aphelion, apsidal precession, axial precession, Earth's orbit, Earth's Wobble, eccentricity, Global warming, Ice Ages, invariable plane, Milankovitch Cycles, Milution Milankovic, obliquity, orbit, orbital inclination, orbital plane, perihelion, solar system, Tilt, Warm Ages
PART II – Understanding the Milankovitch Cycles, Clues to Earth’s Climate Changes
RECAP: In Part I of this series, we looked at how scientists have determined that Earth has experienced regular cycles of cold climates followed by brief periods of warm climates during the last 400,000 years. We learned that the current cycle has been different because the warm period has persisted when past warm climates have rapidly dropped back into a cold climate. We also discussed how the Sun acts as a ‘battery charger’ for Earth’s climate.
Milutin Milanković proposed possible mechanisms related to Earth’s tilt and orbit that could be the root cause of the regular cycle of Ice Ages. His theory, outlined in several papers from 1912 to 1920, is now referred to as the Milankovitch Cycles¹. This theory outlines four factors that change the amount of solar radiation received by the Sun, which could explain why Earth experiences dramatic changes in its climate over a 100,000 year cycle. In addition, there is a fifth factor that has been added to the Milankovitch Cycle theory, which also follows a 100,000 year cycle and may also be contributing triggering our Ice/Warm Age cycles.
Eccentricity or Earth’s Orbit – From a Circle to Oval and Back
Earth’s orbit changes from a nearly perfect circle, to an oval (technically, an ellipse) over a period of thousands of years. The cause of this elongation or eccentricity of our orbit is due to the gravity influences of Jupiter and Saturn, which are much farther away from the Earth than the Sun, but exert enough pull to periodically stretch our orbit out of its circular shape.
In an orbit that is a perfect circle the amount of energy the Earth receives is relatively constant throughout the year, assuming the Sun is generating a constant amount of energy (which it doesn’t.) However, when Earth’s orbit is an ellipse (or more eccentric) the Earth receives more energy when it is closer to the Sun than when it is farther away.
Earth’s orbit eccentricity varies from .005 (low) to .058 (high) and the cycle of low to high eccentricity is roughly 100,000 years. Our orbit had major peak eccentricities of .04 to .06 at approximately 120, 220, 320 thousand years ago. These peaks fall about 10,000 to 20,000 years before the start of the last three Warm Ages.
Currently the eccentricity of our orbit is .017 and it is falling from a minor peak of about .02. That means that Earth’s orbit is about one-third the way from our lowest eccentricity and becoming more circular. The eccentricity of Earth’s current orbit creates about a three million mile difference between its closest and farthest approach to the Sun. Earth at perihelion (closest to the Sun) is 91.4 million miles away from the Sun. At aphelion the Earth is 94.5 million miles away.
Obliquity or the Severity of Earth’s Tilt
Currently the Earth is tilted at 23.44 degrees from the Sun’s orbital plane, but that is not constant. The Earth’s tilt or obliquity is decreasing from the high obliquity (tilt angle) of 24.4° towards the low of 22.1º. It will take Earth about 10,000 years to reach the low point in a 41,000 year cycle.
For comparison, Mars’ current obliquity is 25.19° and varies from 10° to 40° over hundreds of thousands of years. Venus’ obliquity is 177.4°, which means that the planet is so tilted that its ‘north’ is facing south. That may seem strange to have an obliquity greater than 90°; however, since Venus’ rotation is retrograde (Venus turns in the opposite direction of Earth) scientists consider its ‘up’ side to have been literally turned upside down.
Higher obliquity is believed to result in the Earth absorbing more solar radiation (insolation) because the higher latitudes receive more sunshine in the summer. Earth’s current Warm Age began at about the same time as our peak obliquity, so there is evidence that this theory is valid.
Axial Precession or Earth’s Wobble
Currently the north pole, or axis, points towards the star called Polaris. That is temporary because the Earth wobbles. This wobble is called the Axial Precession. Over time our north axis will no longer be aimed at Polaris, but instead will leave us without a ‘North Star’ until Earth’s north axis points to Vega, Deneb, or another bright star or galaxy.
It takes about 26,000 years for the wobble to complete one full cycle and during that cycle the Earth’s wobble will cause a slow change in the seasons. This is because the axis wobble alters the direction of our tilt during every orbit of the Sun. When the Earth returns to the same relative position in its orbit, the axis will point to a slightly different place than it did the prior year. The axis will have reached that point earlier, so our seasons slowly move backward.
Apsidal Precession or The Hula Hoop Effect
One of the more interesting factors is Apsidal Precession. If you think of Earth’s orbit as a hula hoop and your waist as the Sun (no, it’s not that big,) as the hula hoop goes around, the ‘orbit’ shifts. Any particular point on the hula hoop will move from being closest to your waist and then it will shift to be the farthest away from you waist. Our seasons do the same thing as Earth’s orbit slowly shifts or precesses.
Currently, summer in the northern hemisphere occurs when the Earth is the farthest away (aphelion) and in winter we are closest to the Sun (perihelion.) In the southern hemisphere it is exactly opposite. During the summer in the southern hemisphere (Earth at perihelion) it receives 23% more solar radiation than the northern hemisphere does during its summer, which occurs at the aphelion. It takes about 21,000 years for the Apsidal Precession to cause the seasons to make a full cycle, so in about 10,000 years, the northern hemisphere will experience summer at perihelion.
Orbital Inclination or Our Orbits Tilt From the Orbital Plane
By averaging the orbits of the eight planets scientists have created one plane that is considered the invariable plane. Jupiter is almost on this invariable plane; however, Earth and the other six planet’s orbital planes are tilted or inclined from the invariable plane.
Not only is Earth’s orbital inclination 1.57° off the invariable plane, the amount of tilt changes on a cycle that repeats every 100,000 years. Earth’s variance during that cycle can be as much as 3° off the invariable plane, which is additive to Earth’s obliquity or tilt on its axis. That means that increased orbital inclination magnifies the effect of Earth’s obliquity.
This factor was not part of Milankovic’s original theory; however, scientists have added it to the Milankovitch Cycle because it impacts the amount of insolation the Earth receives and because it follows the 100,000 year cycle.
In Part II, we have discussed five cyclical factors that change the amount of insolation the Earth receives and where Earth is in all five cycles. In Part III we look at how Earth’s climate seems to be on a hair-trigger and why we should or should not be in an Ice Age now.
PART III – Should We Be In An Ice Age Now?
NOTES AND REFERENCES
¹Wikipedia – The Free Encyclopedia. (2011). Milankovitch Cycles. Retrieved November 13, 2011, from http://en.wikipedia.org/wiki/Milankovitch_cycles.
IMAGE 1.0 – Image thanks to http://apollo.lsc.vsc.edu/classes/met130/notes/chapter16/mil_cycles.html
IMAGE 1.1 – Image Copyright 1999 by Calvin J. Hamilton. Found at http://www.solarviews.com/cap/misc/obliquity.htm
IMAGE 1.2 – Image thanks to http://tomsastroblog.com/archives/8047
IMAGE 1.3 – Graphic thanks to Wikimedia Commons at http://en.wikipedia.org/wiki/File:Precessing_Kepler_orbit_280frames_e0.6_smaller.gif