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

  1. Introduction
  2. Solar energy
  3. Equinox
  4. Insolation
  5. Orbital cycles
  6. Precession
  7. Axial precession of the earth
  8. Precession of the equinoxes
  9. Obliquity
  10. Eccentricity
  11. Inclination
  12. Supporting Evidence
  13. Spectral Analysis
  14. Deep-sea cores
  15. Atmospheric Vostok Ice Core
  16. 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 sea-level 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. Science151, 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. Science194, 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, 1093-1112.

7)Berger, A. L. (1977) Support for the astronomical theory of climatic change. Nature, 269, 44-45.

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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. (

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)