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

Main Points:

• Small cyclic variations in the earth’s orbital characteristics affect the distribution of solar radiation on earth that, in turn, initiate 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 18O record.

• Reconstructions of sea level change from coral reefs and the 18O-CaCO3 record indicate the extent and timing of ice sheet growth and retreat in the past.

Orbital Effects on Solar Insolation

1. Variations in the tilt of the earth’s axis.

2. Variations in the shape of the earth’s elliptical orbit around the sun.

3. Variations in the position of the earth’s tilt in its elliptical orbit.

• All three of these orbital variations can be accurately reconstructed and have affected the distribution of solar insolation on earth over the last 4.5B years.

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, no seasons.

Periodicity of Tilt

Elliptical Orbit

Shape of earth’s elliptical orbit oscillates from more circular to less circular (called eccentricity).

Variations in eccentricity affect the seasonality.

Periodicity of Eccentricity

Variations in Axial Wobble

-axial precession

-affects seasonality

-a 25.7K yr period

Orbital Precession

The elliptical shape of the earth’s orbit rotates around the sun.

-affects the seasonality of insolation received

Variations in Precession

-combined effects of axial and elliptical precession affects the position of the equinoxes in the earth’s orbit

-period is 23K yrs

Effect of Precession on Seasonality

The location along the orbit when the earth is at its winter and summer solstice affect the insolation received.

Modulation of Precession by Eccentricity

Modulation of the Precession by Eccentricity over the last 1.5

Myrs

- a 23Kyr periodicity

- but no 100Kyr (or 400Kyr) periodicity

Effect of Orbital Changes of Solar Insolation on Climate

• Milankovitch (1920) hypothesized that the orbital induced change in solar insolation was a primary driver of climate change on earth.

• At the time his theory was not taken too seriously, but as climate records improved, there was clear evidence that orbital variations in solar insolation are an important component of climate change.

Latitudinal Trend of Orbitally Induced

Insolation Change in Summer and Winter

Current Solar Insolation Distribution

H

Milankovitch’s Theory (1940s)

• 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 critical.

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 affects amount of snowfall because warm air holds more moisture than cold air.

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

Effect of Changes in Summer Insolation

on 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

Climate Point-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 changing latitude of Climate Point

Ice Sheet Distribution during the Last Glacial Maximum (LGM) ~20K yrs ago

Ice sheet volume at LGM was about twice modern.

Solar Insolation Changes

Red dashed line marks 20K yrs BP (LGM)

I and II mark the terminations of glacial conditions.

Ice Sheet Response Lags Insolation Change

- Ice Sheet growth rate ~ 0.3 m/yr

- a 3000m high ice sheet would take ~ 10,000 yrs to accumulate.

Record of Variations in Ice Sheet Extent

• There is good geologic evidence for areal extent of ice sheets during last Ice Age (~20Kyrs), but not for previous ones.

• More difficult to accurately estimate height of ice sheets.

• Our best records of variations in ice sheet volume comes from the ocean.

– Sea level change and the 18O of CaCO3 preserved in sediments

18O of CaCO3 in Ocean Sediments

• The 18O of CaCO3 is a proxy for both Ice Sheet Volume and Ocean Temperature that extends back millions of years.

• Ocean Temperature vs 18O-CaCO3 Relationship

ΔTemp/Δ18O = -4.2 ºC per 1 ‰ increase

• Ice Sheet Volume Relationship

-an increase in 18O-CaCO3 implies an increase in Ice Sheet volume (quantify later)

• Increase in 18O-CaCO3 implies colder ocean and greater ice sheet volume (and vice-versa)

Correlation between 18O record deep sea CaCO3 sediments and Orbitally forced

Solar Insolation Changes

Strength of Tilt and Precession Periodicities in a Climate Record

Tilt Period (41K yrs)

Dashed = insolation changes

Solid= Spectral analysis of 18O in deep-sea carbonates

Precession Period (23K yrs)

Dashed = insolation changes

Solid= Spectral analysis of 18O in deep-sea carbonates

Slow Cooling and Change in

Dominant Periodicity in 18O-CaCO3

Record

-transition in dominant periodicity of 18O-CaCO3 record at ~1M yrs.

Three Dominant Periods in 18O-CaCO3 Record

over last 1M yrs

Combining Periodicities

(hypothetically as sine waves)

Spectral Analysis of Climate Records

Spectral Analysis of

Insolation and 18O-CaCO3

Records

- there is no power (strength) in the 100K cycles of insolation yet it dominates the climate record over the last ~1 Myrs

Reconstructing Sea Level Changes

Determine the ages of fossil coral reefs that lived close to the ocean’s surface

Present Elevation (m) of Shorelines from 124K years ago

Benchmark: Mean Sea Level at 124,000 yrs BP = +6m (Ruddiman)

Reconstructing Paleo Sea Level

Sea Level Change and its impact on the 18O of Seawater (and thus CaCO3)

As ice sheets grow, the 18O of seawater increases (and vice versa)

The 18O of CaCO3 precipitated by forams depends on the 18O of seawater. Thus the 18O-CaCO3 sediment record reflects both ice sheet volume change and ocean temperature change.

18O of Seawater and CaCO3

• The precipitation CaCO3

Ca++ + CO3= CaCO3 (solid)

• Equilibrium reaction between CO2, carbonate ion and seawater

CO2 + H2O + CO3= 2HCO3

-

• The 18O of the CaCO3 depends on the 18O of seawater (and temperature of precipitation reaction)

Sea Level Effects on 18O-CaCO3 Record

• At LGM (~20Kyrs BP), sea level was 120m lower than today based on Barbados coral reef record.

• Calculate the 18O change in the ocean due the transfer of 120m of ocean to glacial ice sheets.

Depthintgl*18Ointgl – ΔSea Level* 18Oice = Depthgl * 18Ogl

(3800m)*(0 ‰) – 120m*(-35 ‰) = 3680m*(18Ogl oc)

18O glacial ocean (at LGM) = +1.1 ‰

• Thus the transfer of water from ocean to ice sheets at the LGM left the ocean with a 18O which was 1.1 ‰ higher than today’s ocean.

Ice Volume Correction on 18O-CaCO3 record

18O change due to ocean temperature decrease is 0.65 ‰ after the 1.1 ‰ ice volume correction has been applied to the observed 1.75 ‰ change.

Ice volume change

is 1.1 ‰

Estimating Ocean Temperature from 18O of CaCO3

• Correct total 18O change for ice volume effect and then assume remaining 18O change is due to temperature change

• Use empirically determined relationship between 18O of precipitated CaCO3 and temperature (ΔTemp = -4.2 *Δ18O ) to calc temperature change.

• At 20K yrs ago, the 18O 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. The modern

deep ocean is ~ 2 ºC.

The Impact of a Glacial Threshold

The glacial threshold depends on positions of continents, CO2 levels, ocean circulation rates, etc.

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 which affects ice ablation rates.

• 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 other climate factors.

Conclusions• There is a strong correlation between the periodicity of the 18O-CaCO3 record preserved in deep sea sediments and orbital insolation change at 23K and 41K (but not 100K years).

-supports 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 18O-CaCO3 record (temperature change secondary).

• Deep ocean temperatures were ~2.7 ºC colder during the LGM and sea level was 120m lower than today.

W.S. Broecker’s book (2002) discusses the evidence for and causes of ice age events during the last million years.

The Glacial World According To Wally

(available as .pdf)

Wally Broecker has been a leading guru of unraveling the causes of past climate change.