In 1914 Milutin Milankovitch, a Serbian professor at the University of Belgrade found himself under arrest as World War I broke out, deemed a potential danger by the Austro-Hungarian government. Subsequently transferred to a loose house arrest in Budapest with access to the Hungarian Academy of Sciences library, he laid the foundations of his life's work: understanding the interaction between Earth's motion in space and the periods in the distant past when glaciers descended from the North pole - times popularly known as ice ages.
The truths he found are still with us today. They constitute the theory that very long-term cycles in the Earth's motion cause enormous ice sheets, often a mile or two thick, to descend from the North, covering the land and obliterating anything in their path. Their weight pushes the land downward and their motion grinds valleys into solid rock, leaving new features from the Great Lakes to fertile soils made of pulverized rock. These cycles also govern the periodic retreat of these glacial ice sheets. Glacial advances and retreats have been occurring for millions of years and are projected to continue for millions more, until the continents themselves drift away from the higher latitudes and thus remove a substrate for ice to build up on. In honor of his work, these motions of the Earth and its orbit are collectively named Milankovitch cycles. There are three major cycles with periodicities ranging from about 21,000 years long to about 100,000.
The shortest Milankovitch cycle. Consider a spinning top. As it slows down, the axis of rotation, indicated by its stem, starts to trace a circle (if you have one handy, why not spin it now and see for yourself?). The period of this circle is long enough that the top spins many times in the time it takes for the axis to move around the circle once. This is called axial precession. While a toy top may precess all the way around in a second or less, bigger tops precess more slowly. The Earth is a very, very big top. Its axis is currently tilted away from vertical by 23.44 degrees (where "vertical" is relative to the plane of the Earth's orbit). The Earth's axis precesses one time around about every 25,772 years. If the Earth was the only object in the solar system besides the sun, those 25,772 years would constitute a Milankovitch cycle. But it's not so simple.
Even ignoring the relatively small non-planets like Pluto, there are 7 planets besides Earth, and their gravities all affect Earth's orbital behavior to some degree. One effect of this is to create another kind of precession, orbital precession (also called perihelion and apsidal precession). To visualize orbital precession, picture the orbit of the Earth. It is an oval (more precisely, an ellipse) around the sun with the sun nearer to one end than the other. The end nearer the sun is called the perihelion, peri- meaning "near" and "helios" meaning sun in Greek. The other end is relatively far from the sun and is called the aphelion (ap- from the Greek apo, "away"). The easiest way to picture orbital precession is to imagine the Earth swinging along its orbit counterclockwise, one year per trek all the way around, and reaching its aphelion at some point during the year. The Earth then continues on its way and a year later has gone all the way 'round the sun again and is back at its aphelion again. But the aphelion is no longer in the same place! It has moved a tiny bit clockwise because the entire ellipse has rotated. If the aphelion was at 9 o'clock this year, then it would have precessed to 10 o'clock in about 9,330 years. For the aphelion to precess all the way around once requires a good 112,000 years. I find the images of the relatively far away aphelion gradually swinging around the sun more vivid, but of course the perihelion at the other end of the elliptical orbit, opposite the aphelion and relatively near to the sun is doing exactly the same thing. Hence the term "perihelion precession." (And since both ap- and perihelion are "apsids," it is also called apsidal precession.)
Apsidal precession does not itself create a Milankovitch climate cycle, but apsidal and axial precession, acting in concert, do. In brief, while the perihelion gradually marches clockwise, once around in 112,000 years, the Earth's rotational axis marches counterclockwise around its own circle once every 25,772 years. They meet in the middle about every 21,700 years. This meeting point has two characteristics: the axis tilts toward the sun, and it is also the perihelion. Half a year later the Earth is on the other side of its orbit, the Earth is at aphelion, and while the axis is pointing in the same direction as it did before, that direction is now leaning away instead of toward the sun because the sun in essence is on the other side of the Earth. (Actually the Earth is on the other side of the sun but, relative to each other, this is the same thing.) This 21,700-year periodicity is a Milankovich cycle, because the configuration affects the climate. Here's how.
