Orbital variations

The past and future of Milanković cycles helps to understand the prediction of past and future orbital parameters with great precision. The figure shows variations in the orbital elements, such as the Orbital Inclination (orbital Inclination), the Eccentricity, the Longitude of the Periaster and the equinoccial Precession Index, which, together with the obliqueness, controls the seasonal cycle of insolation. Likewise, the amount of insolation calculated daily in the upper area of the atmosphere during the summer solstice appears at a level of latitude of 65o N. Two different levels appear for sea level and ocean temperature, both obtained from the marine sediments and the Antarctic ice, extracted from the benthic deposits and from the ice core in the Russian Antarctic base of Vostok. The vertical grey line shows the current conditions towards 2000 D.C.

The orbital variations or Milanković cycles describe the joint effects that changes in the Earth's motion have on the climate over thousands of years. The term was coined after studies carried out by the Serbian astronomer and geophysicist Milutin Milanković. In the 1920s, he theorized that the resulting variations caused cyclical changes in solar radiation reaching the Earth's surface and that this greatly influenced the patterns of climate change on Earth.

Similar astronomical theories had been advanced during the 19th century by Joseph Adhemar, James Croll, and others, but their verification was difficult due to the lack of relevant fossil data and because it was also unclear which periods in the past were important for testing..

Currently, geological materials on the Earth's surface that have not changed for thousands of years are being studied by specialists to find out about changes in the Earth's climate. Despite the fact that many of them are consistent with the hypothesis of Milankovitch's theories, there is a set of them that predictable hypotheses are not capable of explaining.

Movements of the Earth

The rotation of the Earth around its own axis and its translation around the Sun are disturbed over time by other astronomical bodies present in the solar system. These variations are highly complex, but a few particular cycles dominate over others.

Earth's orbit varies from nearly circular to nearly elliptical, so its eccentricity changes. When the orbit is more elongated, there is more distance between the Earth and the Sun and the global set of solar radiation changes at different times of the year. Also, the Earth's tilt (its obliquity) varies slightly. A great inclination causes more extreme seasons at the climatic level. Finally, the direction in which the Earth's axis of rotation is pointing also changes over time (the so-called Precession of the Equinoxes) as the elliptical orbit around the Sun revolves over time. The combined effect of both causes the greater or lesser proximity to the Sun to vary during the different seasons over time.

Milanković studied the changes in these movements of the Earth, which cause alterations in the amount of solar radiation that reaches its surface. This phenomenon is known as radiative forcing. Milanković placed special emphasis on the changes experienced over 65º north latitude due to the large amount of land surface emerged at that latitude. Large continental land masses change temperature more rapidly than in the oceans, because in large water masses the exchange between the surface and the great liquid depths retards the heating or cooling of the surface, regardless of whether the land surface it has less volumetric warming capacity than the oceans.

Orbital shape (eccentricity)

Earth's translational orbit nearly approximates an ellipse. The orbital eccentricity measures the difference of said ellipse with respect to a perfect circle. The type of Earth's orbit varies between a nearly circular shape (with its lowest eccentricity of 0.000055) and another half-elliptical (highest eccentricity of 0.0679). Its main mean eccentricity is 0.0019. The main change in these variations occurs over a period of approximately 413,000 years (with an eccentricity variation of ±0.012). Other changes occur with a cycle sequence of 95,000 and 125,000 years (with a cyclic rate of 400,000). These movements are combined with each other with variations from −0.03 to +0.02. The current eccentricity is 0.017 and is decreasing.

The eccentricity varies mainly due to the gravitational pull of Jupiter and Saturn. However, the semi-major axis of the orbit of the ellipse remains unchanged, although according to the astronomical theory of perturbation that records its evolution, said axis is an adiabatic invariant. The orbital period (the length of the sidereal year) has not changed either, because, according to Kepler's Third Law of planetary motion, it is determined by the semimajor orbital axis.

Effects on temperature

The semimajor axis is a constant. Therefore, as the Earth's orbit becomes more eccentric, the semi-minor axis becomes shorter. This causes the increase in the magnitude of seasonal changes.

