Milankovitch cycles describe the collective effects of changes in the Earth's movements on its climate over thousands of years. The term is named for Serbian geophysicist and astronomer Milutin Milanković. In the 1920s, he hypothesized that variations in eccentricity, axial tilt, and precession resulted in cyclical variation in the solar radiation reaching the Earth, and that this orbital forcing strongly influenced climatic patterns on Earth.
Similar astronomical hypotheses had been advanced in the 19th century by Joseph Adhemar, James Croll and others, but verification was difficult because there was no reliably dated evidence, and because it was unclear which periods were important.
Now, materials on Earth that have been unchanged for millennia (obtained via ice, rock, and deep ocean cores) are being studied to indicate the history of Earth's climate. Though they are consistent with the Milankovitch hypothesis, there are still several observations that the hypothesis does not explain.
The Earth's rotation around its axis, and revolution around the Sun, evolve over time due to gravitational interactions with other bodies in the Solar System. The variations are complex, but a few cycles are dominant.
The Earth's orbit varies between nearly circular and mildly elliptical (its eccentricity varies). When the orbit is more elongated, there is more variation in the distance between the Earth and the Sun, and in the amount of solar radiation, at different times in the year. In addition, the rotational tilt of the Earth (its obliquity) changes slightly.
A greater tilt makes the seasons more extreme. Finally, the direction in the fixed stars pointed to by the Earth's axis changes (axial precession), while the Earth's elliptical orbit around the Sun rotates (apsidal precession). The combined effect is that proximity to the Sun occurs during different astronomical seasons.
Milankovitch studied changes in these movements of the Earth, which alter the amount and location of solar radiation reaching the Earth. This is known as solar forcing (an example of radiative forcing). Milankovitch emphasized the changes experienced at 65° north due to the great amount of land at that latitude. Land masses change temperature more quickly than oceans, because of the mixing of surface and deep water and the fact that soil has a lower volumetric heat capacity than water.
The Earth's orbit approximates an ellipse. Eccentricity measures the departure of this ellipse from circularity. The shape of the Earth's orbit varies between nearly circular (with the lowest eccentricity of 0.000055) and mildly elliptical (highest eccentricity of 0.0679).Its geometric or logarithmic mean is 0.0019. The major component of these variations occurs with a period of 413,000 years (eccentricity variation of ±0.012). Other components have 95,000-year and 125,000-year cycles (with a beat period of 400,000 years). They loosely combine into a 100,000-year cycle (variation of −0.03 to +0.02). The present eccentricity is 0.017 and decreasing.
Eccentricity varies primarily due to the gravitational pull of Jupiter and Saturn. However, the semi-major axis of the orbital ellipse remains unchanged; according to perturbation theory, which computes the evolution of the orbit, the semi-major axis is invariant. The orbital period (the length of a sidereal year) is also invariant, because according to Kepler's third law, it is determined by the semi-major axis.
The semi-major axis is a constant. Therefore, when Earth's orbit becomes more eccentric, the semi-minor axis shortens. This increases the magnitude of seasonal changes.
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 or 3.2 million mi or 0.034 au).
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.
|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|
The seasons are quadrants of the Earth's orbit, marked by the two solstices and the two equinoxes. Kepler's second law states that a body in orbit traces equal areas over equal times; its orbital velocity is highest around perihelion and lowest around aphelion. The Earth spends less time near perihelion and more time near aphelion. This means that the lengths of the seasons vary.
Perihelion currently occurs around January 3, so the Earth's greater velocity shortens winter and autumn in the northern hemisphere. Summer in the northern hemisphere is 4.66 days longer than winter, and spring is 2.9 days longer than autumn.
Greater eccentricity increases the variation in the Earth's orbital velocity. However, currently, the Earth's orbit is becoming less eccentric (more nearly circular). This will make the seasons more similar in length.
The angle of the Earth's axial tilt with respect to the orbital plane (the obliquity of the ecliptic) varies between 22.1° and 24.5°, over a cycle of about 41,000 years. The current tilt is 23.44°, roughly halfway between its extreme values. The tilt last reached its maximum in 8,700 BCE. It is now in the decreasing phase of its cycle, and will reach its minimum around the year 11,800 CE.
Increased tilt increases the amplitude of the seasonal cycle in insolation, providing more solar radiation in each hemisphere's summer and less in winter. However, these effects are not uniform everywhere on the Earth's surface. Increased tilt increases the total annual solar radiation at higher latitudes, and decreases the total closer to the equator.
