Planetary Forcing of Global Climate?
Earth-Sun Orbital Cycles and Global Climate Change
The Milankovitch/Croll Theory
Jackie Smith
Global Climate Change-2003
Abstract
Global climate has fluctuated cyclically throughout geologic time. The Pleistocene epoch
spans back 2 millions years when the waxing and waning of extensive ice caps began to
occur with fascinating regularity. Milankovitch and Croll have shown mathematically
that the orbital geometries of earth around the sun occur with sinusoidal regularity. The
theory goes on to suggest that these orbital patterns are inducing global climate cycles.
The orbital patterns are eccentricity, obliquity, precession and inclination, all of which
have an effect on the amount of solar insolation that reaches earth’s atmosphere. Tests of
this theory require analysis of the rock record. Geologists search for evidence of
Milankovitch cycles by studying atmospheric gases trapped in ice, oxygen isotope ratios,
fossil assemblages in deep-sea sediments and coastal response to change in sea level.
Outline
I.
II.
III.
IV.
Introduction
Solar energy
a. Equinox
b. Insolation
Orbital cycles
a. Precession
i. Axial precession of the earth
ii. Precession of the equinoxes
b. Obliquity
c. Eccentricity
d. Inclination
Supporting Evidence
a. Spectral Analysis
b. Deep-sea cores
c. Atmospheric Vostok Ice Core
d. Coral reef terraces
“The trouble about arguments is they ain’t nothin’ but theories, after all, and theories
don’t prove nothin’. They only give you a place to rest on a spell when you are tuckered
out buttin’ around trying to find out somethin’ there ain’t no way to find out. And there’s
another thing about theories: there’s always a hole in ‘em sure, if you look close enuf.”
Huckleberry Finn, in Tom Sawyer Abroad by Mark Twain (Muller, 2001)
Introduction
The task of identifying and explaining fluctuations in global paleoclimate is at
hand and it is complicated. The problem has been approached theoretically in 1864 by
James Croll and later, in 1924 by Milutin Milankovitch. Their theory proposes that the
forces driving global cycles of glaciation are patterns in earth’s orbit around the sun. The
periodicities of these orbital patterns; eccentricity, inclination, obliquity and precession
are described mathematically to be regular and predictable. Therefore, the incidence of
glaciation as a response to galactic forcing is expected to be regular and predictable. The
Milankovitch/Croll theory is the only one of its kind that can be tested from the rock
record. Other proposed mechanisms, which will not be discussed here but may influence
large-scale global climate include differences in the amount of energy emitted by the sun
(i.e. sunspots), insolation interference by interstellar dust, volcanic dust and earth’s
magnetic field (Hays, 1976).
At a time when the astronomical theory was being developed, researchers had
begun to describe geologic evidence of ancient ice ages throughout Northern Europe and
America. The old school of thought in support of a great flood having deposited till and
erratic boulders was eliminated thanks to the observations of Louis Agassiz’s (among
others) of striations, loess, and glacial till. Geologists were changing their views to
understand major glaciations that had occurred several times throughout Earth’s history.
The next step toward understanding paleoclimate was to assign ages to the glacial
epochs. Here in lies the problem because the older the geologic formation, the more
complicated it is to decipher and assign accurate ages. Early attempts at dating the last
glacial epoch were based on erosion and recession rates at Niagara Falls and St. Anthony
Falls since the rivers were formed from melt-waters during the last glacial retreat. The
estimates calculated were not accurate enough, however. Brunhes and Matuyama
detected a reversal of the magnetic poles occurring 700 kya and this provided an
excellent time reference that could be found throughout the globe. Finally, with the
advent of radiocarbon and U-series dating techniques, it was possible to estimate the
timing of glacial cycles with precision and to test the periodicities proposed in
Milankovitch's astronomical theory. (Imbrie and Imbrie, 1979)
Solar Energy
Vernal and autumnal equinox occur when the poles are perpendicular to the sun.
