Earth Orbit, Solar Insolation Variability & Sea Level Change

Is there a way that Earth's climate can oscillate on time scales not likely to be attributed to pure tectonics (i.e., time scales less than a million years)? What mechanism(s) can help the earth system return to warmer climate conditions?

A cyclic/repetitive process can be invoked when one considers mechanisms that vary the amount of insolation (incoming solar radiation) delivered to the Earth according to a specific tempo or beat. As noted in the Hansen curve, there appears to have been a regular spacing of glacial maximum events, at roughly 120 thousand years (ky). As it turns out, variations in the amount of light energy delivered to the Earth's surface per unit area might be driven, at least in part, by an extrinsic variable related to the distance of the Earth from the sun, the orientation of the Earth relative to the incoming solar energy, and the uniformity of Earth's rotational pattern. National Geographic Channel has a great short video that explains how these "Milankovitch Cycles" work ^{(When last checked unavailable)}.

Scientists have determined that the Earth exhibits variations in its solar orbit from a more elliptical to a more spherical-shaped orbit on scales of roughly 120,000 years. This is termed eccentricity. In addition, the Earth experiences a wobble in its rotation axis much the way that a basketball begins to wobble as it slows its spin on a globe trotter's finger. This is termed axial precession and has durations of up to 20,000 years or so.

Finally, the tilt angle of the rotation axis of the Earth also contributes to variations in the amount of light energy that reach the Earth's surface at different latitudes. The Earth's polar axis can tilt toward or away from the sun. This results in more energy delivery to higher latitudes when the tilt is toward the sun, or more energy delivery concentrated at the equator when the tilt is less severe. You may recall from prior Earth Science classes that the tilt angle of the Earth is responsible for our annual seasons. Currently, during the northern hemisphere summer, the northern hemisphere is tilted toward the sun, and vice versa during the southern hemisphere's summer. Termed obliquity, the angle of tilt varies on scales of approximately 40,000 years. Thus when the axial tilt is more vertical, more energy is concentrated on the equator, less is delivered to the poles, and cooler climates prevail. When the axial tilt is more toward the horizontal, more energy is delivered to higher latitudes and this produces severe seasons as the Earth makes its annual trip around the sun.

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Figure 4.27: This figure is a composite of diagrams designed to show each of the three types of cycles outlined here and in the National Geographic Video. From left to right these are Eccentricity, Obliquity, and Precession.

Credit: Public Domain downloaded from Wikipedia: Milankovitch Cycles

Collectively, variations in earth's orbit (eccentricity, obliquity, and precession) can either reinforce signatures of cooling or warming, or they can work to counteract each other and produce less severe or ameliorated climate change. When the multiple variables reinforce each other, the amount of climate change can be substantive. As a result, so, too, can the amount of sea level change. When variations in earth's orbit produce repetitive changes in climate and sea level, the observed cycles are often referred to as Milankovitch Cycles. Many sedimentary rock sequences have been shown to have stacking patterns that reflect these time scales, as do ice core data.

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Figure 4.28: Photograph of Late Ordovician rocks exposed on the southern side of the Ohio River at Maysville, Kentucky. The rocks in this road cut show beautiful cycles of repetitive layers that are stacked in patterns (i.e., 5 mini cycles inside of one larger cycle, etc.). Are these ~20 ky precession cycles embedded within ~100 ky eccentricity cycles? This outcrop and many others in Cincinnati and Ohio, have been used as evidence for high-frequency sea level oscillations and changes in the frequency and intensity of major storms during the Late Ordovician, a time that was transitioning from greenhouse climates to a time of ice house.

Credit: Photo courtesy of Sean Cornell.

For more information: check out this Climate Data Information webpage on Milankovitch Cycles