The current trend of decreasing tilt, by itself, will promote milder seasons (warmer winters and colder summers), as well as an overall cooling trend. Because most of the planet's snow and ice lies at high latitude, decreasing tilt may encourage the onset of an ice age for two reasons: There is less overall summer insolation, and also less insolation at higher latitudes, which melts less of the previous winter's snow and ice.
Axial precession is the trend in the direction of the Earth's axis of rotation relative to the fixed stars, with a period of 25,771.5 years. This motion means that eventually Polaris will no longer be the north pole star. It is caused by 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 inclining the southern hemisphere toward the Sun and (2) the Earth's proximity to the Sun, both reach maximum during the southern summer and both reach minimum during the southern winter. Their effects on heating are thus additive, which means that seasonal variation in irradiation of the southern hemisphere is 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 effects work in opposite directions, resulting in less extreme variations in insolation.
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. Axial precession will promote more extreme variation in irradiation of the northern hemisphere and less extreme variation 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.
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.
Apsidal precession combines with the 25,771.5-year cycle of axial precession (see above) to vary the position in the year that the Earth reaches perihelion. Apsidal precession shortens this period to 23,000 years on average (varying between 20,800 and 29,000 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. This three-dimensional movement is known as "precession of the ecliptic" or "planetary precession". Earth's current inclination relative to the invariable plane (the plane that represents the angular momentum of the Solar System, approximately the orbital plane of Jupiter) is 1.57°.
Milankovitch did not study planetary precession. It was discovered more recently and measured, relative to Earth's orbit, to have a period of about 70,000 years. However, when measured independently of Earth's orbit, but relative to the invariable plane, precession has a period of about 100,000 years. This period is very similar to the 100,000-year eccentricity period. Both periods closely match the 100,000-year pattern of glacial events.
Materials taken from the Earth have been studied to infer the cycles of past climate. Antarctic ice cores contain trapped air bubbles whose ratios of different oxygen isotopes are a reliable proxy for global temperatures around the time the ice was formed. Study of this data 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 of lake depths, 1,700 ft (520 m) core of rock drilled in Arizona show a pattern synchronized with Earth's eccentricity, and cores drilled in New England match it, going back 215 million years.and a seminal paper by Hays, Imbrie, and Shackleton provide additional validation through physical evidence. Climate records contained in a
Of all the orbital cycles, Milankovitch believed that obliquity had the greatest effect on climate, and that it did so by varying the summer insolation in northern high latitudes. Therefore, he deduced a 41,000-year period for ice ages.However, subsequent research has shown that ice age cycles of the Quaternary glaciation over the last million years have been at a period of 100,000 years, which matches the eccentricity cycle.
Various explanations for this discrepancy have been proposed, including frequency modulationor 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.
Jung-Eun Lee of Brown University proposes that precession changes the amount of energy that Earth absorbs, because the southern hemisphere's greater ability to grow sea ice reflects more energy away from Earth. Moreover, Lee says, "Precession only matters when eccentricity is large. That's why we see a stronger 100,000-year pace than a 21,000-year pace."
Some have argued that the length of the climate record is insufficient to establish a statistically significant relationship between climate and eccentricity variations.
In fact, from 1–3 million years ago, climate cycles did match the 41,000-year cycle in obliquity. After 1 million years ago, the Mid-Pleistocene Transition (MPT) occurred with switch to the 100,000-year cycle matching eccentricity. The transition problem refers to the need to explain what changed 1 million years ago.The MPT can now be reproduced in numerical simulations that include a decreasing trend in carbon dioxide and glacially induced removal of regolith.
Even the well-dated climate records of the last million years do not exactly match the shape of the eccentricity curve. Eccentricity has component cycles of 95,000 and 125,000 years. However, some researchers say the records do not show these peaks, but only show a single cycle of 100,000 years.
Deep-sea core samples show that the interglacial interval known as marine isotope stage 5 began 130,000 years ago. This is 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.)
Physical evidence shows that the variation in Earth's climate is much more extreme than the variation in the intensity of solar radiation calculated as the Earth's orbit evolves. If orbital forcing causes climate change, science needs to explain why the observed effect is amplified out of linear proportion to the theoretical cause.
Some climate systems exhibit amplification (positive feedback) and others exhibit damping responses (negative feedback). As an illustration, if during an ice age the northern land masses were covered in year-round ice, solar energy would be reflected away, counteracting the eventual warming effect from orbital forcing and extending the ice age.