Suppose for a moment that the Earth's axis of rotation was exactly vertical, at a perfect right angle to the plane of the Earth's orbit and thus not tilted at all. Then the sun would shine squarely on the equator from directly overhead at mid-day, and as one traveled north or south the sun would shine more and more glancingly on the ground as the surface curved backward from the equator. Thus the equator would be hot from the sun directly upward, and it would get colder as one traveled north or south, because the sun's closest approach to directly overhead, at mid-day, would become progressively further away from it and, thus, closer to the horizon.
Next suppose the upper (northern) end of the axis tilts toward the sun, as it always does for part of the year. Now the sun shines squarely, not on the equator, but on a point north of the equator. This tends to heat up the northern hemisphere and cool the southern. In the north, this is called "summer." But far south of the equator, it's winter, because with the sun shining squarely on a point north of the equator, south of the equator the sun is not blazing directly overhead anywere. On the other hand, a half-year later the Earth will be on the other side of its orbit, the axial tilt will be the same but now leaning away from the sun, and we call that "winter" (in the northern hemisphere), and summer (in the southern hemisphere).
As a final factor, we now consider how the locations of the ap- and perihelions affect the seasons. If summer in the north happens near aphelion, the northern summer will be on the cool side because the Earth is relatively far from the sun (and southern winter will also be cooler). The northern summer, southern winter will also be a few days longer because the Earth, being further from the sun, is traveling slower than when it is nearer. On the other hand if northern summers are instead occurring near perihelion, the summer will be relatively hot (and southern winter mild). Also the northern summer and southern winter will be a few days shorter because, with the Earth relatively near the sun it will be moving a bit faster, thus spending less time in the portion of the orbit corresponding to this season. Why less time? Because the Earth travels faster the nearer it gets to the sun, for exactly the same reason that if you drop something, it speeds up as it goes down. The mathematical details are governed by Johannes Kepler's Second Law of planetary motion, generalized by Isaac Newton's Law of Universal Gravitation, which in turn is generalized still more in Albert Einstein's General Theory of Relativity. So there's no need to take my word for it: if you drop your toast, it will speed up as the buttered side heads for the floor.
Currently, aphelion coincides with northern hemisphere summer, so summers are on the cool but long side while winters, occurring at perihelion, are relatively warm and short (in the southern hemisphere, winters are colder than they would be otherwise and summers warmer). Northern summers will start warming and shortening, and winters cooling and lengthening, while the opposite will occur in the south, but this process is too slow to make a noticeable difference in your lifetime, and is dwarfed by the current greenhouse effect global warming trend in any case. Although this cycle takes a hefty 21,700 years to repeat, it is merely the shortest Milankovitch climate cycle.
The medium-length Milankovitch cycle. The theory that cool Northern summers tend to trigger the ice age descent of glaciation from the far north across what is now normal land is hard to square with axial precession as a reason, because axial precession tends to compensate for the cool summer by making it longer. The theory is supported, however, by a 41,000-year Milankovitch cycle in something called "obliquity." I will explain obliquity. But first, consider its importance.
Starting about 2.6 million years ago the continents had drifted into an "ice age position," putting the Earth officially in an ice age. In this ice age, glacial advances, covering large amounts of land, intersperse with warmer glacial retreats. That ice age still continues today, however we are fortunately in a warm spell between glacial advances that began around 11,000 years ago, and will end...sometime in the future, perhaps in 50,000 years, perhaps farther off than that. (Often the term "ice age" is used colloquially to refer to a single period of glacial advancement, but technically the term for that is "glaciation.") Prior to the current ice age there were others at different times in Earth's prehistory. The worst of them are thought to have caused "snowball Earth" situations, where ice coverage extended to the equator. But details about ancient ice ages hundreds of millions or billions of years ago are tricky to nail down.
When the most recent ice age began, its glaciations occurred fairly regularly every 41,000 years! Scientists tried to make sense of this remarkable fact based, naturally, on the effects of differences in obliquity, because of its 41,000 year cycle. Obliquity is a meta-characteristic of the axial precession described earlier. Recall the stem of the spinning top. As the top slows down, the stem begins describing a circle: axial precession. The circle gets wider as the top slows, until finally it is so wide that the side of the top touches the floor and it stops. In the case of the Earth, this circle gets bigger, but then gets smaller, then bigger, smaller, on and on, with a period of 41,000 years. When the circle is big, it is because the Earth's axis is more tilted than when it is small. The amount of tilt is called the obliquity. At its widest, the axis is 24.5 degrees away from vertical, and at its smallest, just over 21 degrees, though most cycles do not quite reach those extremes. This range of less than 2.5 degrees might not seem like much, but to the finely tuned climate system of Earth it means a lot. Here's why.