The relative increase in solar radiation at its closest approach to the Sun (perihelion) compared to the irradiation at Earth's greatest distance from the Sun (aphelion) is slightly greater by four times the total eccentricity. For the current eccentricity of the Earth, the incoming solar radiation varies by about 6.8%, while the distance between the Sun and the Earth is only 3.4% (5.1 million km). Currently, perihelion coincides approximately with January 3, while aphelion occurs around July 4. When the orbit is at its most eccentric point, the amount of solar radiation at perihelion can be 23% greater than at aphelion. Therefore, the terrestrial eccentricity is always so small that the variation in the amount of solar radiation is a minor factor in the variation of seasonal changes compared to the axial tilt of the axis and even to the heating that occurs over the great masses. continental northern hemisphere.

Effect on the duration of the seasons

The seasons are quadrants of the Earth's orbit separated by the two solstices and the two equinoxes. Kepler's second Law of planetary motion determines that an orbiting body traces areas of equal size in identical times; although its orbital speed is greater during perihelion than during aphelion, when it decreases due to less gravity. Earth spends less time near perihelion and more time near aphelion. This means that the length of the seasons varies.

Perihelion occurs around January 3, so Earth's increased speed shortens winter and fall in the Northern Hemisphere. Northern Hemisphere summer is 4.66 days longer than winter and spring 2.9 days longer than fall.

Greater eccentricity increases the variation of Earth's orbital speed. Therefore, the Earth's orbit is becoming less eccentric (closer to circular). This eventually causes the seasons to be more similar in length.

Axial inclination (obliquity)

From 22.1 to 24.5. Land oblique range

The angle of the inclination of the Earth's axial axis with respect to the orbital plane (the obliquity of the ecliptic) varies from 22.1° to 24.5° in a cycle of approximately 41,000 years. The current inclination is 23.44°, roughly a middle ground between the two extreme values. The inclination reached its maximum in the year 8,700 B.C. Currently, it is in the decreasing phase of its cycle and will reach its minimum in the year 11,800 of our current era.

The higher inclination increases the amplitude of the seasonal cycle in its amount of insolation, providing more amount of solar radiation in each hemisphere during the summer and less during the winter. Therefore, these effects are not uniform over the Earth's surface. Greater inclinations of the axis increase the total solar radiation at high latitudes and decrease the same the closer they are to the Equator.

The current downward trend by itself leads to less extreme seasons with warmer winters and cooler summers as well as a general cooling trend. Because most of the planet's snow and ice is found at high latitudes on land land surface, the decreasing tilt could trigger the onset of an ice age for two reasons: there is less total insolation in summer and also less insolation at higher latitudes, it would melt less of the previous winter's snow and ice.

Axial Precession

Movement of the equinoccial precession

Axial precession or equinoctial precession is the tendency to change the direction of the Earth's axis of rotation with respect to the fixed stars in a turn with a period of 25,771.5 years. This movement means that for a while Polaris will no longer be the pole star of the northern hemisphere. This movement is caused by the tidal forces exerted by the Sun and the Moon on the Earth and both contribute approximately equally to the generation of this effect.

Currently, perihelion occurs during the southern hemisphere summer. This means that the solar radiation due to (A) the axial tilt pointing the southern hemisphere towards the Sun and (B) the proximity of the Earth to the Sun, both reach their maximum during the summer and reach a minimum during the winter. Its effects on warming are additive, meaning that the seasonal variation in southern hemisphere irradiance is more extreme. In the Northern Hemisphere, these two factors peak at opposite times of the year: North is tilted toward the Sun when Earth is furthest from it. The two forces work in opposite directions, resulting in less extreme variation.

In about 13,000 years, the north pole will tilt toward the Sun when Earth is at perihelion. The axial tilt and orbital eccentricity will contribute to its maximum increase in solar radiation during the northern hemisphere summer. Axial precession will promote a more extreme variation in irradiation in the northern hemisphere and a less extreme variation in the south.

When the Earth's axis is aligned such that aphelion and perihelion occur near the equinoxes, the axial tilt will not be aligned with or against the eccentricity.

Apsidal Precession

Planets orbiting the Sun follow elliptical (ovaled) orbits that gradually rotate over time (apsidal precession). The eccentricity of this ellipse is exaggerated for visualization. Most orbits in the Solar System have a much smaller eccentricity, which makes them almost circular.

In addition, the orbital ellipse itself precesses irregularly in space, completing one cycle every 112,000 years relative to the fixed stars. Apsidal precession occurs in the plane of the ecliptic and alters the orientation of Earth's orbit relative to the ecliptic. This happens mainly as a result of interactions with Jupiter and Saturn. Small disturbances are also caused by the flattening of the sun and by general relativity effects that are well known thanks to the planet Mercury.

