solar Insolation on Climate • Milankovitch (1940s) hypothesized that the orbital induced change in solar insolation was a primary driver of climate change on earth. • His theory was not taken seriously until climate records improved and demonstrated variations that had similar periods and amplitudes as orbital variations.

14. Latitudinal Trend of Orbitally Induced Solar Insolation Change in Summer and Winter - high latitudes show the most temporal variations in insolation received

15. Current Solar Insolation Distribution H

16. Milankovitch’s Theory • Variations in summer insolation at high latitudes in the northern hemisphere caused by variations in earth’s orbital characteristics resulted in temperature changes which in turn affected the growth and retreat of ice sheets. • Milankovitch theorized that the amount of summer insolation received at 65ºN was a critical factor affecting ice sheet growth and retreat.

17. Mean Annual Temperature at High Latitudes (key to Ice Sheet Growth/Retreat) • Ice Sheet growth depends on two primary characteristics • Mean annual temperature and snowfall rate • If summer temps are cold enough, then snowfall during previous winter doesn’t entirely melt, snow/ice accumulates and ice sheets grow. • If summer temps are warm enough, then snowfall during previous winter plus additional snow/ice melts and ice sheets retreat. • Temperature also can affect amount of snowfall because warm air holds more moisture than cold air.

18. Ice Sheet Mass Balance: Temperature Dependence -ablation has a much stronger temperature dependence than accumulation -thus summer temperatures are important for ice sheet growth/decay -equilibrium temperature around –10º C

19. Effect of Changes in Summer Insolationon Ice Sheet Growth -Milankovitch’s theory is based on the premise that insolation variations are sufficient to yield mean annual temperature changes at high latitudes that cause swings between ice sheet growth and retreat

20. -Climate Point where equilibrium line intersects earth’s surface -regions poleward of equilibrium line accumulate ice and regions south of the line lose ice -insolation variations affect the latitude of Climate Point - current CO2 increase is increasing the latitude of Climate Point Climate Point

21. Ice Sheet Distribution during the Last Glacial Maximum (LGM) ~20K yrs ago Ice sheet volume at LGM was about twice modern.

22. Solar Insolation Changes Red dashed line marks 20K yrs BP (LGM) I and II mark the terminations of glacial conditions.

23. Ice Sheet Response Lags Insolation Change - Ice Sheet growth rate ~ 0.3 to 1 m/yr - a 3000m high ice sheet would take ~3,000 to 10,000 yrs to accumulate.

24. Record of Variations in Ice Sheet Extent • There is good geologic evidence (moraines) for areal extent of ice sheets during last Ice Age (~20Kyrs), but not for previous ones. • It is more difficult to accurately estimate height (and thus volume) of ice sheets. • Our best records of past variations in ice sheet volume comes from the ocean. • Sea level change and the d18O of CaCO3 preserved in sediments are used to estimate changes in Ice Sheet volume.

25. d18O of CaCO3 in Ocean Sediments • The d18O of CaCO3 is a proxy for bothIce Sheet Volume and Ocean Temperature that extends back millions of years. • Ocean Temperature vs d18O-CaCO3 Relationship ΔTemp/Δd18O = -4.2 ºC per 1 ‰ increase • Ice Sheet Volume Relationship -an increase in d18O-CaCO3 implies an increase in Ice Sheet volume (quantify later) • Increase ind18O of CaCO3 implies colder ocean and greater ice sheet volume (and vice-versa)

26. Correlation between d18O record deep sea CaCO3 sediments and Orbitally forced Solar Insolation Changes d18O = +2 ‰ d18O = 0 ‰

27. Strength of Tilt and Precession Periodicities in Insolation and d18O Carbonate Record Tilt Period (41K yrs) Dashed = insolation changes Solid= Spectral analysis of d18O in deep-sea carbonates Precession Period (23K yrs) Dashed = insolation changes Solid= Spectral analysis of d18O in deep-sea carbonates

28. Slow Cooling and Change in Dominant Periodicity in d18O-CaCO3 Record • transition in dominant periodicity of d18O-CaCO3 record at ~1M yrs from 41K yrs to 100K yrs. • Why?

29. Three Periods in d18O-CaCO3 Record Observedover last 1M yrs

30. Combining Periodicities (sine waves of equal amplitudes)

31. Spectral Analysis of Climate Records

32. Spectral Analysis of Insolation and d18O-CaCO3 Records - Enigma: there is no power (strength) in the 100K cycles of insolation, yet it dominates the climate record over the last ~1 Myrs

33. Reconstructing Sea Level Changes Determine the ages of fossil coral reefs that lived close to the ocean’s surface in the past.