Recall that cooler northern summers are believed to encourage glaciation by not melting off preceding winters' snow covers, at least if the summer is not compensating for being cooler by being longer. When as a consequence the otherwise dark ground stays white year-round, less sunlight is absorbed, compounding the coolness problem and making it even harder to melt the white coating. The result is a fabled "vicious cycle," or positive feedback loop, and things can get a lot worse before they get better - as in moving sheets of solid ice, a mile or more thick, covering the landscape. This has happened countless times in the past, many within the current ice age. "The congealed venomous streams continued to send out frost," in the words of Norse myth. What is it about the obliquity cycle that causes summers to cool, warm, and then repeat, every 41,000 years? The reasoning is similar, yet simpler, than for the 21,000 year axial precession cycle. Summer is defined as when the axis tilts toward the sun. In the northern hemisphere, that means the higher latitudes face the sun more squarely than in other seasons. The sun beats down more strongly because it is more directly overhead. Days are longer - up to 6 months without a sunset at the north pole. The result is more warmth. That's why we call it summer. This effect is accentuated by greater axial tilt: greater tilt, warmer summers. More to the point, less tilt therefore means cooler summers, which melt less snow and ice, allowing glaciers to form, move, and cover the land. Every 41,000 years...until about 800,000 years ago.
The pattern changes. 800,000 years ago the alternating glaciations and warm periods changed to what looked more like a 100,000 year cycle time. Although the amount of obliquity still cycles at 41,000 years, and axial precession still has a 21,000 year interval, a third Milankovitch cycle appeared to have taken control. The third major cycle is in the amount of eccentricity in the Earth's orbit.
The Earth's orbit is sometimes somewhat eccentric (elliptical rather than circular) and other times it is nearly circular. Currently its eccentricity is .0167 and will trend toward circular for the next 26,000 years or so. When close to circular the Earth gets the same amount of sunlight overall, year-round. When more eccentric, the Earth is closer to the sun for part of the year and farther away for another part, getting more or less sunlight accordingly. Currently the sun is about 7% brighter at its nearest during the year compared to its furthest.
A cycle through both the more circular and the more eccentric phases takes about 100,000 years. And ice age glaciations have occurred about 100,000 years apart since 800,000 years ago. Far from solving the ice age timing problem and thus enabling predicting the next one, however, this leads to puzzling questions.
Question 1. The effects of the 100,000 year cycle on solar energy input at 65 degrees north - the pressure point of the northern hemisphere from the standpoint of glaciations and glacial terminations - are small compared to those of axial precession and obliquity. So why would it control the glacial cycle?
Question 2. The 41,000 year obliquity cycle characterized the glaciation cycle until 800,000 years ago. So why would it stop?
Considerable effort in the paleoclimatology community has gone into trying to figure out these questions. However, there is growing evidence that, in fact, the 41,000 year obliquity cycle actually still dominates. For example the glacial era before the most recent one ending 11,000 years ago tipped into its own termination phase 123,000 years earlier - three obliquity cycles apart. To explain the apparent 100,000 year cycle one need merely posit glacial retreats at intervals of 2 or 3 obliquity cycles (about 82,000 and 123,000 years apart), averaging about 100,000 years and thus by coincidence giving a false impression of being driven by the 100,000 year eccentricity cycle. As for why obliquity is so important, it appears that glacial terminations are triggered by a peak in the total summer solar energy impinging on the upper northern hemisphere (65 degrees in latitude is the typical proxy for that). This happens at high obliquity - every 41,000 years in the obliquity cycle.