Apsidal precession combines with the 25,771.5-year cycle of axial precession (see above) to vary position during the year Earth reaches perihelion. Apsidal precession shortens this period to 23,000 years on average (varying between 20,800 and 29,000 years).

Effects of the precession in the stations (using the values of the Northern Hemisphere).

As the orientation of Earth's orbit changes, each season will gradually start earlier each year. Precession means that the non-uniform movement of the Earth will affect the different seasons of the year. Winter, for example, will be in a different section of the orbit. When Earth's apses are aligned with the equinoxes, the length of spring and summer combined will be equal to that of fall and winter. When they are aligned with the solstices, the difference in the duration of these seasons will be greater.

Orbital inclination

The tilt of Earth's orbit shifts up and down relative to its current orbit. This three-dimensional movement is known as "precession of the ecliptic" or "planetary precession". The current tilt of the Earth is 1.57°.

Milankovitch did not study apsidal precession. It was discovered more recently and is estimated to have a period of 70,000 years relative to Earth's orbit.

However, when measured independently of Earth's orbit, but relative to the unchanging plane (the plane representing the angular momentum of the Solar System, roughly the orbital plane of Jupiter), the precession has a period of about 100 000 years. This period is very similar to the eccentricity period of 100,000 years. Both periods closely match the pattern of ice age cycles every 100,000 years.

Problems

The nature of the sediments can vary cyclically and these cycles can be shown in the sedimentary registry. Here, you can see cycles in coloring and resistance of different layers.

Samples taken from Earth's sediments have been systematically studied to infer past climate cycles. A study of the chronology of Antarctic ice cores using oxygen-nitrogen ratios in ice-trapped bubbles, which appear to respond directly to local insolation, concluded that the documented climate response in ice cores was induced by hemisphere insolation. North as proposed by Milankovitch's hypothesis. Analyzes of deep ocean sedimentary deposits by Hays, Imbrie, and Shackleton have provided additional validity to Milankovitch's theories by obtaining concrete physical samples.

These studies fit orbital periods so well that they support Milankovitch's theories that variations in Earth's orbit influence climate. However, the pattern is not perfect and the problems remain with reconciling the set of all theories with the observations.

The 100,000-year problem

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 at high northern latitudes. Thus, he deduced a period of 41,000 years for the great ice ages. However, subsequent research has shown that the ice age cycles of the Quaternary glaciation over the last million years are in line with a period of 100,000 years, which coincides much better with the eccentricity cycle.

Various explanations have been proposed for this discrepancy, including frequency modulation or various reactions (from carbon dioxide, to cosmic rays, or from ice sheet dynamics).

Some models can reproduce 100,000-year cycles as a result of nonlinear interactions between small changes in Earth's orbit and internal oscillations of the climate system.

Jung-Eun Lee, a professor at Brown University, proposes that precession changes the amount of energy absorbed by Earth, because the southern hemisphere's increased ability to produce sea ice reflects more energy from Earth back into space. Furthermore, Lee argues that: "Precession only matters when the eccentricity is large. For this reason we see a stronger rhythm of 100,000 years than a rhythm of 21,000".

Some have argued that the length of the climate record is insufficient to establish a statistically significant relationship between climate and eccentricity variations.

The problem of Transition

Cycle variations, curves determined from oceanic sediments

In fact, from a period between 1 and 3 million years ago, climatic cycles coincided with the 41,000-year cycle in relation to obliquity. However, 1 million years ago there was a change in cycles to 100,000 due to the effect of eccentricity. This is known as the Middle Pleistocene Transition or MPT. Although it was recently discovered that the 41,000 yr cycle did not disappear and the 100,000 yr cycle appeared, but that the 41,000 yr cycle continued to happen but was eclipsed by the 100,000 yr cycle and that is why only the 100,000 yr cycle appears in the geological record.

Unresolved key issue

Even well-dated climate records for the last million years do not exactly match the shape of the eccentricity curve. The eccentricity has cycles of 95,000 and 125,000 years. However, some researchers say that the records do not show these spikes, since only 100,000-year cycles actually show up.