34. Elevation (m) of Shorelines from 124K yrs ago (relative to current sea level) Benchmark: Mean Sea Level at 124,000 yrs BP = +6m (relative to today’s sea level)

35. Reconstructing Paleo Sea Level Assumes reefs formed 124K yrs ago were at 6m higher than present sea level.

36. Changes in Sea Level due to Ice Sheet Growth and Retreat affect the d18O of Seawater (and thus CaCO3) Remember: d18O of SMOW = 0 ‰ As ice sheets grow, the d18O of seawater increases (and vice versa) . As more ice is formed on land, more of the low d18O water is removed from the ocean and thus the remaining seawater has a higher d18O.

37. d18O of CaCO3 depends on d18O of Seawater • The precipitation of CaCO3 Ca++ + CO3=  CaCO3 (solid) • Equilibrium reaction between CO2, carbonate ion and seawater CO2 + H2O + CO3=   2HCO3- • Thus the d18O of CaCO3 precipitated by forams depends on the d18O of CO3= which in turn depends on the d18O of seawater. • As mentioned previously, the d18O of CaCO3 precipitated by forams also depends on temperature of precipitation reaction (seawater).

38. Sea Level (Ice Volume) Effects on d18O of CaCO3 Record • At LGM (~20Kyrs BP), sea level was 120m lower than today based on Barbados coral reef record. • Calculate the d18O change in the ocean due the transfer of 120m of ocean to glacial ice sheets. Depthtoday*d18Otoday – ΔSea Level* d18Oice = DepthLGM * d18OLGM (3800m)*(0 ‰) – 120m*(-35 ‰) = 3680m*(d18OLGM) d18O seawater at LGM = +1.1 ‰ (relative to today) • Thus the transfer of water from ocean to ice sheets at the LGM left the ocean with a d18O which was 1.1 ‰ higher than today’s ocean.

39. Ice Volume Correction on d18O-CaCO3 record Ice volume change is 1.1 ‰ d18O of CaCO3 (‰) Only 0.65 ‰ of the total d18O-CaCO3 increase of 1.75 ‰ (at LGM) is due to an ocean temperature decrease, whereas 1.1 ‰ of the d18O increase is due to ice sheet volume increase (the latter effect dominates).

40. Estimating Ocean Temperature from d18O of CaCO3 • Correct total d18O change for ice volume effect and then assume remaining d18O change is due to temperature change • Use empirically determined relationship between d18O of precipitated CaCO3 and temperature (ΔTemp = -4.2 *Δd18O ) to calc temperature change. • At 20K yrs ago, the d18O of CaCO3 was 1.75 ‰ higher of which 1.1 ‰ was ice volume effect. This leaves 0.65 ‰ as temperature effect. • implies that ocean was 2.7 ºC colder at LGM than the modern deep ocean (which is ~ 2 ºC).

41. The Impact of a Glacial Threshold The glacial threshold depends on all climate factors (e.g. positions of continents, greenhouse gas concentrations, ocean circulation rates, etc.)

42. Conclusions • Earth’s orbital changes affect the distribution of solar insolation (especially important at high northern latitudes). • Ice sheet growth is likely impacted by changes in summertime insolation at high (northern) latitudes which affects ice ablation rates. •

43. Conclusions • There is a strong correlation between the periodicity of the d18O changes in the CaCO3 record preserved in deep sea sediments and orbital insolation change at 23K and 41K (but not 100K years). -key support for Milankovich’s theory that orbitally induced changes in solar insolation are a trigger for climate change on earth. • Reconstruction of paleo sea levels from coral reef positions indicate that changes in ice sheet volume had the dominant impact on the d18O of CaCO3 in the sedimentary record (temperature change secondary). • Deep ocean temperatures were ~2.7 ºC colder during the LGM and sea level was 120m lower than today.

44. Conclusions • Whether or not orbital changes in solar insolation are sufficient to initiate the growth or retreat of ice sheets depends on the earth’s glacial threshold at the time, which in turn depends on all of earth’s climate factors. • Thus, despite the relatively low solar insolation rates at present, the increased level of GHGs due to anthropogenic activity would likely prevent the onset of an Ice Age because it has changed the earth’s glacial threshold.

45. Wallace S. Broecker has been the guru of the paleoclimate community for the last 30 or so years. In 2002 , Wally wrote The Glacial World According To Wallywhich discusses the evidence for and causes of ice age events during the last million years. It is available as a pdf.