mechanisms of past climate change (16:107:553) fall 2007 ice ages and changes in earth’s orbit
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Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Ice Ages and Changes in Earth’s Orbit
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Topic Outline
• Introduction to the Quaternary
• Oxygen isotopes as an indicator of ice volume
• Temporal variations in ice volume
• Periodic changes in Earth’s orbit
• Relationship between orbital changes and variations in ice volume
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Topic Outline
• Introduction to the Quaternary
• Oxygen isotopes as an indicator of ice volume
• Temporal variations in ice volume
• Periodic changes in Earth’s orbit
• Relationship between orbital changes and variations in ice volume
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Geologic Time Scale
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Geologic Time Scale
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Geologic Time Scale
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
The Quaternary Period
• In the first half of the 19th century, Louis Agassiz argued that widespread glaciation was the explanation for various unusual geologic features in much of North America and Europe.
• A lengthy scientific debate ensued, but the evidence for a number of continental glaciations gradually became accepted.
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Moraines• As a glacier advances,
its leading edge acts like the blade of a bulldozer, pushing rock and debris in advance.
• These remnants of glaciation, called terminal moraines, mark the location of maximum ice extent.
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Moraines• As a glacier advances,
its leading edge acts like the blade of a bulldozer, pushing rock and debris in advance.
• These remnants of glaciation, called terminal moraines, mark the location of maximum ice extent.
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Moraines• As a glacier advances,
its leading edge acts like the blade of a bulldozer, pushing rock and debris in advance.
• These remnants of glaciation, called terminal moraines, mark the location of maximum ice extent.
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
The Surface of the Ice Age Earth
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
LGM Ice Extent in the Northeastern United States
Moraines from earlier glaciations are most often destroyed by subsequent glaciations, so moraines are generally evidence of the most recent glacial advance.
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Topic Outline
• Introduction to the Quaternary
• Oxygen isotopes as an indicator of ice volume
• Temporal variations in ice volume
• Periodic changes in Earth’s orbit
• Relationship between orbital changes and variations in ice volume
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Oxygen Isotopes
• A small fraction of water molecules contain the heavy isotope 18O instead of 16O.
• 18O/16O ≈ 1/500
• This ratio is not constant, but varies over a range of several percent.
• Vapor pressure of H218O is lower than that
of H216O, thus the latter is more easily
evaporated.
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
18O
• As water vapor is transported poleward in the hydrologic cycle, each cycle of evaporation and condensation lowers the ratio of H2
18O to H216O, in a process called
fractionation.
• This ratio is expressed as 18O.
10001618
1618161818
std
stdsample
OO
OOOOO
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
18O vs. Temperature
• As a consequence of fractionation, 18O in precipitation decreases with decreasing temperature.
• Ice sheets have very low 18O values.
Observed 18O in average annual precipitation as a function of mean annual air temperature (Dansgaard 1964). Note that all the points in this graph are for high latitudes (>45°). (From Broecker 2002)
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
18O and Global Ice Volume
• As ice sheets grow, the water removed from the ocean has lower 18O than the water that remains.
• Thus the 18O value of sea water in the global ocean is linearly correlated with ice volume (larger 18O → larger ice sheets).
• A time series of global ocean 18O is equivalent to a time series of ice volume.
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Obtaining a 18O Time Series
• Microscopic marine organisms called foraminifera incorporate oxygen into their shells in the form of CaCO3.
• When these organisms die, their shells fall to the sea floor and are deposited in deep sea sediments.
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Obtaining Sediment Cores
• As sediments accumulate, the properties of the overlying ocean are recorded sequentially.
• Sediment cores are obtained by drilling into the sea floor.
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Obtaining Sediment Cores
• The sediments are analyzed, using both chemical and visual analysis.
• To produce a time series of ocean properties, a chronology or “age model” must be developed.
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Chronology
• A simple age model can be obtained by assuming a constant accumulation rate.
• Reversals in Earth’s magnetic field can be used for benchmarks.
• Magnetic reversals have been radiometrically dated.
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Chronology
• A simple age model can be obtained by assuming a constant accumulation rate.
• Reversals in Earth’s magnetic field can be used for benchmarks.
• Magnetic reversals have been radiometrically dated.
Brunhes-Matuyamamagneticreversal
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Other Sources of 18O Variation
• Complicating factor: Changes in ice volume are the largest contributor to 18O variations, but they are not the only one.
• Regions of the ocean in which evaporation exceeds precipitation are enriched in 18O, and vice versa.
• Isotope separation between water oxygen and shell oxygen depends on temperature.
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Solution
• Changes in 18O driven by variations inP-E are largest near the ocean surface, so 18O from benthic (i.e., deep dwelling) forams are more representative of global ocean 18O.
• The Pacific deep ocean temperature is very close to freezing, so it could not have been much colder during glacial periods.