The Earth's current orbital inclination is 1.57° (see above). Earth presently moves through the invariable plane around January 9 and July 9. At these times, there is an increase in meteors and noctilucent clouds. If this is because there is a disk of dust and debris in the invariable plane, then when the Earth's orbital inclination is near 0° and it is orbiting through this dust, materials could be accreted into the atmosphere. This process could explain the narrowness of the 100,000-year climate cycle.
Since orbital variations are predictable,any model that relates orbital variations to climate can be run forward to predict future climate, with two caveats: the mechanism by which orbital forcing influences climate is not definitive; and non-orbital effects can be important (for example, the human impact on the environment principally increases greenhouse gases resulting in a warmer climate ).
An often-cited 1980 orbital model by Imbrie predicted "the long-term cooling trend that began some 6,000 years ago will continue for the next 23,000 years." N summer insolation over the next 25,000 years. [ failed verification ] Earth's orbit will become less eccentric for about the next 100,000 years, so changes in this insolation will be dominated by changes in obliquity, and should not decline enough to permit a new glacial period in the next 50,000 years.More recent work suggests that orbital variations should gradually increase 65°
Other bodies in the Solar System undergo orbital fluctuations like the Milankovitch cycles. Any geological effects would not be as pronounced as climate change on the Earth, but might cause the movement of elements in the solid state.
Mars has no moon large enough to stabilize its obliquity, which has varied from 10 to 70 degrees. This would explain recent observations of its surface compared to evidence of different conditions in its past, such as the extent of its polar caps.
Saturn's moon Titan has a cycle of approximately 60,000 years that could change the location of the methane lakes.Neptune's moon Triton has a variation similar to Titan's, which could cause its solid nitrogen deposits to migrate over long time scales.
Scientists using computer models to study extreme axial tilts have concluded that high obliquity could cause extreme climate variations, and while that would probably not render a planet uninhabitable, it could pose difficulty for land-based life in affected areas. Most such planets would nevertheless allow development of both simple and more complex lifeforms.Although the obliquity they studied is more extreme than Earth ever experiences, there are scenarios 1.5 to 4.5 billion years from now, as the Moon's stabilizing effect lessens, where obliquity could leave its current range and the poles could eventually point almost directly at the Sun.
An ice age is a long period of reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages and greenhouse periods, during which there are no glaciers on the planet. Earth is currently in the Quaternary glaciation, known in popular terminology as the Ice Age. Individual pulses of cold climate within an ice age are termed "glacial periods", and intermittent warm periods within an ice age are called "interglacials" or "interstadials", with both climatic pulses part of the Quaternary or other periods in Earth's history.
In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, or, equivalently, the angle between its equatorial plane and orbital plane. It differs from orbital inclination.
Milutin Milanković was Serbian mathematician, astronomer, climatologist, geophysicist, civil engineer and popularizer of science.
James Croll, FRS, was a 19th-century Scottish scientist who developed a theory of climate variability based on changes in the Earth's orbit.
Earth orbits the Sun at an average distance of 149.60 million km, and one complete orbit takes 365.256 days, during which time Earth has traveled 940 million km. Ignoring the influence of other solar system bodies, Earth's orbit is an ellipse with the Earth-Sun barycenter as one focus and a current eccentricity of 0.0167; since this value is close to zero, the center of the orbit is close, relative to the size of the orbit, to the center of the Sun.
A glacial period is an interval of time within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate between glacial periods. The last glacial period ended about 15,000 years ago. The Holocene epoch is the current interglacial. A time with no glaciers on Earth is considered a greenhouse climate state.
The orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values between 0 and 1 form an elliptic orbit, 1 is a parabolic escape orbit, and greater than 1 is a hyperbola. The term derives its name from the parameters of conic sections, as every Kepler orbit is a conic section. It is normally used for the isolated two-body problem, but extensions exist for objects following a Klemperer rosette orbit through the galaxy.
Orbital forcing is the effect on climate of slow changes in the tilt of the Earth's axis and shape of the Earth's orbit around the sun. These orbital changes vary the total amount of sunlight reaching the Earth by up to 25% at mid-latitudes. In this context, the term "forcing" signifies a physical process that affects the Earth's climate.
In many cases astronomical phenomena viewed from the planet Mars are the same or similar to those seen from Earth but sometimes they can be quite different. For example, because the atmosphere of Mars does not contain an ozone layer, it is also possible to make UV observations from the surface of Mars.
