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

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 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.
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 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
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 d18O of CaCO3
preserved in sediments
d18O of CaCO3 in Ocean Sediments
• The d18O of CaCO3 is a proxy for both Ice 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 in d18O-CaCO3 implies colder ocean and
greater ice sheet volume (and vice-versa)
Correlation between d18O 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
d18O in deep-sea carbonates
Precession Period (23K yrs)
Dashed = insolation changes
Solid= Spectral analysis of
d18O in deep-sea carbonates
Slow Cooling
and Change in
Dominant
Periodicity in
d18O-CaCO3
Record
-transition in dominant
periodicity of d18OCaCO3 record at ~1M
yrs.
Three Dominant
Periods in d18OCaCO3 Record
over last 1M yrs
Combining
Periodicities
(hypothetically
as sine waves)
Spectral Analysis of Climate Records
Spectral
Analysis of
Insolation and
d18O-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
d18O of Seawater (and thus CaCO3)
As ice sheets grow, the d18O of seawater increases (and vice versa)
The d18O of CaCO3 precipitated by forams depends on the d18O of
seawater. Thus the d18O-CaCO3 sediment record reflects both ice
sheet volume change and ocean temperature change.
d18O of Seawater and CaCO3
• The precipitation CaCO3
Ca++ + CO3=  CaCO3 (solid)
• Equilibrium reaction between CO2, carbonate ion
and seawater
CO2 + H2O + CO3=   2HCO3• The d18O of the CaCO3 depends on the d18O of
seawater (and temperature of precipitation reaction)
Sea Level Effects on d18O-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.
Depthintgl*d18Ointgl – ΔSea Level* d18Oice = Depthgl * d18Ogl
(3800m)*(0 ‰) – 120m*(-35 ‰) = 3680m*(d18Ogl oc)
d18O glacial ocean (at LGM) = +1.1 ‰
• 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.
Ice Volume Correction on d18O-CaCO3 record
Ice volume change
is 1.1 ‰
d18O 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.
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. 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 d18O-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 d18O-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.