Clearly though, obliquity cannot be the full explanation because deglaciations averaging 100,000 years apart naturally have to skip many obliquity cycles. The whole story is thus more complex. Something - perhaps a gradually cooling climate over the past few million years (despite the current global warming spike caused by human activity) is causing glaciation cycles to both skip obliquity cycles and be more severe than previously (Figure 1). Furthermore, the solar energy effects of all the Milankovitch cycles act simultaneously, sometimes adding together and sometimes tending to cancel out. This leads to considerable variation in the degree of solar forcing of glacial terminations, even at peak obliquity (Figure 2). Also, keep in mind that vast glacial coverings cannot terminate unless they build up first, and that occurs more slowly than the usually faster, more dramatic termination events (Figure 3). Buildup, too, is influenced by Milankovitch cycles.
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Looking much further into the future, troughs (low points) in solar energy striking the northern hemisphere will occur because of Milankovitch cycles. The resulting chill will encourage ice sheet formation over vast expanses of otherwise habitable land. Likewise, peaks (high points) will correlate with ice sheet termination events (Figure 2). A low trough with the potential to bring about a glaciation can be predicted by orbital simulations and will not occur for another 50,000 years, unfortunately far too late to solve the current global warming problem. A big trough is predicted for 620,000 years out. No need to pay for rush delivery on your own copy of "Igloo Making for Dummies"! Yet, many important details of how Milankovitch cycles, geological and climatological processes translate into glacial ice sheet formation and termination events are still poorly understood. Thus predictions of future ice age events are more uncertain than they should be. Similarly, predictions that a major glacial advance won't occur for another 50,000 years are likewise uncertain. More paleoclimatology research is needed to clarify and improve predictions. Such work is likely to have the additional payoff of better understanding of current weather and climate processes, hence better weather and climate forecasting on scales of months to years. This would be immensely useful to business and government in our own time because, from food production to flood control, so many activities could benefit greatly from improved long term term weather forecasting.
References
"...he laid the foundations of his life's work: understanding the interaction between Earth's motion in space and the periods in the distant past when glaciers descended from the North pole - times popularly known as ice ages." M. Milankovitch, O pitanju astronomskih teorija ledenih doba (Astronomical theory of periods of increased glaciation), University of Zagreb, 1914. Http://scr.digital.nb.rs/document/II-037625 (in Serbian).
"The congealed venomous streams continued to send out frost." K. Mortensen, trans. by A.C. Crowell, A Handbook of Norse Mythology, chap. 1, p. 49, Thomas Y. Crowell Co., e.g. www.scribd.com/doc/54300132/2/I-HOW-THE-WORLD-WAS-CHEATED
"For example the glacial era before the most recent one ending 11,000 years ago tipped into its own termination phase 123,000 years earlier - three obliquity cycles apart." R. N. Drysdale, J. C. Hellstrom, G. Zanchetta, A. E. Fallick, M. F. Sanchez Goni, I. Couchoud, J. McDonald, R. Maas, G. Lohmann, and I. Isola, Evidence for obliquity forcing of glacial termination II, Science, vol. 325, pp. 1527-1531, 2009. Http:sciencemag.org/content/325/5947/1527.full.html.
"...averaging about 100,000 years and thus by coincidence giving a false impression of being driven by the 100,000 year eccentricity cycle." P. Huybers and C. Wunsch, Obliquity bpacing of the late Pleastocene glacial terminations, Nature, vol. 434, pp. 491-494, 2005. Http://www.people.fas.harvard.edu/~phuybers/Doc/pace_nature2005.pdf.
"Something - perhaps a gradually cooling climate over the past few million years (despite the current global warming spike caused by human activity) is causing glaciation cycles to both skip obliquity cycles and be more severe than previously." P. J. Huybers, Early pleistocene glacial cycles and the integrated summer insolation forcing, Science, vol. 313, pp. 508-511, 2006. Http://nrs.harvard.edu/urn-3:HUL.InstRepos:3382981.
The truths he found are still with us today. They constitute the theory that very long-term cycles in the Earth's motion cause enormous ice sheets, often a mile or two thick, to descend from the North, covering the land and obliterating anything in their path. Their weight pushes the land downward and their motion grinds valleys into solid rock, leaving new features from the Great Lakes to fertile soils made of pulverized rock. These cycles also govern the periodic retreat of these glacial ice sheets. Glacial advances and retreats have been occurring for millions of years and are projected to continue for millions more, until the continents themselves drift away from the higher latitudes and thus remove a substrate for ice to build up on. In honor of his work, these motions of the Earth and its orbit are collectively named Milankovitch cycles. There are three major cycles with periodicities ranging from about 21,000 years long to about 100,000.