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Topic Outline
• Introduction to the Quaternary
• Oxygen isotopes as an indicator of ice volume
• Temporal variations in ice volume
• Periodic changes in Earth’s orbit
• Relationship between orbital changes and variations in ice volume
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Topic Outline
• Introduction to the Quaternary
• Oxygen isotopes as an indicator of ice volume
• Temporal variations in ice volume
• Periodic changes in Earth’s orbit
• Relationship between orbital changes and variations in ice volume
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Earth’s Orbit Can Vary
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Earth’s Orbit Can Vary
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Earth’s Orbit Can Vary
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Eccentricity
Eccentricity =(distance from focus to center) / (length of semimajor axis)
Eccentricity of Earth’s orbit varies from 0 to 0.05, with 100-kyr, 400-kyr and 2 Myr periodicities.
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Eccentricity
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Obliquity
Obliquity (i.e., tilt) of Earth’s axis varies from 22° to 24.5°, with a 41-kyr periodicity.
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Obliquity
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Precession
The Earth’s axis precesses, or wobbles, with periodicities of 19 kyr and 23 kyr.
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Precession
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Astronomical Theory of Ice Ages
• In 1842, J. Adhémar suggested that slow variations in Earth’s orbit could be responsible for climatic changes by altering the lengths of the seasons.
• In 1875, J. Croll hypothesized that orbital variations might lead to substantial changes in climate. (Colder winters → larger snow cover → glaciation)
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
• Renewed interest inorbital forcing of glacialcycles occurred whenM. Milankovitch (1941) computed long-term variations in insolation.
• Milankovitch believed that cold summers led to glaciation by allowing snow to survive into the next year.
Milankovitch
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Three Conceptual Models of
Orbital Effects on Glacial
Cycles
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Temporal Variation of Orbital Parameters
• Eccentricity: Relatively low for the past 60 kyr.
• Obliquity: Variations have been quite regular; current value of 23.5° near mean.
• Precession: Perihelioncurrently occurs near NH winter solstice.
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
In the N. Hemisphere, the effects of tilt and distance act in opposite directions, although tilt dominates.
In the S. Hemisphere, the effects of tilt and distance are in phase, yielding an amplified seasonal cycle of insolation.
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Insolation at 65°N
• High latitude summer insolation (June, 65°N) has been regarded as an index of orbital forcing of glaciation. (This is the original Milankovitch hypothesis: Cool summers are beneficial to ice growth.)
• Note that the effects of precession are modulated by eccentricity.
• For low summer insolation: Aphelion in summer (esp. with high eccentricity), low obliquity.
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Topic Outline
• Introduction to the Quaternary
• Oxygen isotopes as an indicator of ice volume
• Temporal variations in ice volume
• Periodic changes in Earth’s orbit
• Relationship between orbital changes and variations in ice volume
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Turning Point for Astronomical Theory of Ice Ages
• Hays, J. D., J. Imbrie, and N. J. Shackleton, 1976: Variations in the Earth’s orbit: Pacemaker of the ice ages. Science, 194, 1121-1132.
• “It is concluded that changes in the earth’s orbital geometry are the fundamental cause of the succession of Quaternary ice ages.”
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Peaks in 18O Spectrum
Correspond to Orbital
Frequencies
Variance spectra for marine oxygen isotopes for the last 700 kyr (lower curve) compared with spectra for Earth’s orbital parameters (Imbrie,1985). (From Broecker, 2002)
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Spectral Analysis of SPECMAP Stacked 18O Record
• Distinct peaks in ice volume record at orbital frequencies are present.
• These peaks are robust, even when more powerful spectral methods are used.
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
The 100-kyr Problem
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Model 1: Calder (1974)
0iikdt
dV
V = ice volumei = summer insolation at 65°Ni0 = insolation thresholdk = kA (accumulation) if i < i0
k = kM (melting) if i > i0
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Model 2: Imbrie and Imbrie (1980)
• Written in dimensionless form (i.e., variables are divided by a scaling value)
VV
dt
dV i
V = ice volumeVi = equil. ice volume at insolation ii = summer insolation at 65°N = M if V > i (melting) = A otherwise
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Model 3: Paillard (1998)
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Model 3: Paillard (1998)
• Very good agreement with record, both in time and frequency domain.
• Weakness: Highly nonlinear, with a number of adjustable parameters.
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Ice Core Paleoclimatology
• As snow falls on very cold glaciers or ice sheets and gradually is converted to ice, air is trapped in bubbles.
• This “fossil air” can be chemically analyzed to determine past atmospheric composition.
• Other paleoclimatic proxies (isotopes, dust, acidity) can also be determined from the ice, providing information about temperature, sulfate aerosols, precipitation.
Mechanisms of Past Climate Change (16:107:553)
Fall 2007
Multiproxy Analysis of Glacial Cycles
• Glacial-interglacial cycles are evident in a variety of paleoclimatic and paleoceanographic proxies.
• The shapes of the cycles vary somewhat among the different proxies.
• Glacial-interglacial variations in atmospheric CO2 concentration are substantial. (But what causes them?)
• There are uncertainties in time scales.