Orbital-Scale Climate
Change
Ice Age Model
• When ice growing
–
–
–
–
ocean saltier (not freeze in winter)
ocean warmer (fewer ice bergs & less cold runoff
thus more evaporation to provide winter precipitation
albedo of ice make it more difficult to melt in summer
• When ice melting
–
–
–
–
water from melt is fresh so ocean freezes in winter
ocean cold from melt water so less evaporation
less evaporation means less snowfall in winter
albedo decreases in summer so more melting
Climate Needed for Ice Age
• Warm winter and non-frozen oceans so lots
of evaporation and snowfall
• Cool summer so that ice does not melt
Milankovitch Theory
• Obliquity of Ecliptic - variation of tilt of
earth relative to plane of orbit
– caused by torque of Sun & Moon on Earth
• Eccentricity of Earth’s orbit (ellipticity)
– caused by pull of Jupiter on Earth
• Precession of Equinoxes - date of Perihelion
– caused by torque of Sun & Moon on Earth
1
Tilt of Earth’s Axis
gives us seasons
Obliquity of Ecliptic
For Ice Age need less tilt
Tilt presently high, but decreasing
Effect of Tilt on Polar Regions
Long Term
Changes in Tilt
41,000 Year
Cycle
Precession of Equinoxes
Precession
of the
Earth’s
Axis
2
Precession
of the
Ellipse
Milankovitch Cycles
Precession of the Equinoxes
• Combined effect of Precession of the
Earth’s Axis and Precession of the Ellipse
• Period = 23,000 years
Precession of Equinoxes
Precession
of the
Equinoxes
Need Summer Solstice near aphelion
Near now, but tilt wrong
3
Eccentricity of Earth’s Orbit
Change in
Eccentricity of
Earth’s Orbit
100,000 year cycle
Change of Eccentricity
High eccentricity enhances precession difference
Low eccentricity today, but also low 20,000 yr ago
Extreme Solstice Positions
Precession of the Equinox
Angle between the Perihelion
and Vernal Equinox
Eccentricity Modulating Precession
4
Maximizing Three Cycles
Eccentricity
Modulated
Precession
Over last 1.65 Million Years
June Insolation Variations
Combining
Cycles
December Insolation Variations
5
Solar Insolation
Solar Insolation
Summer Solstice = 90, Winter Solstice = 270
Summer Solstice = 90, Winter Solstice = 270
Geological Features vs Irradiance
Irradiance vs Oxygen Ratio
Temperature
Power
Spectrum
6
Ice Volume
Lags
Insolation
Ice
Volume
Lags Tilt
and
Precession
Bedrock Sinking
Bedrock
(Positive)
Feedback to ice
growth and
melting
Solar
Radiation
versus
Ice
Volume
Oxygen
Isotope
Record
During Last
Ice Age
7
Oxygen Ratio Temperature
Cores from Eastern Pacific
Climate Change
Evidence of
Ice Sheet
Evolution
Determining
Past
Atmospheric
Composition
from air
trapped in
Glacial Ice
Widespread Northern Hemisphere Glaciations Begin
Carbon Dioxide vs Ice Volume
Carbon Dioxide
versus
Ice Volume
8
Methane in Past Atmospheres
Marine
versus
Ice Core
Temperatures
Methane versus Temperature
(Fig. 15-14)
Decreased Carbon Dioxide
during Glaciations
• More CO2 dissolves in cold water
– Atmospheric CO2 drops by 9 ppm for oC cooling
– Total drop during Ice Age about 22 ppm
• CO2 dissolves easier in less salty water
– During Ice Age salinity increased about 1.2%
– Thus Atmospheric CO2 increased by about 11 ppm
Other Causes of CO2 Change
• Pollen suggests drier and less vegetation
– About 25% less so carbon not stored here
• If less CO2 in Atmosphere then less in near
surface ocean (about 25%)
• Not enough time to exchange with rocks
• Thus must be in deep ocean
9
Causes of CO2 Increase in Deep Ocean
Carbon
Transfer
to Deep
Oceans
During
Glaciations
• Greater Winds Cause Greater Upwelling
• More Arid so Winds Blow Nutrients into
Ocean where Photosynthesis increases and
Dead Organic Matter Falls to Ocean Floor
• Deep Ocean Currents Decrease and
corrosive water dissolves Carbonates that
are brought to the surface where they
combine with CO2 to form Bicarbonates
• Less Surface Water CO2 makes for less
Atmospheric CO2
During Glaciation (Fig. 13-11)
Ice Sheets (Fig 9-4)
North American Ice Extent
Response
to
Changes
in
Atlantic
Conveyor
Belt
(Fig. 15-13)
10
Extent of Last 4 Glaciers
Vegetation
during last
Ice Age
North American Ice Retreat
Retreat of
Glacier in
North
America
(Fig. 13-2)
Great Lakes Ice Withdrawal
Lakes Formed
During
Deglaciation
(Fig. 13-10)
11
Routes of Meltwater Flow (Fig. 13-8)
Colder Temperatures accumulated
less snow in Greenland and made
Stronger winds in Venezuela
Pollen in
Minnesota
after last
Ice Age
Polar
Front at
various
Times
(Fig. 12-9)
(Fig. 13-6)
European Ice Age
European
Vegetation
(Fig. 12-15)
12
Ice Age in Europe
European Ice
Retreat
Changes
in
Europe
Ice Volume
and
Temperature
versus
Pollen in
France
Last 80,000
years
(Fig. 14-6)
(Fig. 11-9)
African Monsoon
Sand Dunes
Now versus
Last Glaciation
(Fig. 12-5)
13
Glacial Australia and South
America were Drier
Antarctica Winter Sea Ice (Fig. 12-17)
14