The shortest Milankovitch cycle. Consider a spinning top. As it slows down, the axis of rotation, indicated by its stem, starts to trace a circle (if you have one handy, why not spin it now and see for yourself?). The period of this circle is long enough that the top spins many times in the time it takes for the axis to move around the circle once. This is called axial precession. While a toy top may precess all the way around in a second or less, bigger tops precess more slowly. The Earth is a very, very big top. Its axis is currently tilted away from vertical by 23.44 degrees (where "vertical" is relative to the plane of the Earth's orbit). The Earth's axis precesses one time around about every 25,772 years. If the Earth was the only object in the solar system besides the sun, those 25,772 years would constitute a Milankovitch cycle. But it's not so simple.
Even ignoring the relatively small non-planets like Pluto, there are 7 planets besides Earth, and their gravities all affect Earth's orbital behavior to some degree. One effect of this is to create another kind of precession, orbital precession (also called perihelion and apsidal precession). To visualize orbital precession, picture the orbit of the Earth. It is an oval (more precisely, an ellipse) around the sun with the sun nearer to one end than the other. The end nearer the sun is called the perihelion, peri- meaning "near" and "helios" meaning sun in Greek. The other end is relatively far from the sun and is called the aphelion (ap- from the Greek apo, "away"). The easiest way to picture orbital precession is to imagine the Earth swinging along its orbit counterclockwise, one year per trek all the way around, and reaching its aphelion at some point during the year. The Earth then continues on its way and a year later has gone all the way 'round the sun again and is back at its aphelion again. But the aphelion is no longer in the same place! It has moved a tiny bit clockwise because the entire ellipse has rotated. If the aphelion was at 9 o'clock this year, then it would have precessed to 10 o'clock in about 9,330 years. For the aphelion to precess all the way around once requires a good 112,000 years. I find the images of the relatively far away aphelion gradually swinging around the sun more vivid, but of course the perihelion at the other end of the elliptical orbit, opposite the aphelion and relatively near to the sun is doing exactly the same thing. Hence the term "perihelion precession." (And since both ap- and perihelion are "apsids," it is also called apsidal precession.)
Apsidal precession does not itself create a Milankovitch climate cycle, but apsidal and axial precession, acting in concert, do. In brief, while the perihelion gradually marches clockwise, once around in 112,000 years, the Earth's rotational axis marches counterclockwise around its own circle once every 25,772 years. They meet in the middle about every 21,700 years. This meeting point has two characteristics: the axis tilts toward the sun, and it is also the perihelion. Half a year later the Earth is on the other side of its orbit, the Earth is at aphelion, and while the axis is pointing in the same direction as it did before, that direction is now leaning away instead of toward the sun because the sun in essence is on the other side of the Earth. (Actually the Earth is on the other side of the sun but, relative to each other, this is the same thing.) This 21,700-year periodicity is a Milankovich cycle, because the configuration affects the climate. Here's how.
Suppose for a moment that the Earth's axis of rotation was exactly vertical, at a perfect right angle to the plane of the Earth's orbit and thus not tilted at all. Then the sun would shine squarely on the equator from directly overhead at mid-day, and as one traveled north or south the sun would shine more and more glancingly on the ground as the surface curved backward from the equator. Thus the equator would be hot from the sun directly upward, and it would get colder as one traveled north or south, because the sun's closest approach to directly overhead, at mid-day, would become progressively further away from it and, thus, closer to the horizon.
Next suppose the upper (northern) end of the axis tilts toward the sun, as it always does for part of the year. Now the sun shines squarely, not on the equator, but on a point north of the equator. This tends to heat up the northern hemisphere and cool the southern. In the north, this is called "summer." But far south of the equator, it's winter, because with the sun shining squarely on a point north of the equator, south of the equator the sun is not blazing directly overhead anywere. On the other hand, a half-year later the Earth will be on the other side of its orbit, the axial tilt will be the same but now leaning away from the sun, and we call that "winter" (in the northern hemisphere), and summer (in the southern hemisphere).
