Milankovitch cycles describes the collective effects of changes in the Earth's movements upon its climate, named after Serbian geophysicist and astronomer Milutin Milanković, who in the 1920s had theorized that variations in eccentricity, axial tilt, and precession of the Earth's orbit strongly influenced climatic patterns on Earth through orbital forcing.
The Earth's axis completes one full cycle of precession approximately every 26,000 years. At the same time, the elliptical orbit rotates more slowly. The combined effect of the two precessions leads to a 21,000-year period between the astronomical seasons and the orbit. In addition, the angle between Earth's rotational axis and the normal to the plane of its orbit (obliquity) oscillates between 22.1 and 24.5 degrees on a 41,000-year cycle. It is currently 23.44 degrees and decreasing at a rate of about 0.013° per century.
Similar astronomical theories had been advanced in the 19th century by Joseph Adhemar, James Croll and others, but verification was difficult due to the absence of reliably dated evidence and doubts as to exactly which periods were important. Study of materials on Earth that have been unchanged for millennia now indicate the history of Earth's climate. A study of the chronology of Antarctic ice cores using oxygen-nitrogen ratios in air bubbles trapped in the ice, which appear to respond directly to the local insolation, concluded that the climatic response documented in the ice cores was driven by northern hemisphere insolation as proposed by the Milankovitch hypothesis. Analysis of deep-ocean cores and a seminal paper by Hays, Imbrie, and Shackleton provide additional validation of the Milankovitch hypothesis through physical artifacts. However, there are still several observations that the hypothesis does not explain.
- 1 Earth’s movements
- 2 Problems
- 3 Present and future conditions
- 4 Effects beyond Earth
- 5 References
- 6 Further reading
- 7 External links
As the Earth rotates around its axis and revolves around the Sun, several quasi-periodic variations occur due to gravitational interactions. Although the curves have a large number of sinusoidal components, a few components are dominant. Milankovitch studied changes in the orbital eccentricity, obliquity, and precession of Earth's movements. Such changes in movement and orientation alter the amount and location of solar radiation reaching the Earth. This is known as solar forcing (an example of radiative forcing). Changes near the north polar area, about 65 degrees North, are considered important due to the great amount of land. Land masses respond to temperature change more quickly than oceans, which have a higher effective heat capacity, because of the mixing of surface and deep water and the fact that the specific heat of solids is generally lower than that of water.
Orbital shape (eccentricity)
The Earth's orbit is an ellipse. The eccentricity is a measure of the departure of this ellipse from circularity. The shape of the Earth's orbit varies in time between nearly circular (with the lowest eccentricity of 0.000055) and mildly elliptical (highest eccentricity of 0.0679) with the mean eccentricity of 0.0019 as geometric or logarithmic mean. The major component of these variations occurs on a period of 413,000 years (eccentricity variation of ±0.012). A number of other terms vary between components 95,000 and 125,000 years (with a beat period 400,000 years), and loosely combine into a 100,000-year cycle (variation of −0.03 to +0.02). The present eccentricity is 0.017 and decreasing.
If the Earth were the only planet orbiting our Sun, the eccentricity of its orbit would not perceptibly vary even over a period of a million years. The Earth's eccentricity varies primarily due to interactions with the gravitational fields of Jupiter and Saturn. As the eccentricity of the orbit evolves, the semi-major axis of the orbital ellipse remains unchanged. From the perspective of perturbation theory used in celestial mechanics to compute the evolution of the orbit, the semi-major axis is an adiabatic invariant. According to Kepler's third law the period of the orbit is determined by the semi-major axis. It follows that the Earth's orbital period, the length of a sidereal year, also remains unchanged as the orbit evolves.
Orbital shape and temperature
|2005||Winter solstice||Summer solstice||21 December 2005 18:35||88.99 days|
|2006||Spring equinox||Autumn equinox||20 March 2006 18:26||92.75 days|
|2006||Summer solstice||Winter solstice||21 June 2006 12:26||93.65 days|
|2006||Autumn equinox||Spring equinox||23 September 2006 4:03||89.85 days|
|2006||Winter solstice||Summer solstice||22 December 2006 0:22||88.99 days|
|2007||Spring equinox||Autumn equinox||21 March 2007 0:07||92.75 days|
|2007||Summer solstice||Winter solstice||21 June 2007 18:06||93.66 days|
|2007||Autumn equinox||Spring equinox||23 September 2007 9:51||89.85 days|
|2007||Winter solstice||Summer solstice||22 December 2007 06:08|
As the semi-major axis is a constant, eccentricity works by varying the semi-minor axis. This decreases as eccentricity increases. When the semi-major axis decreases, the magnitude of seasonal changes increases.