The Quaternary glaciation, also known as the Pleistocene glaciation, is an alternating series of glacial and interglacial periods during the Quaternary period that began 2.58 Ma, and is ongoing. Although geologists describe the entire time period as an "ice age", in popular culture the term "ice age" is usually associated with just the most recent glacial period during the Pleistocene. Since the planet Earth still has ice sheets, geologists consider the Quaternary glaciation to be ongoing, with the Earth now experiencing an interglacial period.
John Imbrie was an American paleoceanographer best known for his work on the theory of ice ages. He was the grandson of William Imbrie, an American missionary to Japan.
The 100,000-year problem of the Milankovitch theory of orbital forcing refers to a discrepancy between the reconstructed geologic temperature record and the reconstructed amount of incoming solar radiation, or insolation over the past 800,000 years. Due to variations in the Earth's orbit, the amount of insolation varies with periods of around 21,000, 40,000, 100,000, and 400,000 years. Variations in the amount of incident solar energy drive changes in the climate of the Earth, and are recognised as a key factor in the timing of initiation and termination of glaciations.
Throughout the history of the Earth, the planet's climate has been fluctuating between two dominant climate states: the greenhouse Earth and the icehouse Earth. These two climate states last for millions of years and should not be confused with glacial and interglacial periods, which occur only during an icehouse period and tend to last less than 1 million years. There are five known great glaciations in Earth's climate history; the main factors involved in changes of the paleoclimate are believed to be the concentration of atmospheric carbon dioxide, changes in the Earth's orbit, long-term changes in the solar constant, and oceanic and orogenic changes due to tectonic plate dynamics. Greenhouse and icehouse periods have profoundly shaped the evolution of life on Earth.
James D. Hays is a professor of Earth and environmental sciences at Columbia University's Lamont-Doherty Earth Observatory. Hays founded and led the CLIMAP project, which collected sea floor sediment data to study surface sea temperatures and paleoclimatological conditions 18,000 years ago.
In celestial mechanics, apsidal precession is the precession of the line connecting the apsides of an astronomical body's orbit. The apsides are the orbital points closest (periapsis) and farthest (apoapsis) from its primary body. The apsidal precession is the first time derivative of the argument of periapsis, one of the six main orbital elements of an orbit. Apsidal precession is considered positive when the orbit's axis rotates in the same direction as the orbital motion. An apsidal period is the time interval required for an orbit to precess through 360°.
Lorraine Lisiecki is an American paleoclimatologist. She is a professor in the Department of Earth Sciences at the University of California, Santa Barbara. She has proposed a new analysis of the 100,000-year problem in the Milankovitch theory of climate change. She also created the analytical software behind the LR04, a "standard representation of the climate history of the last five million years".
André Léon Georges Chevalier Berger is a Belgian professor and climatologist. He is best known for his significant contribution to the renaissance and further development of the astronomical theory of paleoclimates and as a cited pioneer of the interdisciplinary study of climate dynamics and history.
North African climate cycles have a unique history that can be traced back millions of years. The cyclic climate pattern of the Sahara is characterized by significant shifts in the strength of the North African Monsoon. When the North African Monsoon is at its strongest, annual precipitation and consequently vegetation in the Sahara region increase, resulting in conditions commonly referred to as the "green Sahara". For a relatively weak North African Monsoon, the opposite is true, with decreased annual precipitation and less vegetation resulting in a phase of the Sahara climate cycle known as the "desert Sahara".
There are various solar/celestial effects that exist which have an effect on Earth's climate. These effects usually occur in cycles, and primarily include how Earth's obliquity, the eccentricity of Earth's orbit, and the precession of the equinoxes and solstices affect Earth's climate. In addition to these effects, there are also other factors that have an effect on Earth's climate. These other factors include how sun activity affects climate and how celestial phenomena, such as meteors, affect Earth's climate. Some of these factors aren't yet well understood, for instance the ice ages occur on 100,000 year cycles, and it's not completely understood why the various effects with this periodicity have such a strong effect on glaciation.
The dwarf planet Pluto has an unusual set of climate zones, due to its atypical axial configuration. Five climate zones are assigned on the dwarf planet: tropics, arctic, tropical arctic, diurnal, and polar. These climate zones are delineated based on astronomically defined boundaries or sub-solar latitudes, which are not associated with the atmospheric circulations on the dwarf planet. Charon, the largest moon of Pluto, is tidally locked with it, and thus has the same climate zone structure as Pluto itself.
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.CS1 maint: uses authors parameter (link)