The Milankovitch cycles are long term cycles (many thousands to tens of thousands of years) that result in changes in three factors which are the Earth’s tilt, the orientation of the Earth’s axis and the eccentricity of the orbit of the Earth around the Sun. The Earth spins like a top but the top of the top is tilted at an angle. If not tilted, the Earth’s tilt would be perpendicular to the direction of which sunlight is approaching the Earth (such as in diagram below labeled straight if the Sun is at a position directly right or left of the Earth). The Earth is currently tilted by 23.5 degrees. Thus, as the Earth goes around the Sun, during half of the year the tilt is toward the Sun while during the other half of the year, the tilt is away from the Sun. This is what primarily causes the seasons. This tilt of 23.5 degrees is not a constant however and changes from about 22 to 24.5 degrees over a 41,000 year cycle. A higher tilt results in greater seasonality while a smaller tilt results in less seasonality. Currently the tilt is in that part of the cycle where it will be moving from 23.5 to about 22 degrees over the next several thousand years.  The second cyclic factor to consider is called precession. Another term for this is tilt wobble. If you watch a spinning top, it can be noticed that the top of the top where the tilted axis is rotating can itself wobble in circles. Thus, the direction in space where the titled axis is pointing is changing in a circular motion. This orientation of the tilt changes in about a 25,000 year cycle. This cycle has an influence on seasonality. The perihelion is the date of the year that the Earth is closest to the Sun and the aphelion is the date of the year that the Earth is farthest from the Sun. When the Earth is tilted toward the Sun at a similar time as which perihelion occurs, then that hemisphere of the Earth will maximize solar intensity in summer. When the Earth is tilted away from the sun at a similar time as which aphelion occurs, then that hemisphere of the Earth will minimize solar intensity in winter. Currently (in Northern Hemisphere) the Earth is closest to the Sun in winter while it is farther away in summer. This has a minor impact of dampening the seasonal extremes. It is minor since the tilt of the Earth plays a more significant role in causing the seasons as compared to when the Earth is closest or farthest from the Sun since the eccentricity is close to circular in this day in age.  The third cyclic factor is called eccentricity. This is the shape of Earth’s orbit around the Sun. Generally this shape is a circle but it is not a perfect circle. The shape of Earth’s orbit around the Sun is an ellipse. Through geologic time, the orbit varies between being less elliptical (closer to a perfect circle) and more elliptical. The diagram below shows an example of a nearly circular and a more elliptical orbit. The current eccentricity is nearly circular. The current perihelion (date Earth is closest to the Sun) is around January 3rd and the current aphelion is around July 4th (date Earth is farthest to the Sun). The change is eccentricity is very slow and the cycle can take 100,000 to several 100,000 years to complete. When the Earth’s orbit is more elliptical then the climate tends to be cooler since the Earth spends more time in its orbit farther from the Sun. When considering an orbit, the orbit is faster when the planet is closer to the Sun and slower when it is farther away. It is due to a slower moving planet when it is farther from the Sun that results in the overall cooler climate during the course of a year when the orbit is more elliptical.  These three cycles overlap each other to create a variation in the amount of solar radiation that reaches the Earth over geologic time. Since the three cycles each have a different time scale, each individual factor can combine with the other factors to enhance them or dampen them with respect to changing the solar radiation received on Earth during the course of a year. The Milankovitch cycles are one factor that can produce climate change on Earth over many thousands of years and geologic time. Other factors include actual solar output from the sun, the orientation of the Earth’s continents, orientation of ocean currents, volcanic activity and the chemical makeup of the Earth’s atmosphere. |