At equinox, late March and September, day and night are of equal length. Summer and
winter solstice occur when the poles are tilted furthest away from the sun, resulting in the
longest and shortest day within a year on either hemisphere.
Insolation is the amount of incident solar radiation received by the upper
atmosphere. Insolation values are derived from orbital cycles of precession, obliquity
and eccentricity as a function of latitude, season and time (Hays, 1976). When
comparing geologic data to the orbital periodicities, it is necessary to know what season
is most crucial to the survival of a glacier. Initially, Croll believed that the occurrence of
wintertime solstice along with maximum eccentricity triggered an ice age (Imbrie and
Imbrie, 1979). However, the earth’s obliquity causes longer summer days and shorter
winter days at higher latitudes, allowing greater insolation during summer months when
the ice is most vulnerable to destruction. It is now accepted that the amount of
summertime solar radiation received at different latitudes is a controlling factor in the
advance and retreat of glaciers. So long as summertime melt does not exceed wintertime
accumulation, a glacier will continue to grow. The spectra of July insolation in northern
latitudes are correlated to orbital periodicities (Figure 1)(Muller, 2001). Milankovitch
proposed a critical factor to be the summertime insolation at 65oN but matches have been
made with insolation from different latitudes.
There is an insolation peak at 48,000 years that has caused some controversy over
the validity of the Milankovitch theory. One must remember that there are four different
orbital variables and they are not necessarily occurring in tune with each other. For
instance, a high precessional index occurring with a low obliquity could result in the two
phases canceling out any dramatic climate response. (Broecker, 1966)
Orbital Cycles
The idea that the sun and earth’s orbit around it can drive ice ages first came
about in 1842 with a book by Joseph Alphonse Adhemar called Revolutions of the Sea.
Here it is argued that due to precession of the equinoxes, one-hemisphere experiences
more hours of darkness per year and consequently is colder. Since earth completes one
cycle of precession every 22,000 years, ice ages would have occurred in alternating
hemispheres every 11,000 years. This idea was corrected when von Humbolt
demonstrated that any solar radiation that is lost in one season is compensated for in the
next and each hemisphere receives equal amounts of heat. Croll realized that the albedo
of the ice sheets acts to lower temperatures by decreasing solar radiation, described as
“positive feedback”. (Imbrie and Imbrie, 1979)
Imbrie and Imbrie (1979) discuss precession in terms of the axial precession of
the earth and the precession of the equinoxes (sin). Axial precession is the circular
motion of earth about its axis of rotation due to the gravitational pull of the sun and moon
on earth’s equatorial bulge. (Figure 2) The axis of rotation wobbles about the tilt of its
axis, comparable to a spinning top. This motion is difficult to envision since we already
describe variations in the axis of tilt as the obliquity. Precession of the equinoxes is the
delay between perihelion and summer. The rotation of precession travels against the
eccentricity rotation, like two conveyor belts moving in opposite directions with
eccentricity moving at a very slow rate compared to precession. (Muller, 2001) The
rotation of Earth about its elliptical orbit causes variability in the distance between the
sun and the timing of the equinoxes.