As a final factor, we now consider how the locations of the ap- and perihelions affect the seasons. If summer in the north happens near aphelion, the northern summer will be on the cool side because the Earth is relatively far from the sun (and southern winter will also be cooler). The northern summer, southern winter will also be a few days longer because the Earth, being further from the sun, is traveling slower than when it is nearer. On the other hand if northern summers are instead occurring near perihelion, the summer will be relatively hot (and southern winter mild). Also the northern summer and southern winter will be a few days shorter because, with the Earth relatively near the sun it will be moving a bit faster, thus spending less time in the portion of the orbit corresponding to this season. Why less time? Because the Earth travels faster the nearer it gets to the sun, for exactly the same reason that if you drop something, it speeds up as it goes down. The mathematical details are governed by Johannes Kepler's Second Law of planetary motion, generalized by Isaac Newton's Law of Universal Gravitation, which in turn is generalized still more in Albert Einstein's General Theory of Relativity. So there's no need to take my word for it: if you drop your toast, it will speed up as the buttered side heads for the floor.
Currently, aphelion coincides with northern hemisphere summer, so summers are on the cool but long side while winters, occurring at perihelion, are relatively warm and short (in the southern hemisphere, winters are colder than they would be otherwise and summers warmer). Northern summers will start warming and shortening, and winters cooling and lengthening, while the opposite will occur in the south, but this process is too slow to make a noticeable difference in your lifetime, and is dwarfed by the current greenhouse effect global warming trend in any case. Although this cycle takes a hefty 21,700 years to repeat, it is merely the shortest Milankovitch climate cycle.
The medium-length Milankovitch cycle. The theory that cool Northern summers tend to trigger the ice age descent of glaciation from the far north across what is now normal land is hard to square with axial precession as a reason, because axial precession tends to compensate for the cool summer by making it longer. The theory is supported, however, by a 41,000-year Milankovitch cycle in something called "obliquity." I will explain obliquity. But first, consider its importance.
Starting about 2.6 million years ago the continents had drifted into an "ice age position," putting the Earth officially in an ice age. In this ice age, glacial advances, covering large amounts of land, intersperse with warmer glacial retreats. That ice age still continues today, however we are fortunately in a warm spell between glacial advances that began around 11,000 years ago, and will end...sometime in the future, perhaps in 50,000 years, perhaps farther off than that. (Often the term "ice age" is used colloquially to refer to a single period of glacial advancement, but technically the term for that is "glaciation.") Prior to the current ice age there were others at different times in Earth's prehistory. The worst of them are thought to have caused "snowball Earth" situations, where ice coverage extended to the equator. But details about ancient ice ages hundreds of millions or billions of years ago are tricky to nail down.
When the most recent ice age began, its glaciations occurred fairly regularly every 41,000 years! Scientists tried to make sense of this remarkable fact based, naturally, on the effects of differences in obliquity, because of its 41,000 year cycle. Obliquity is a meta-characteristic of the axial precession described earlier. Recall the stem of the spinning top. As the top slows down, the stem begins describing a circle: axial precession. The circle gets wider as the top slows, until finally it is so wide that the side of the top touches the floor and it stops. In the case of the Earth, this circle gets bigger, but then gets smaller, then bigger, smaller, on and on, with a period of 41,000 years. When the circle is big, it is because the Earth's axis is more tilted than when it is small. The amount of tilt is called the obliquity. At its widest, the axis is 24.5 degrees away from vertical, and at its smallest, just over 21 degrees, though most cycles do not quite reach those extremes. This range of less than 2.5 degrees might not seem like much, but to the finely tuned climate system of Earth it means a lot. Here's why.
