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Now Gavin Schmidt of NASA mentions that 8,000 years ago when the Sahara turned into a desert the tilt was 24.1 degrees and that today’s tilt is 23.5 degrees. 24.1-23.5 = 0.6 degree. The Milankovitch cycle for the axial tilt is 41,000 years from 21.5 degrees -> 24.5 to 21.5 degrees which is 6 degrees. Each degree takes 41,000/6 = 6833 years to complete. Then from 24.1 degrees to 23.5 degrees = 0.6 degree which is 0.6*6833 = 4,099 years. So it has to be 24.1 degrees over the hump 24.5 degrees in the cycle when the Sahara turned into a desert. 24.1 degrees -> 24.5 ->24.1 to 23.5 degrees i.e. 1.6 degrees. 1.6 * 6833 = 10,932 years. But I ignored the precession which gives the cycle that frilly appearance. Anyway it is approximate.
The LGM (Last Glacial Maximum) was 20,000 to 19,000 years ago and the Younger Dryas was 12,500 to 11,500 years ago. So we can try to envision which part of the cycle the LGM was in.
Today’s axial tilt is 23.5 degrees so 20,000 years ago is nearly equal to half of the axial cycle. As shown above the 8,000 years for drying of the Sahara was just over the hump 24.5 degrees so it was from 21.5 degrees to 24.5 degrees when LGM and the Younger Dryas (drying period) occurred as well of the desertification of the Sahara. Note the tilt is measured from the vertical of the elliptic plane so increasing the tilt means going down from the North pole. So the cooling period was shrinking of the tropical region as the sun oscillated from 21.5 degrees north (Tropic of Cancer) to 21.5 degrees south (Tropic of Capricorn).
Now we are at 23.5 degrees and going to 21.5 degrees which is increasing the tropical zone as the Tropics of Cancer and Capricorn are moving outwards so there is a warming of the earth till the sun reaches 21.5 degrees and starts increasing in axial tilt from 21.5 degrees to 24.5 degrees which would be the cooling period. 23.5 – 21.5 = 2 degrees which translate to 6833*2 = 13,766 years. So the cooling starts approximately 14,000 years from now and hopefully not another Ice Age.
Now this calculation I did only shows the energy input to earth from the sun. The eccentricity shows the path the earth takes around the sun oscillating from near circle to an ellipse. This shows the sun’s radiation would vary as the earth distance from the sun varies.
The axial tilt shows where the sun’s path is along the earth’s surface in effect dictates the tropical region's size and the precession is the North pole going in circles every 23,000 per cycle which gives a zig-zag pattern to the path of the sun as it oscillates between the Tropics.
The Milankovitch cycles only shows the energy input of the sun to the earth so it is not a complete view of the Ice Ages. The distribution of energy in the form of ocean and wind currents which change with the configuration of the continents which are not constant due to tectonic plate mechanics. Vegetation affects distribution as it moderates temperature over land surfaces as it absorbs sunlight and animals start living there so it is warmed by the animals at night. Then there is the heat loss from cloud and snow cover and greenhouse gases (which slows the heat loss). These would be difficult to include in the equations but must be adjusted for each period in earth’s history.
The first of the three 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.
Axial TiltAxial tilt, the second of the three Milankovitch Cycles, is 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.
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.
PrecessionThe third and final 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 Earthis furthest from the Sun and summer when the Earth is closest to the Sun. This coincidence will result in greater seasonal contrasts. At present, theEarth is at perihelion very close to the winter solstice.
http://www.indiana.edu/~geol105/images/gaia_chapter_4/milankovitch.htm
The LGM (Last Glacial Maximum) was 20,000 to 19,000 years ago and the Younger Dryas was 12,500 to 11,500 years ago. So we can try to envision which part of the cycle the LGM was in.
Today’s axial tilt is 23.5 degrees so 20,000 years ago is nearly equal to half of the axial cycle. As shown above the 8,000 years for drying of the Sahara was just over the hump 24.5 degrees so it was from 21.5 degrees to 24.5 degrees when LGM and the Younger Dryas (drying period) occurred as well of the desertification of the Sahara. Note the tilt is measured from the vertical of the elliptic plane so increasing the tilt means going down from the North pole. So the cooling period was shrinking of the tropical region as the sun oscillated from 21.5 degrees north (Tropic of Cancer) to 21.5 degrees south (Tropic of Capricorn).
Now we are at 23.5 degrees and going to 21.5 degrees which is increasing the tropical zone as the Tropics of Cancer and Capricorn are moving outwards so there is a warming of the earth till the sun reaches 21.5 degrees and starts increasing in axial tilt from 21.5 degrees to 24.5 degrees which would be the cooling period. 23.5 – 21.5 = 2 degrees which translate to 6833*2 = 13,766 years. So the cooling starts approximately 14,000 years from now and hopefully not another Ice Age.
Now this calculation I did only shows the energy input to earth from the sun. The eccentricity shows the path the earth takes around the sun oscillating from near circle to an ellipse. This shows the sun’s radiation would vary as the earth distance from the sun varies.
The axial tilt shows where the sun’s path is along the earth’s surface in effect dictates the tropical region's size and the precession is the North pole going in circles every 23,000 per cycle which gives a zig-zag pattern to the path of the sun as it oscillates between the Tropics.
The Milankovitch cycles only shows the energy input of the sun to the earth so it is not a complete view of the Ice Ages. The distribution of energy in the form of ocean and wind currents which change with the configuration of the continents which are not constant due to tectonic plate mechanics. Vegetation affects distribution as it moderates temperature over land surfaces as it absorbs sunlight and animals start living there so it is warmed by the animals at night. Then there is the heat loss from cloud and snow cover and greenhouse gases (which slows the heat loss). These would be difficult to include in the equations but must be adjusted for each period in earth’s history.
Eccentricity
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.
Axial Tilt
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.
Precession
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 Earthis furthest from the Sun and summer when the Earth is closest to the Sun. This coincidence will result in greater seasonal contrasts. At present, theEarth is at perihelion very close to the winter solstice.
EccentricityThe first of the three 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.
Axial TiltAxial tilt, the second of the three Milankovitch Cycles, is 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.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.
PrecessionThe third and final 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.
http://www.indiana.edu/~geol105/images/gaia_chapter_4/milankovitch.htm
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