Eccentricity (e) is the shape of earth’s orbit around the sun (Figure 2). It is
measured by specifying the distance between two foci as a percentage of the long axis of
the ellipse. The earth is 93 million miles from the sun and receives only 1/2 of one
billionth of the total emitted solar energy. Slight variation in the circular shape of earth's
orbit effects the amount of solar radiation that reaches the surface of earth. A perfectly
circular eccentricity = 0. Earth's eccentricity ranges from 0.005-0.06 and it is presently
0.0167 (site). There is a three million-mile difference between aphelion and perihelion
due to the elliptical shape of earth’s orbit. (Imbrie and Imbrie, 1979) James Croll added
that in addition to precessional cycles, eccentricity variations cause solar radiation to
change enough to influence ice sheet growth. When eccentricity is low, as it is today
(~1%), winters are mild and high eccentricity (up to 6%) results in harsher winters. The
periodicity of eccentricity is 400 kyr, 125 kyr, 105 kyr and 95 kyr (often listed as one 100
kyr cycle). (Muller, 2001)
Obliquity () is the tilt of Earth's axis relative to the orbital plane of the sun
(Figure 2). Currently, earth is tilted 23.5o and ranges between 22-24.5o. One cycle of
obliquity is completed every 41 kyr. (Imbrie and Imbie, 1979) Earth's obliquity is why
we have different seasons. For half of the year (summer), when the Northern Hemisphere
is pointed toward the sun, the days are longer and the amount of solar radiation received
is greater. When the Northern Hemisphere is pointing away from the sun (winter), the
days are shorter and temperatures become lower. The timing is exactly the opposite in
the Southern Hemisphere and the seasons are offset by six months.
Orbital inclination is the plane of earth’s ecliptic orbit relative to the reference
plane of the sun (Figure 2). The torques of Jupiter, Venus and Saturn influence the
inclination of the plane. It has a periodicity of 100 kyr. Depending on the orbital
inclination and shape of eccentricity, the earth may be brought into a field of interstellar
dust. Upon entering the atmosphere, the dust interferes with solar energy and decreases
insolation. Less understood is the effect of interstellar dust in the upper atmosphere on
the glacial budget. (Muller, 2001)
Spectral Analysis
The data can be manipulated through spectral analysis to find regular frequency
peaks. It is a superposition of a few pure frequencies plus background where the data is a
sum of individual oscillations. The data often require a tuning adjustment made to the
spectra for errors in time scale due to sedimentation rate variability or a lag response of
the system to the inducing factor. (Muller, 2001)
Deep Sea Cores-18O-SPECMAP
The oceanic oxygen isotope ratio (18O) is essentially a reflection of the global ice
budget (Figure 3). This is because the vapor pressure of H216O is higher causing it to
evaporate more readily (Muller, 2001). Weather systems feed the poles with water that is
enriched in oxygen-16. During an ice age, large volumes of 16O are trapped in glaciers
and ocean water becomes 18O-rich. When global ice volume is low, the oceans are rich in
both oxygen-16 and oxygen-18. This fundamental principle is applied in isotope studies
of benthic foraminifera tests collected from deep-sea sediment cores (Figure 4) (Muller,
2001; Imbrie, 1979; Hays, 1976). The 18O of pelagic forams reflect both water
composition and sea surface temperature where it may not be possible to quantify
individual variables. Alternatively, benthic forams are forming their tests in an
environment where oxygen isotope ratios should not be effected by temperature.
Vostok Ice Core-18O
Pockets of atmospheric gas are trapped in glaciers where they are stored until the
ice melts. There is residual ice located in the most remote glacial regions, such as
Antarctica, that has survived the long interglacial periods. The Vostok ice core contains
423 kyrs of ice accumulation at Antarctica. Atmospheric concentrations of oxygen and
methane change in response to the extent of glacial ice cover. The respiration of plants
and animals change atmospheric gas concentrations and biologic activity is affected by
climate fluctuation. When there is greater ice volume and sea level is low, biologic
activity is decreased so that there are fewer coastal marshes producing methane and
selective consumption of oxygen isotopes is decreased. (Muller, 2001)
The atmospheric record and spectra of methane (Figure 5) show a periodicity of
100 kyr. Muller (2001) has linked the inclination effect to it because the spectra do not
detect a 400-kyr-eccentricity peak. Oxygen concentrations have a cycle occurring at 105
kyr, 41 kyr, 23 kyr and 19 kyr (Figure 6). These periodicities are comparable to the
SPECMAP marine isotope record; the only discrepancy being the intensity of spectral
peaks. Factors considered to effect atmospheric carbon and oxygen do not sufficiently
account for all atmospheric variability. (Muller, 2001) The marine isotope record is
currently the best indicator of glacial volume and paleoclimate.