Recall that cooler northern summers are believed to encourage glaciation by not melting off preceding winters' snow covers, at least if the summer is not compensating for being cooler by being longer. When as a consequence the otherwise dark ground stays white year-round, less sunlight is absorbed, compounding the coolness problem and making it even harder to melt the white coating. The result is a fabled "vicious cycle," or positive feedback loop, and things can get a lot worse before they get better - as in moving sheets of solid ice, a mile or more thick, covering the landscape. This has happened countless times in the past, many within the current ice age. "The congealed venomous streams continued to send out frost," in the words of Norse myth. What is it about the obliquity cycle that causes summers to cool, warm, and then repeat, every 41,000 years? The reasoning is similar, yet simpler, than for the 21,000 year axial precession cycle. Summer is defined as when the axis tilts toward the sun. In the northern hemisphere, that means the higher latitudes face the sun more squarely than in other seasons. The sun beats down more strongly because it is more directly overhead. Days are longer - up to 6 months without a sunset at the north pole. The result is more warmth. That's why we call it summer. This effect is accentuated by greater axial tilt: greater tilt, warmer summers. More to the point, less tilt therefore means cooler summers, which melt less snow and ice, allowing glaciers to form, move, and cover the land. Every 41,000 years...until about 800,000 years ago.
The pattern changes. 800,000 years ago the alternating glaciations and warm periods changed to what looked more like a 100,000 year cycle time. Although the amount of obliquity still cycles at 41,000 years, and axial precession still has a 21,000 year interval, a third Milankovitch cycle appeared to have taken control. The third major cycle is in the amount of eccentricity in the Earth's orbit.
The Earth's orbit is sometimes somewhat eccentric (elliptical rather than circular) and other times it is nearly circular. Currently its eccentricity is .0167 and will trend toward circular for the next 26,000 years or so. When close to circular the Earth gets the same amount of sunlight overall, year-round. When more eccentric, the Earth is closer to the sun for part of the year and farther away for another part, getting more or less sunlight accordingly. Currently the sun is about 7% brighter at its nearest during the year compared to its furthest.
A cycle through both the more circular and the more eccentric phases takes about 100,000 years. And ice age glaciations have occurred about 100,000 years apart since 800,000 years ago. Far from solving the ice age timing problem and thus enabling predicting the next one, however, this leads to puzzling questions.
Question 1. The effects of the 100,000 year cycle on solar energy input at 65 degrees north - the pressure point of the northern hemisphere from the standpoint of glaciations and glacial terminations - are small compared to those of axial precession and obliquity. So why would it control the glacial cycle?
Question 2. The 41,000 year obliquity cycle characterized the glaciation cycle until 800,000 years ago. So why would it stop?
Considerable effort in the paleoclimatology community has gone into trying to figure out these questions. However, there is growing evidence that, in fact, the 41,000 year obliquity cycle actually still dominates. For example the glacial era before the most recent one ending 11,000 years ago tipped into its own termination phase 123,000 years earlier - three obliquity cycles apart. To explain the apparent 100,000 year cycle one need merely posit glacial retreats at intervals of 2 or 3 obliquity cycles (about 82,000 and 123,000 years apart), averaging about 100,000 years and thus by coincidence giving a false impression of being driven by the 100,000 year eccentricity cycle. As for why obliquity is so important, it appears that glacial terminations are triggered by a peak in the total summer solar energy impinging on the upper northern hemisphere (65 degrees in latitude is the typical proxy for that). This happens at high obliquity - every 41,000 years in the obliquity cycle.
Clearly though, obliquity cannot be the full explanation because deglaciations averaging 100,000 years apart naturally have to skip many obliquity cycles. The whole story is thus more complex. Something - perhaps a gradually cooling climate over the past few million years (despite the current global warming spike caused by human activity) is causing glaciation cycles to both skip obliquity cycles and be more severe than previously (Figure 1). Furthermore, the solar energy effects of all the Milankovitch cycles act simultaneously, sometimes adding together and sometimes tending to cancel out. This leads to considerable variation in the degree of solar forcing of glacial terminations, even at peak obliquity (Figure 2). Also, keep in mind that vast glacial coverings cannot terminate unless they build up first, and that occurs more slowly than the usually faster, more dramatic termination events (Figure 3). Buildup, too, is influenced by Milankovitch cycles.
