Climate Cycles-Part 2 Earth’s Obital Cycles Control Glacial Periods

There are several theories for the cause of glacial periods–Cycles of glaciers followed by  interglacial warm periods and then repeating that cycle. Of those theories, the Milankovitch Cycles theory seems to have a broad base of adherents who believe it to have the best answer that question.   The Milankovitch theory has some weaknesses.  So this posting remains skeptical, however, it appears that there are good reasons why it is probably the most accepted theory.  That Richard Lindzen is a supporter of the theory is one of the good reasons.

The Milankovitch theory says that moving in and out of glacial periods is a result of variation in the Earth’s orbit and orientation.  Three parameters—Earth’s eccentric orbit around the Sun, the planet’s axial tilt and the procession of its axis are the basis of the theory.   These parameters are pretty well defined.   The coincidence with certain combinations of the three parameters and the paleohistory of glacial periods is reasonably close.   A posting by Doug Hoffman on his blog, the Resilient Earth “Confirmed! Orbital Cycles Control Ice Ages” is very good.  I could not say it as well, so this posting  will lift much from his.  

From Hoffman’s posting:

Earth’s orbit goes from measurably elliptical to nearly circular in a cycle that takes around 100,000 years. Presently, Earth is in a period of low eccentricity, about 3%. This causes a seasonal change in solar energy of 7%. The difference between summer and winter is a 7% difference in the energy a hemisphere receives from the Sun. When Earth’s orbital eccentricity is at its peak (~9%), seasonal variation reaches 20-30%. Additionally, a more eccentric orbit will change the length of seasons in each hemisphere by changing the length of time between the vernal and autumnal equinoxes. (Click on the Chart to enlarge.)

Variation in Axial Obliquity, Orbital Eccentricity, and Polar Precession.NOAA.

The variation in eccentricity doesn’t change regularly over time, like a sine wave. This is because Earth’s orbit is affected by the gravitational attraction of the other planets in the solar system. There are two major cycles; one every 100,000 years and a weaker one every 413,000 years.

The second Milankovitch cycle involves changes in obliquity, or tilt, of Earth’s axis. Presently Earth’s tilt is 23.5°, but the 41,000 year cycle varies from 22.1° to 24.5°. This tilt is depicted in the upper-left panel of the illustration above. The smaller the tilt, the less seasonal variation there is between summer and winter at middle and high latitudes. For small tilt angles, the winters tend to be milder and the summers cooler. Cool summer temperatures are thought more important than cold winters for the growth of continental ice sheets. This implies that smaller tilt angles lead to more glaciers.

The concept is that the cooler summers are unable to melt the previous winters ice, so it accumulates and begins glacier building.

The third cycle is due to precession of the spin axis. As a result of a wobble in Earth’s spin, the orientation of Earth in relation to its orbital position changes. This occurs because Earth, as it spins, bulges slightly at its equator. The equator is not in the same plane as the orbits of Earth and other objects in the solar system, as shown in the illustration below.

Precession of Earth’s axis of rotation.

The gravitational pull of the Sun and the Moon on the equatorial bulge tries to bring Earth’s spin axis into perpendicular alignment with the orbital plane. Earth’s rotation is counter clockwise; gravitational forces make Earth’s rotational axis move clockwise in a circle around its orbital axis. This is called precession of the equinoxes because, over time, the retrograde axial rotation causes the seasons to shift.

Until recently, variations in the intensity of high-latitude Northern Hemisphere summer insolation, driven largely by precession, were widely thought to control the timing of glacial terminations. However, it has been suggested that changes in Earth’s obliquity may be a more important mechanism. According to the paper by R. N. Drysdale et al., “Our record reveals that Terminations I and II are separated by three obliquity cycles and that they started at near-identical obliquity phases.”

During the Late Pleistocene, the period of glacial-to-interglacial transitions (or ‘terminations’) has increased when compared to the Early Pleistocene. The length of the cold glacial periods shifted from ~40,000 years to ~100,000 years in length some 700,000 years ago.   Although many different explanations have been proposed for the shift, the most widely accepted one invokes changes in the intensity of high-latitude Northern Hemisphere summer insolation (NHSI). These changes were thought to be primarily driven by precession, which produces relatively large seasonal and hemispheric insolation variation.

The new work by Drysdale et al. claims that obliquity, not precession, is the proximate cause of glacial terminations. Moreover, based on a detailed study of the last two terminations (T-I and T-II), it was found that glacials can span multiple obliquity cycles. The researchers make the case for obliquity as the forcing mechanism:

Based on our results, both T-I and T-II commence at the same phase of obliquity and the period between them is exactly equivalent to three obliquity cycles (~123 ky). Obliquity is clearly very important during the Early Pleistocene, and recently a compelling argument has been advanced that Late Pleistocene terminations are also forced by obliquity, but that they bridge multiple obliquity cycles. Under this model, predominantly obliquity-driven total summer energy is considered more important in forcing terminations than the classical precession-based peak summer insolation model, primarily because the length of summer decreases as the Earth moves closer to the sun.

Timing of the Termination II was established by matching a uranium–thorium (U–Th) chronology derived from a high-resolution speleothem δ¹⁸O time series to the T-II marine sediment record from the Iberian margin in the northeast Atlantic Ocean. A speleothem is a secondary mineral deposit formed in caves.

Figure 3 from Drysdale et al., Science express 13 Aug 2009.

Shown above is a comparison of the benthic δ¹⁸O record through T-I (orange crosses; plotted on the upper timescale) and T-II (black crosses; plotted on the lower timescale), showing similarities in the duration of both terminations. Southern Hemisphere summer insolation at 65°S (blue) and obliquity curves (red) for T-I (dashed lines) and T-II (solid lines), and obliquity. The gray vertical bar marks the commencement points for both terminations, revealing an age difference of ~123,000 years, which is equivalent to three obliquity cycles of ~41,000 years each. As the author’s put it: “Our record reveals that Terminations I and II are separated by three obliquity cycles and that they started at near-identical obliquity phases.”

While the earlier paper by Hönisch et al. showed that CO₂ could not be the driver of glacial-interglacial transitions (again see “Change In Ice Ages Not Caused By CO₂”), this paper shows that change in obliquity is the probable trigger for the onset of global warming. Indeed, obliquity also nicely matches the previous ~40,000 year glacial-interglacial cycle that had been dominant prior to ~700,000 years ago.

If you want to see additional discussion on obliquity and precession, click on Milankovitch Cycles Precession and Obliquity.

See Climate Cycles-Part 1 Glacial Periods part 1 by clicking here.

cbdakota