Long-term climate change is one of the most unexplainable things in science; we have theories for why it happens, but we don’t know for sure why it happens. The best theories we have favor changes in patterns in Earth’s orbit and changes in axial tilt, alongside long-term atmospheric and ocean circulation changes. In the early 1900s, Milutin Milankovic, a Serbian astronomer proposed a set of theories for why Earth goes into glacial and interglacial periods within the present ice age (last 2 million years). The theories are based on cyclical changes in Earth’s revolution and rotation around the Sun on an inconsistent axis on various time scales; they are eccentricity, axial tilt, and precession.
These cycles have been one of the biggest causes for glacial and interglacial periods over the Quaternary period (last 2.5 million years). They even help explain the last 10,000 to 20,000 years of Earth’s climate history, beginning with the end of the last glacial period.
These cycles alone do not have direct effect on the climate system, but rather they influence the climate indirectly. These changes alter the amount of solar radiation that the Earth receives at its surface. Decreased solar radiation is associated with glaciation and increased solar radiation is associated with the retreat of glaciers and ice sheets. However, despite these changes resulting in decreased or increased solar radiation, they do not reduce the total amount of solar energy. Rather, they impact the “seasonality” and location of the solar radiation around Earth.
Eccentricity is perhaps the most widely known Milankovitch Cycle. Eccentricity refers to the shape and extent of the elliptical orbit around the Sun. The shape of the orbit is constantly fluctuating as the elliptical shape ranges from 0% to 5% ellipticity on a time scale of +/- 100,000 years (Figure 1).
The Earth’s orbit changes from more elliptic to less elliptic. This change is the main reason for glaciation and glacial retreat on long-term time scales. Eccentricity alters the distance of the Sun’s short wave radiation to reach Earth’s surface.
Our orbit is currently 3% elliptical, which means that we are right in the middle of where we are in the cycle. This 3% means that we have seen a 6% increase in solar radiation in January, than in July.
AXIAL TILT / OBLIQUITY
Axial tilt is perhaps the second most widely known Milankovitch Cycle. It is the tilt in degrees of the Earth’s axis as it makes its revolution around the Sun. The Earth’s axis is inconsistent in the sense that it isn’t always tilted at the same degree. The inclination of the Earth’s axis changes from 21.5 degrees to 24.5 degrees and vice versa on 41,000 year time scales (Figure 2).
We are currently tilted at 23.5 degrees, which is the main reason that we currently have very distinct seasons in both hemispheres. When the axial tilt is less, such as when it is at its minimum 21.5 degrees, the solar radiation becomes more evenly distributed around the globe between winter and summer, while in general creating a large difference in the radiation between the equatorial and polar regions.
Less axial tilt has been theorized to result in the growth of glaciers, because of the warmer winters and cooler summers associated with this degree of tilt. Warmer winters mean that the oceans are also warmer, which means that there is an increase in evaporation during these times. With more water vapor in the atmosphere, that means that there will be more precipitation, and during the winter, that means that there will be more snowfall in the poleward regions. The increase in snowfall promotes the growth of ice sheets and glaciers. With cooler summers, less glacier and ice sheet melt occur.
The lesser known cycle is the Earth’s precession. Precession is simply the Earth’s slow wobble as it rotates on its axis and revolves around the Sun. While we have the axial tilt cycle on 41,000 year time scales, we have small variations in the axial tilt, known as the wobble, on 23,000 year time scales. The Earth’s axis will go from pointing at the North Star, Polaris, to pointing at the star Vega (Figure 3).
Despite being such a small variation, precession has large effects on global climate on long-term time scales. When the axis is pointed toward Vega, the Northern Hemisphere winter and the Southern Hemisphere summer will occur at the same time as the aphelion and perihelion, respectively. Simply put, this means that the Northern Hemisphere’s winter takes place when the Earth is farthest from the Sun and summer occurs when the Earth is closest to the Sun. When this occurs, it causes large seasonal contrasts. Currently, we are in the opposite stage, since the axis is currently tilted at the star Polaris. This means that the Northern Hemisphere winter occurs when Earth is closest to the Sun and summer occurs when Earth is farthest from the Sun.
Precession also changes the timing of the Earth’s seasons. This is referred to as elliptical precession. Despite eccentricity changing the shape of Earth’s orbit on 100,000 year time scales, elliptical precession also plays a role in the shape of the Earth’s orbit around the Sun as it rotates around a focus point (Figure 4).
WHAT CAUSES THESE CYCLES?
These changes in Earth’s orbit and are mainly due to gravitational pulls by other planets, which have similar cycles too. Jupiter, for example, has moderate eccentricity, thus if larger, it would have larger impact on Earth’s.
However, changes in Earth’s axial tilt have been theorized to exist because of the torque exerted by the gravitational pull between the Sun and the moon on Earth. Precession is also made possible by the tilt of the Earth’s orbital plane.
The timing of these cycles are constantly changing, very slowly, and they are getting ever so slightly slower with each passing cycle due to different reasons.
Colose, Chris. “Climate Science Glossary.” Skeptical Science, Skeptical Science, 22 July 2011, skepticalscience.com/Milankovitch.html.
“Milankovitch Cycles and Glaciation.” Milankovitch Cycles and Glaciation, Indiana University, http://www.indiana.edu/~geol105/images/gaia_chapter_4/milankovitch.htm.