Net radiation key
All of the cycles described below are important because over time they have an affect on the net radiation received during a year and perhaps even more importantly, during summer seasons in the Northern and Southern Hemispheres. The higher that net radiation (Rn) during the summer season, especially the Northern Hemisphere summer, the warmer this planet will be. At times the three cycles (eccentricity, obliquity, precession) have been aligned to allow for more sun during the Northern Hemisphere summer and CO2 levels were high enough that our planet had a very high net radiation and was completely free of ice. At one point the Earth was entirely covered by ice.
Eccentricity
Eccentricity measures how much the shape of Earth’s orbit departs from a perfect circle. These variations affect the distance between Earth and the Sun. Eccentricity is the reason why our seasons are slightly different lengths, with summers in the Northern Hemisphere currently about 4.5 days longer than winters, and springs about three days longer than autumns.
As eccentricity decreases, the length of our seasons gradually evens out. The difference in the distance between Earth’s closest approach to the sun (known as perihelion), which occurs on or about January 5 each year, and its farthest departure from the sun (known as aphelion) on or about July 6, is currently about 5.1 million kilometers (about 3.2 million miles), a variation of 3.4 percent. That means each January, about 6.8 percent more incoming solar radiation reaches Earth than it does each July.
Currently, Earth’s eccentricity is near its least elliptic (most circular) and is very slowly decreasing, in a cycle that spans about 100,000 years. Currently, Earth’s eccentricity is near its least elliptic (most circular) and is very slowly decreasing, in a cycle that spans about 100,000 years. The total change in global annual insolation due to the eccentricity cycle is very small. Because variations in Earth’s eccentricity are fairly small, they’re a relatively minor factor in annual seasonal climate variations.
Figure 1. Images depicting the differences in obliquity, precession, and eccentricity over time.
Obliquity
The angle Earth’s axis of rotation is tilted as it travels around the sun is known as obliquity. Obliquity is why Earth has seasons. Over the last million years, it has varied between 22.1 and 24.5 degrees with respect to Earth’s orbital plane. Currently it is at 23.4 degrees and slowly decreasing. The greater Earth’s axial tilt angle, the more extreme our seasons are, as each hemisphere receives more solar radiation during its summer, when the hemisphere is tilted toward the sun, and less during winter, when it is tilted away. Favors deglaciation. As obliquity decreases, it gradually helps make our seasons milder, resulting in increasingly warmer winters, and cooler summers that gradually, over time, allow snow and ice at high latitudes to build up into large ice sheets. As ice cover increases, it reflects more of the Sun’s energy back into space, promoting even further cooling. Positive feedback!
Precession
As Earth rotates, it wobbles slightly upon its axis, like a slightly off-center spinning toy top. This wobble is due to tidal forces caused by the gravitational influences of the Sun and Moon that cause Earth to bulge at the equator, affecting its rotation. Axial precession makes seasonal contrasts more extreme in one hemisphere and less extreme in the other. Currently perihelion occurs during winter in the Northern Hemisphere and in summer in the Southern Hemisphere. This makes Southern Hemisphere summers hotter and moderates Northern Hemisphere seasonal variations. Every 13,000 years, axial precession will cause these conditions to flip, with the Northern Hemisphere seeing more extremes in solar radiation and the Southern Hemisphere experiencing more moderate
seasonal variations.
Figure 1. Basic information on Milankovitch cycles
