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Milankovitch Cycles

One of many factors affecting Earth Climate

The Earth's wobble and the dry and wet Sahara 

The Sahara was once green, with rivers and large lakes. There were plenty of animals, fishes, and people.

Fished swam across the wet Sahara. These rivers and lakes were not to last.

The door slammed shut on the green corridor. But scientists know that the Earth's wobble, the Milankovitch climatic precession cycle, makes the Sahara like a pendulum. It goes from desert to grassland and from wet to dry every 23,000 years like clockwork.

The Earth's wobble will bring the Sahara back to the old days wet and green again in another 15,000 years unless is operational before. What will happen to other regions? The answer is simple; dryer climate for some (lack of H2O), cold, very cold (lack of heat), wet and very wet (H2O abundance) for other zones with cities risking being washed to the seas or being abandoned as repairs will be costly.

To know which climate each region was, just see how and when they were developed;

  • North Hemisphere was cold (lack of sun heat and heat distribution),
  • South West America was dry (lack of H2O),
  • East Asia cities by river deltas were wetlands (H2O over-abundance). Example Bangkok, Saigon, Shanghai, and others were a wetland. Few thousands years after people had spreaded from Africa to as far as East Asia, they established civilisation like the Khmer at Angkok. Once those areas were flooded, they abandoned they cities and the Khmer emigrated to higher grounds, fighting each others for survial and expansion. Example in Laos, the Khmer established at the plain of Jars as river basins were regularly flooded.

During this industrial evolution, civil engineers had omitted huge volume of advancing ice sheets, their melting and increase rain and snowfalls that inevitably will destroy many of North Hemispheric cities and infrastructures of less than 23,000 years old. That's include the majority of them. Oroville dam overspill was a WARNING.

Our initial phase of will gradually eliminate risks above saving resources and time for next phases.

The episodic nature of the Earth's glacial and interglacial periods within the present Ice Age (the last couple of million years) have been caused primarily by cyclical changes in the Earth's circumnavigation of the Sun. Variations in the Earth's eccentricity, axial tilt, and precession comprise the three dominant cycles, collectively known as the Milankovitch Cycles for Milutin Milankovitch, the Serbian astronomer and mathematician who is generally credited with calculating their magnitude. Taken in unison, variations in these three cycles creates alterations in the seasonality of solar radiation reaching the Earth's surface. These times of increased or decreased solar radiation directly influence the Earth's climate system, thus impacting the advance and retreat of Earth's glaciers.
It is of primary importance to explain that climate change, and subsequent periods of glaciation, resulting from the following four variables is not due to the total amount of solar energy reaching Earth. The four Milankovitch Cycles impact the seasonality and location of solar energy around the Earth, thus impacting contrasts between the seasons.


Periodicity: 41,000 years

The first of the four Milankovitch Cycles is Axial tilt, it's the inclination of the Earth's axis in relation to its plane of orbit around the Sun. Oscillations in the degree of Earth's axial tilt occur on a periodicity of 41,000 years from 21.5 to 24.5 degrees.It is of primary importance to explain that climate change, and subsequent periods of glaciation, resulting from the following three variables is not due to the total amount of solar energy reaching Earth. The three Milankovitch Cycles impact the seasonality and location of solar energy around the Earth, thus impacting contrasts between the seasons.

Today the Earth's axial tilt is about 23.5 degrees, which largely accounts for our seasons. Because of the periodic variations of this angle the severity of the Earth's seasons changes. With less axial tilt the Sun's solar radiation is more evenly distributed between winter and summer. However, less tilt also increases the difference in radiation receipts between the equatorial and polar regions.

One hypothesis for Earth's reaction to a smaller degree of axial tilt is that it would promote the growth of ice sheets. This response would be due to a warmer winter, in which warmer air would be able to hold more moisture, and subsequently produce a greater amount of snowfall. In addition, summer temperatures would be cooler, resulting in less melting of the winter's accumulation. At present, axial tilt is in the middle of its range.


