Milankovitch Cycles and Glaciation- Clarification and Further Explanation
Definition from lecture: Milankovitch cycles describe cyclic variation in Earth’s orbit and axis position, causing periodicities in solar insolation.
Milankovitch cycles were mentioned in lecture as one of the various earth-orbit periodicity theories that is predominant in explaining why glaciation occurs (also, why glaciation occurs in a cyclical manner). The Milankovitch theory suggests that normal cyclical variations in three of the Earth's orbital characteristics is most likely responsible for some past climatic change and glaciation.
Variations in the Earth's eccentricity, axial tilt (or obliquity), and precession make up the three dominant cycles, which are collectively known as the Milankovitch Cycles. The three Milankovitch Cycles impact the seasonality and location of solar energy around the Earth, thus impacting contrasts between the seasons.
So all the variations in all three cycles influence changes in the seasonality of solar radiation reaching the Earth's surface. These times of increased or decreased solar radiation directly influence the Earth's climate system, thus impacting the advance and retreat of glaciers.
The three Milankovich Cycles:
1) Eccentricity of Earth’s orbit
Eccentricity is the shape of the Earth's orbit around the Sun. The Earth's orbit is an ellipse, but constantly fluctuates- the orbit gradually changes from being elliptical to being nearly circular and then back to elliptical on a cycle of about 100,000 years. As mentioned in lecture, this periodicity correlates well with “ice age” occurrences over the last 700 to 800 ka BP. These oscillations, from more elliptic to less elliptic, are of important to glaciation in that they alter the distance from the Earth to the Sun, thus changing the distance the Sun's short wave radiation must travel to reach Earth, subsequently reducing or increasing the amount of radiation received at the Earth's surface in different seasons. Currently, the Earth is experiencing a period of low eccentricity.
2) Obliquity/tilt of axis
Axial tilt or obliquity, the second cyclical variation, results from the fact that as the Earth rotates on its polar axis it wobbles like a spinning top. This changes the orbital timing of the equinoxes and solstices. Oscillations in the degree of Earth's axial tilt occur on a periodicity of 41,000 years from 21.5 to 24.5 degrees. Today the Earth's axial tilt is about 23.5 degrees, which largely accounts for our seasons. Because of the periodic variations of this angle the severity of the Earth's seasons changes. With less axial tilt the Sun's solar radiation is more evenly distributed between winter and summer. However, less tilt also increases the difference in radiation receipts between the equatorial and polar regions. One hypothesis for Earth's reaction to a smaller degree of axial tilt is that it would promote the growth of ice sheets. This response would be due to a warmer winter, in which warmer air would be able to hold more moisture, and subsequently produce a greater amount of snowfall. In addition, summer temperatures would be cooler, resulting in less melting of the winter's accumulation. At present, axial tilt is in the middle of its range.
3) Precession of axis
Precession is the Earth's slow wobble as it spins on axis. This wobbling of the Earth on its axis can be likened to a top running down, and beginning to wobble back and forth on its axis. The precession of Earth wobbles from pointing at Polaris (North Star) to pointing at the star Vega. This top-like wobble, or precession, has a periodicity of 23,000 years.
When the tilt is small there is less climatic variation between the summer and winter seasons in the middle and high latitudes. Winters tend to be milder and summers cooler. Warmer winters allow for more snow to fall in the high-latitude regions. When the atmosphere is warmer, it has a greater ability to hold water vapor and therefore more snow is produced at areas of uplift. Cooler summers cause snow and ice to accumulate on the Earth's surface because less of this frozen water is melted. Thus, the net effect of a smaller tilt would be more extensive formation of glaciers in the polar latitudes.
Note: even when all of these variables favor glaciation, it would barely enough to trigger glaciation, not to cause large ice sheets to grow. Ice sheet growth requires the support of positive feedback loops, the most obvious of which is that snow and ice have a much higher albedo than ground and vegetation, thus ice masses tend to reflect more radiation back into space, thus cooling the climate and allowing glaciers to expand.
Information taken from http://www.homepage.montana.edu/~geol445, http://www.eoearth.org/article/Causes_of_climate_change, and
Manuel C. Molles Jr. Ecology Concepts and Applications, Fourth Edition.