19.6: What are Milankovitch Cycles?

What Are Milankovitch Cycles?

Over a century ago, Serbian scientist Milutin Milankovitch Links to an external site. hypothesized the long-term, collective effects of changes in Earth’s position relative to the Sun were a strong driver of Earth’s long-term climate and responsible for triggering the beginning and end of glacial periods Links to an external site. (Ice Ages). Milankovitch determined that over time, Earth’s movement in space affects the amount of radiation received from the sun. Collectively, his ideas are known as the Milankovitch Cycles Links to an external site. and include the following patterns of motion.

Eccentricity Links to an external site., the overall shape of Earth’s orbit around the Sun, alternates between more circular and more elliptical in a cycle that spans about 100,000 years, in part due to the gravitational influence of the gas giants, Jupiter and Saturn. Currently, Earth’s eccentricity is near its least elliptic (i.e. most circular) and is very slowly decreasing (Figure 19.9). For GIFS visit this NASA page Links to an external site..

On 100,000 year cycles, Earth’s orbit around the sun varies between almost a perfect circle to slightly elliptical.

Figure 19.9: Eccentricity, orbit shape. (Public Domain, NASA Links to an external site.)

Obliquity Links to an external site. is the angle of Earth’s axis of rotation relative to its ecliptic plane around the Sun. Obliquity is the reason Earth has seasons and is a very strong control on Earth’s climate and glacial periods. Typically, Earth’s obliquity varies between 22.1 and 24.5 degrees in a cycle that spans about 41,000 years, with minimum tilt favoring glaciation and maximum tilt promoting interglacial periods. Earth’s axis is currently tilted 23.4 degrees and is very slowly decreasing towards minimum tilt (Figure 19.10). For GIFS visit this NASA page Links to an external site..

On 41,000 year cycles, Earth’s axial tilt changes between 22.1 to 24.5 degrees.

Figure 19.10: Obliquity, axial tilt. (Public Domain, NASA Links to an external site.)

Precession Links to an external site. is the slight wobble of Earth as it rotates on its axis. This wobble is due to tidal forces caused by the gravitational influences of the Sun and Moon. The direction of wobble is determined relative to the fixed positions of stars; Earth is currently fixed to Polaris in the northern hemisphere but may wobble toward other North Stars, including Thuban and Kochab. The cycle of axial precession spans about 25,000 years (Figure 19.11). For GIFS visit this NASA page Links to an external site..

On 26,000 year cycles, Earth’s axis wobbles, like that of a slow moving toy top.

Figure 19.11: Axial precession, wobble. (Public Domain, NASA Links to an external site.)

When the cycles occur together at the same interval, they can influence Earth’s climate, and with other factors in place may encourage a glacial period or glaciation Links to an external site. to occur.