Figure 1. Cycles of glacial advances and retreats have increased in severity, while becoming less frequent. Height in this graph is a proxy for total world ice mass. (Adapted from: http://www.globalwarmingart.com/wiki/File:Five_Myr_Climate_Change_Rev_png)
Figure 2. Peak solar energy incident upon the Earth at latitude 65 north. (Adapted from: http://en.wikipedia.org/wiki/File:SummerSolstice65N-future.png)
Figure 3. World glacial buildup is relatively slow; terminations are relatively dramatic. (Adapted from: http://www.globalwarmingart.com/wiki/File:Ice_Age_Temperature_Rev_png)
What should be done. There are many glacial termination events in Earth's future, just as many have occurred in Earth's past (Figures 1, 3). In fact we are undergoing one right now due to human-caused global warming. Although ice sheets have not been extensive since after the last termination event ended 11,000 years ago, the glaciation that remains is disappearing at a rapid clip. This will cause major changes in sea level and other climate patterns world wide that will lead to mass species extinction, coastal flooding, dislocations, hardship and suffering for countless people world wide, many of whom can barely hang on even under normal circumstances. Climate change denialists - and those who fund them - are perpetrating a hoax and will have blood on their hands if they succeed in preventing effective steps to control global warming or compensate for its effects.
Looking much further into the future, troughs (low points) in solar energy striking the northern hemisphere will occur because of Milankovitch cycles. The resulting chill will encourage ice sheet formation over vast expanses of otherwise habitable land. Likewise, peaks (high points) will correlate with ice sheet termination events (Figure 2). A low trough with the potential to bring about a glaciation can be predicted by orbital simulations and will not occur for another 50,000 years, unfortunately far too late to solve the current global warming problem. A big trough is predicted for 620,000 years out. No need to pay for rush delivery on your own copy of "Igloo Making for Dummies"! Yet, many important details of how Milankovitch cycles, geological and climatological processes translate into glacial ice sheet formation and termination events are still poorly understood. Thus predictions of future ice age events are more uncertain than they should be. Similarly, predictions that a major glacial advance won't occur for another 50,000 years are likewise uncertain. More paleoclimatology research is needed to clarify and improve predictions. Such work is likely to have the additional payoff of better understanding of current weather and climate processes, hence better weather and climate forecasting on scales of months to years. This would be immensely useful to business and government in our own time because, from food production to flood control, so many activities could benefit greatly from improved long term term weather forecasting.
References
"...he laid the foundations of his life's work: understanding the interaction between Earth's motion in space and the periods in the distant past when glaciers descended from the North pole - times popularly known as ice ages." M. Milankovitch, O pitanju astronomskih teorija ledenih doba (Astronomical theory of periods of increased glaciation), University of Zagreb, 1914. Http://scr.digital.nb.rs/document/II-037625 (in Serbian).
"The congealed venomous streams continued to send out frost." K. Mortensen, trans. by A.C. Crowell, A Handbook of Norse Mythology, chap. 1, p. 49, Thomas Y. Crowell Co., e.g. www.scribd.com/doc/54300132/2/I-HOW-THE-WORLD-WAS-CHEATED
"For example the glacial era before the most recent one ending 11,000 years ago tipped into its own termination phase 123,000 years earlier - three obliquity cycles apart." R. N. Drysdale, J. C. Hellstrom, G. Zanchetta, A. E. Fallick, M. F. Sanchez Goni, I. Couchoud, J. McDonald, R. Maas, G. Lohmann, and I. Isola, Evidence for obliquity forcing of glacial termination II, Science, vol. 325, pp. 1527-1531, 2009. Http:sciencemag.org/content/325/5947/1527.full.html.
"...averaging about 100,000 years and thus by coincidence giving a false impression of being driven by the 100,000 year eccentricity cycle." P. Huybers and C. Wunsch, Obliquity bpacing of the late Pleastocene glacial terminations, Nature, vol. 434, pp. 491-494, 2005. Http://www.people.fas.harvard.edu/~phuybers/Doc/pace_nature2005.pdf.
"Something - perhaps a gradually cooling climate over the past few million years (despite the current global warming spike caused by human activity) is causing glaciation cycles to both skip obliquity cycles and be more severe than previously." P. J. Huybers, Early pleistocene glacial cycles and the integrated summer insolation forcing, Science, vol. 313, pp. 508-511, 2006. Http://nrs.harvard.edu/urn-3:HUL.InstRepos:3382981.