The relative increase in solar irradiation at closest approach to the Sun (perihelion) compared to the irradiation at the furthest distance (aphelion) is slightly larger than four times the eccentricity. For Earth's current orbital eccentricity, incoming solar radiation varies by about 6.8%, while the distance from the Sun currently varies by only 3.4% (5.1 million km). Perihelion presently occurs around January 3, while aphelion is around July 4. When the orbit is at its most eccentric, the amount of solar radiation at perihelion will be about 23% more than at aphelion. However, the Earth's eccentricity is always so small that the variation in solar irradiation is a minor factor in seasonal climate variation, compared to axial tilt and even compared to the relative ease of heating the larger land masses of the northern hemisphere.
Kepler's second law says that a body in orbit traces equal areas over time; its orbital velocity increases around perihelion and decreases around aphelion. This means that the lengths of the seasons vary; orbital mechanics states that they are proportional to the areas of the seasonal quadrants. When the Earth's orbit is most eccentric, the Earth's orbital motion and the lengths of the seasons vary the most.
Currently, autumn and winter in the northern hemisphere occur around perihelion. The Earth is moving at its maximum velocity, and therefore these seasons are slightly shorter than spring and summer. Thus, summer in the northern hemisphere is 4.66 days longer than winter and spring is 2.9 days longer than autumn. When the Earth's orbit becomes more eccentric, it will spend more time near aphelion and less time near perihelion.
Axial tilt (obliquity)
The angle of the Earth's axial tilt (obliquity of the ecliptic) varies with respect to the plane of the Earth's orbit. These slow 2.4° obliquity variations are roughly periodic, taking approximately 41,000 years to shift between a tilt of 22.1° and 24.5° and back again. When the obliquity increases, the amplitude of the seasonal cycle in insolation increases, with summers in both hemispheres receiving more radiative flux from the Sun, and winters less. Conversely, when the obliquity decreases, summers receive less insolation and winters more.
But these changes of opposite sign in summer and winter are not of the same magnitude everywhere on the Earth's surface. At high latitude the annual mean insolation increases with increasing obliquity, while lower latitudes experience a reduction in insolation. Cooler summers are suspected of encouraging the onset of an ice age by melting less of the previous winter's precipitation. Because most of the planet's snow and ice lies at high latitude, it can be argued that lower obliquity favors ice ages for two reasons: the reduction in overall summer insolation and the additional reduction in mean insolation at high latitude.
Currently the Earth is tilted at 23.44 degrees from its orbital plane, roughly halfway between its extreme values. The tilt is in the decreasing phase of its cycle, and will reach its minimum value around the year 11,800 CE ; the last maximum was reached in 8,700 BCE. This trend in forcing, by itself, tends to make winters warmer and summers colder (i.e. milder seasons), as well as cause an overall cooling trend.
Precession is the trend in the direction of the Earth's axis of rotation relative to the fixed stars, with a period of roughly 26,000 years. This gyroscopic motion is due to the tidal forces exerted by the Sun and the Moon on the solid Earth; both contribute roughly equally to this effect.
Currently, perihelion occurs during the southern hemisphere's summer. This means that solar radiation due to (1) axial tilt aiming the southern hemisphere toward the Sun and (2) the Earth's proximity to the sun, both reach maximum during the summer and both reach minimum during the winter. Their effects on heating are additive, which means that the southern hemisphere's seasons are relatively more extreme. In the northern hemisphere, these two factors reach maximum at opposite times of the year: The north is tilted toward the Sun when the Earth is furthest from the Sun. The two forces work in opposite directions, resulting in less extreme seasons.
In about 13,000 years, the north pole will be tilted toward the Sun when the Earth is at perihelion. Axial tilt and orbital eccentricity will both contribute their maximum increase in solar radiation during the northern hemisphere's summer. Seasons will be more extreme in the northern hemisphere and less extreme in the south.
When the Earth's axis is aligned such that aphelion and perihelion occur near the equinoxes, axial tilt will not be aligned with or against eccentricity, and they will not make summer or winter more or less extreme.
In addition, the orbital ellipse itself precesses in space, in an irregular fashion, completing a full cycle every 112,000 years relative to the fixed stars. Apsidal precession occurs in the plane of the ecliptic and alters the orientation of the Earth's orbit relative to the ecliptic. This happens primarily as a result of interactions with Jupiter and Saturn. Smaller contributions are also made by the sun's oblateness and by the effects of general relativity that are well known for Mercury.