Coral Reef Terraces
Exposed coral reef terraces on the island of Barbados were formed at highstands
of sea level (interglacial period). The use of Barbados reefs in sea-level studies is
complicated by the fact that the terraces are not in their original position. Tectonic
activity has raised the corals, making it difficult to know where sea level was when the
reefs formed. However, the age of each coral terrace does correlate to the timing of sealevel highstands. Radiometric dating techniques (234U/238U and 230Th/234U) applied to the
terraces yield ca. 80 kyr, 105 kyr, 125 kyr, 170 kyr and 230 kyr. These ages compare
well to Northern Hemisphere summer insolation and eccentricity (Mesolella, 1969;
Imbrie and Imbrie, 1979). During periods of high eccentricity there are interglacial
periods and sea-level highstands. The frequency of insolation and the occurrence of
highstand is higher than eccentricity suggesting that there is another factor influencing
global glaciation cycles. Summer insolation occurs in tune with the coral terraces formed
at interglacial epochs.
Conclusion
Not all evidence supports the Milankovitch theory. The data often must be
tweaked and different sequences compared until there is a match between the periodicity
of geologic record and astronomical cycles. One must also be aware that tweaking can
cause false associations by forcing cycles to compare when there is no true connection.
Uncertainties aside, it still remains that there is a strong eccentricity signal in the data,
although logically the effect of eccentricity is expected to be minimal. The precession
and obliquity cycles are also leaving an imprint on paleoclimate records.
Research Materials
Text:
1) Broecker, W. S. (1966) Absolute Dating and the Astronomical Theory of
Glaciation. Science 151, 299-304.
2) Mesolella, K. J., et al. (1969) The astronomical theory of climatic change:
Barbados data. Journal of Geology, 77, 250-274.
3) Hays, J. D., et al. (1976) Variations in the Earth’s Orbit: Pacemaker of the Ice
Ages. Science 194, 1121-1132.
4) Imbrie, J. and Imbrie, K. P. (1979) Ice Ages: Solving the Mystery. ed. Enslow
Publishers, Short Hills, NJ. 224 p.
5) Muller, R. A. and MacDonald, G. J. (2000) Ice Ages and Astronomical Causes:
Data, Spectral Analysis and Mechanisms. Ed. Praxis Publishing, Chichester, UK.
314 p.
6) Blanchon, P. and Eisenhauer, A. (2001) Multi-stage reef development on
Barbados during the Last Interglaciation. Quaternary Science Reviews, 20, 10931112.
7) Berger, A. L. (1977) Support for the astronomical theory of climatic change.
Nature, 269, 44-45.
Web:
1) http://www.cco.caltech.edu/~bachmann/obliq/obliq.htm.
2) http://www.ldeo.columbia.edu/~polsen/nbcp/cmintro.html
3) http://www.ou.nl.open/dja/Klimaat/System/solar_radiation_and_milank.htm
4) http://www.-istp.gsfc.nasa.gov/stargaze.Sprecess.htm
5) Manchester Metropolitan University
Figure 1. Eccentricity spectral peak is absent (.0025, .008, .0105). Obliqity (.024) and
Precession (.042, .045, .053) signals are strong with obliquity being dominant in southern
hemisphere. (Muller, 2001)
Figure 2. Eccentricity, precession and obliquity of earth.
(www.ldeo.columbia.edu/~polsen/nbcp/cmintro.html)
Figure 3. Oxygen isotope exchange and the effect of glaciers on oceanic isotope ratios.
(Muller, 2001)
Figure 4. Marine isotope stages showing cyclic 100 kyr incidence of glaciation. (Muller,
2001)
Figure 5. Methane record from Vostok ice core. (Muller, 2001)
Figure 6. Comparison of Vostok and SPECMAP 18O. (Muller, 2001)