Periodicity: 100,000 years

Eccentricity, the second of the four Milankovitch Cycles is the Earth's eccentricity. Eccentricity is, simply, the shape of the Earth's orbit around the Sun. This constantly fluctuating, orbital shape ranges between more and less elliptical (0 to 5% ellipticity) on a cycle of about 100,000 years. These oscillations, from more elliptic to less elliptic, are of prime importance to glaciation in that it alters the distance from the Earth to the Sun, thus changing the distance the Sun's short wave radiation must travel to reach Earth, subsequently reducing or increasing the amount of radiation received at the Earth's surface in different seasons.

Today a difference of only about 3 percent occurs between aphelion (farthest point) and perihelion (closest point). This 3 percent difference in distance means that Earth experiences a 6 percent increase in received solar energy in January than in July. This 6 percent range of variability is not always the case, however. When the Earth's orbit is most elliptical the amount of solar energy received at the perihelion would be in the range of 20 to 30 percent more than at aphelion. Most certainly these continually altering amounts of received solar energy around the globe result in prominent changes in the Earth's climate and glacial regimes. At present the orbital eccentricity is nearly at the minimum of its cycle.

Climatic Precession

Periodicity: 23,000 years

The third of the Milankovitch Cycles is Earth's precession. Precession is the Earth's slow wobble as it spins on axis. This wobbling of the Earth on its axis can be likened to a top running down, and beginning to wobble back and forth on its axis.

The precession of Earth wobbles from pointing at Polaris (North Star) to pointing at the star Vega. When this shift to the axis pointing at Vega occurs, Vega would then be considered the North Star. This top-like wobble, or precession, has a periodicity of 23,000 years.

Due to this wobble a climatically significant alteration must take place. When the axis is tilted towards Vega the positions of the Northern Hemisphere winter and summer solstices will coincide with the aphelion and perihelion, respectively. This means that the Northern Hemisphere will experience winter when the Earth is furthest from the Sun and summer when the Earth is closest to the Sun. This coincidence will result in greater seasonal contrasts. At present, the Earth is at perihelion very close to the winter solstice.


The most complex of the four Milankovitch Cycles

FEEDBACKS EFFECTS: Small change in Total insolation, the 100,000 yr cycle produces global insolation change <1%, similar to sunspot cycles.

a. more snow: increased albedo, cooler atm.

b. lower sea level: exposed continental shelf, incr. albedo, aerosols
c. sea ice: fresh water freezes easily, air colder, high albedo
. fresh surface water turns off thermohaline circulation
- no production of Atlantic deep water
- no Gulf Current to warm North Atlantic
slow circulation increases oceanic CO2 (decr. atm. CO2)

cold ocean - water vapor 20% - 33% less
lower biol. productivity, + CO2 in atm
reduced precipitation on glaciers
ice sheet growth
isostatic depression
gravitational attraction of water
increased calving of glaciers

Insolation varies in Space
1. at equator, precession (22 ka) felt much less than tilt (41 ka)
2. amplitude of insolation changes greatest at pole

Insolation varies in Time
1. relationship of perihelion to equinoxes changes ca. 22,000 yr cycle
2. each month receives maximum insolation at different times in Holocene (45°N)
-fall - 5,000 ya
-summer - 9,000 ya
-spring - 15,000 ya

3. Direct Effects processes or phenomena effected by seasonal insolation
- upper tree line highest, July temperature greatest, 9,000 ya. Spruce
(Ritche et al., 1983) Poplar (Bartlein et al 1995)
- continental interiors: L. Baikal biogenic productivity 100, 41, 23, 19 Kyr cyclicity
(Colman et al 1995) Devils H (Winograd et al, 1988)
4. Indirect Effects on atmospheric circulation
- Asian and African Monsoon (Ping et al, 1995) wettest, +85% in Chinese arid regions, 25% drier LGM (Maher & Thompson, 1995)
- Arizona Monsoon

Note the amazing rhythmical similarities of the four cycles, which indicate the very strong solar-orbital influences on Earth's climate. Since Earth's CO2 level do not drive the solar-orbital cycles, your can see why many scientists doubt the currently popular "CO2 CAUSES harmful global warming" argument. On the other hand, an increase in CO2 may provide some slight positive feedback (support) to a warming Earth, but the magnitude or even the direction of positive or negative feedback is still being debated.