The apsidal precession combined with the axial precession means that it takes between 20,800 and 29,000 years for the perihelion to return to the position in the year. On average this shortens the period of the precession of the equinoxes with respect to the perihelion from 25,771.5 to 23,000 years.
As the orientation of Earth's orbit changes, each season will gradually start earlier in the year. Precession means the Earth's nonuniform motion (see above) will affect different seasons. Winter, for instance, will be in a different section of the orbit. When the Earth's apsides are aligned with the equinoxes, the length of spring and summer combined will equal that of autumn and winter. When they are aligned with the solstices, the difference in the length of these seasons will be greatest.
The inclination of Earth's orbit drifts up and down relative to its present orbit. Milankovitch did not study this three-dimensional movement. This movement is known as "precession of the ecliptic" or "planetary precession".
More recent researchers noted this drift and that the orbit also moves relative to the orbits of the other planets. The invariable plane, the plane that represents the angular momentum of the Solar System, is approximately the orbital plane of Jupiter. Currently, the inclination of Earth's orbit is 1.57°. It drifts up and down relative to its present orbit with a cycle having a period of about 70,000 years. The inclination of the Earth's orbit has a 100,000-year cycle relative to the invariable plane. This is very similar to the 100,000-year eccentricity period. This 100,000-year cycle closely matches the 100,000-year pattern of glacial events.
It has been proposed that a disk of dust and other debris exists in the invariable plane, and this affects the Earth's climate through several possible means. The Earth presently moves through this plane around January 9 and July 9, when there is an increase in radar-detected meteors and meteor-related noctilucent clouds.
Because the observed periodicities of climate fit so well with the orbital periods, the orbital theory has overwhelming support. Nonetheless, there are several difficulties in reconciling theory with observations.
The 100,000-year problem is that the eccentricity variations have a significantly smaller impact on solar forcing than precession or obliquity – according to theory – and hence might be expected to produce the weakest effects. However, the greatest observed response in regard to the ice ages is at the 100,000-year timescale, even though the theoretical forcing is smaller at this scale. During the last 1 million years, the strongest climate signal is the 100,000-year cycle. In addition, despite the relatively great 100,000-year cycle, some have argued that the length of the climate record is insufficient to establish a statistically significant relationship between climate and eccentricity variations. Various explanations for this discrepancy have been proposed, including frequency modulation or various feedbacks (from carbon dioxide, cosmic rays, or from ice sheet dynamics). Some models can reproduce the 100,000-year cycles as a result of non-linear interactions between small changes in the Earth's orbit and internal oscillations of the climate system.
Stage 5 problem
Deep-sea core samples show that marine isotope stage 5 began 130,000 years ago, 10,000 years before the solar forcing that the Milankovitch hypothesis predicts. (This is also known as the causality problem, because the effect precedes the putative cause.)
Effect exceeds cause
The effects of these variations are primarily believed to be due to variations in the intensity of solar radiation upon various parts of the globe. Observations show climate behavior is much more intense than the calculated variations. Various internal characteristics of climate systems are believed to be sensitive to the insolation changes, causing amplification (positive feedback) and damping responses (negative feedback).
The unsplit peak problem
The unsplit peak problem refers to the fact that eccentricity has cleanly resolved variations at both the 95 and 125 ka periods. A sufficiently long, well-dated record of climate change should be able to resolve both frequencies. However, some researchers interpret climate records of the last million years as showing only a single spectral peak at 100 ka periodicity
The transition problem
The transition problem refers to the switch in the frequency of climate variations 1 million years ago. From 1–3 million years, climate had a dominant mode matching the 41 ka cycle in obliquity. After 1 million years ago, this switched to a 100 ka variation matching eccentricity, for which no reason has been established.
Identifying dominant factor
Milankovitch believed that decreased summer insolation in northern high latitudes was the dominant factor leading to glaciation, which led him to (incorrectly) deduce an approximate 41 ka period for ice ages. Subsequent research has shown that ice age cycles of the Quaternary glaciation over the last million years have been at a 100,000-year period, leading to identification of the 100 ka eccentricity cycle as more important, although the exact mechanism remains obscure.
Present and future conditions
As mentioned above, at present, perihelion occurs during the southern hemisphere's summer and aphelion during the southern winter. Thus the southern hemisphere seasons should tend to be somewhat more extreme than the northern hemisphere seasons. The relatively low eccentricity of the present orbit results in a 6.8% difference in the amount of solar radiation during summer in the two hemispheres.
Since orbital variations are predictable, if one has a model that relates orbital variations to climate, it is possible to run such a model forward to "predict" future climate. Two caveats, however, are necessary: that anthropogenic effects may modify or even overwhelm orbital effects; and that the mechanism by which orbital forcing influences climate is not well understood. In the most prominent anthropogenic example, orbital forcing from the Milankovitch cycles has been in a cooling phase for millennia, but that cooling trend was reversed in the 20th and 21st centuries due to warming caused by increased anthropogenic greenhouse gas emissions.
The amount of solar radiation (insolation) in the Northern Hemisphere at 65° N seems to be related to occurrence of an ice age. Astronomical calculations show that 65° N summer insolation should increase gradually over the next 25,000 years. A regime of eccentricity lower than the current value will last for about the next 100,000 years. Changes in northern hemisphere summer insolation will be dominated by changes in obliquity ε. No declines in 65° N summer insolation, sufficient to cause a glacial period, are expected in the next 50,000 years.
An often-cited 1980 study by Imbrie and Imbrie determined that, "Ignoring anthropogenic and other possible sources of variation acting at frequencies higher than one cycle per 19,000 years, this model predicts that the long-term cooling trend that began some 6,000 years ago will continue for the next 23,000 years."
Effects beyond Earth
Other planets in the Solar System have been discovered to have Milankovitch cycles. Mostly these cycles are not as intense or complex as the Earth's cycles, but they do have a global geological impact with respect to the movement of mobile solids such as water or nitrogen ices or hydrocarbon lakes. The known affected planets are:
Scientists using computer models to study more extreme axial tilts than Earth ever experiences have concluded that climate extremes at high obliquity would be particularly threatening to advanced forms of life like those on Earth. They noted that high obliquity would not likely sterilize a planet completely, but would make it harder for warm-blooded, land-based life to thrive.
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- Data from United States Naval Observatory
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- The oldest reference for Milankovitch cycles is: M. Milankovitch, Mathematische Klimalehre und Astronomische Theorie der Klimaschwankungen, Handbuch der Klimatologie, Band I, Teil A,Berlin, Verlag von Gebrüder Borntraeger, 1030.
- Roe G (2006). "In defense of Milankovitch". Geophysical Research Letters. 33 (24): L24703. Bibcode:2006GeoRL..3324703R. doi:10.1029/2006GL027817. This shows that Milankovitch theory fits the data extremely well, over the past million years, provided that we consider derivatives.
- Kaufmann R. K.; Juselius K. (2016), "Testing competing forms of the Milankovitch hypothesis", Paleoceanography, 31: 286–297, doi:10.1002/2014PA002767.
- Edvardsson S, Karlsson KG, Engholm M (2002). "Accurate spin axes and solar system dynamics: Climatic variations for the Earth and Mars". Astronomy and Astrophysics. 384 (2): 689–701. Bibcode:2002A&A...384..689E. doi:10.1051/0004-6361:20020029. This is the first work that investigated the derivative of the ice volume in relation to insolation (page 698).
- Zachos J, Pagani M, Sloan L, Thomas E, Billups K (2001). "Trends, Rhythms, and Aberrations in Global Climate 65 Ma to Present". Science. 292 (5517): 686–693. Bibcode:2001Sci...292..686Z. doi:10.1126/science.1059412. PMID 11326091.
This review article discusses cycles and great-scale changes in the global climate during the Cenozoic Era.
- Pälike, H., R.D. Norris, J.O. Herrle, P.A. Wilson, H.K. Coxall, C.H. Lear, N.J. Shackleton, A.K. Tripati, and B.S. Wade (2006). "The Heartbeat of the Oligocene Climate System". Science. 314 (5807): 1894–1898. doi:10.1126/science.1133822.
A 13-million-year continuous record of Oligocene climate from the equatorial Pacific reveals a pronounced “heartbeat” in the global carbon cycle and periodicity of glaciations.
|Wikimedia Commons has media related to Milankovitch cycles.|
|The Wikibook Historical Geology has a page on the topic of: Milankovitch cycles|
- Ice Age – Milankovitch Cycles – National Geographic Channel
- The Milankovitch band, Internet Archive of American Geophysical Union lecture
- Some history of the adoption of the Milankovitch hypothesis (and an alternative)
- More detail on orbital obliquity also matching climate patterns
- "Milutin Milankovitch". On the Shoulders of Giants. Retrieved January 15, 2010.
- The Seasons
- "Climate Forcing Data". NOAA. National Centers for Environmental Information.
Text: includes (calculated) data on orbital variations over the last 50 million years and for the coming 20 million years.
- The Orbital Simulations; by Varadi, Ghil and Runnegar (2003) provide another, slightly different series for orbital eccentricity, and also a series for orbital inclination
- ABC: Earth wobbles linked to extinctions
- Milankovitch Cycles & Ice Ages