5th stage problem

Samples of isotope cores taken in deep water show that the interglacial interval known as the stage 5 marine isotope began 130,000 years ago, or 10,000 years before the solar impulse predicted by Milankovitch's hypothesis. (This is also known as the "causation problem" because the effect precedes the putative cause.)

The effect exceeds the cause

420 000 years of data from the ice core of the Vostok, Antarctica research station

Records show that the variation in Earth's climate is much more extreme than the variation in solar radiation intensity calculated as Earth's orbit evolves. If orbital change causes climate change, science needs to explain why the observed effect is amplified compared to the theoretical effect.

Some weather systems theorize amplifying (positive feedback) and damping (negative feedback) responses. An example of amplification would be if with the land masses around 65° north latitude covered in ice all year round, most of the solar energy would end up being reflected back into space. Such an amplification would mean that an ice age induces terrestrial changes that prevent the orbital change from ending the ice age naturally.

Earth's current orbital inclination is 1.57° (see above). Earth is currently moving through the unchanging plane around January 9 and July 9. At these specific moments there is an increase in the number of meteorites and noctilucent clouds. If this is because there is a disk of dust and debris in the invariant plane, when Earth's orbital inclination is near 0° and it orbits through this dust, materials could accumulate in the atmosphere. This process could explain the narrowness of the 100,000-year climate cycle.

Present and Future Conditions

Past and future of the average daily insolation at the top of the atmosphere on the day of the summer solstice, to 65th North latitude. The green curve has eccentricity e hypothetically set in 0. The red curve uses the real (prognosed) value of e. The blue point is located around the year 2000 d. C.

Since orbital variations are predictable, any model that relates orbital variations to climate can be advanced to predict future climate.

A 1980 orbital model, often cited by Imbrie and Imbrie, predicted "that the long-term cooling trend that began about 6,000 years ago will continue for the next 23,000 years". More recent work suggests that orbital variations should increase summer insolation in the 65° N area over the next 25,000 years. Earth's orbit will become less eccentric over the next 100,000 years, so Changes in insolation will be dominated by changes in obliquity, and should not decrease enough to cause an ice age in the next 50,000 years.

However, the mechanism by which orbital change influences climate is neither well understood nor conclusive: the Earth is not homogeneous. Milankovitch did not relate Earth's ice ages to the total amount of solar radiation (insolation) reaching Earth, but rather because of the particular insolation received in summer at 65° north latitude, due to the relative ease warming of the large land masses of the northern hemisphere. Later studies have suggested that solar radiation striking ice deposited on these large land masses would simply be mostly reflected.

1. The Earth is not inert. Geology affects climate, not only by the heat of the Earth's core, but also by changes in the atmosphere caused by volcanic eruptions. Even the layout of land masses and ice shelves change over time. due to continental drift.

2. The flourishing industrial activity of humanity can affect the climate by contributing (Human impact on the environment) with effects not foreseen by orbital models. Many studies have concluded that detectable increases in greenhouse gases in the 20th and 21st centuries would trap infrared energy resulting in a warmer climate. An earlier theory determined that industrial particulate pollution of the atmosphere would block solar radiation and cause global cooling.

3. The Future of Earth article presents a variety of rare events, such as collisions of bodies within the solar system and encounters with stars outside the solar system, with the potential to cause past or future weather to deviate from the predetermined orbital mathematical model.

Effects beyond Earth

Other bodies in the Solar System experience geologic effects associated with orbital fluctuations like the Milankovitch cycles, though not as intense or complex as those on Earth. These cycles cause the movement of elements in the solid state:

Mars

Mars does not have a large enough moon to stabilize its obliquity, which has varied from 10 to 70 degrees. This would explain recent observations of its surface compared with evidence for different conditions in its past, such as the extent of its polar ice caps.

Saturn

Saturn's moon Titan has a cycle of about 60,000 years that changes the location of its methane lakes.

Neptune

Neptune's moon Triton has similar variation to Titan with respect to the migration of its solid nitrogen deposits over long periods of time.

Exoplanets

Scientists using computer models to study extreme axial tilts have concluded that high obliquity on other planets would cause weather extremes that threaten possible Earth-like life. They noted that a high type of obliquity would probably not sterilize life on a planet entirely, but would substantially hamper Earth-like models of life, at least those that are warm-blooded, such as mammals and birds. Although the obliquity they studied is more extreme than Earth has ever experienced, there are what-if scenarios 1.5 to 4.5 billion years from now, as the Moon's stabilizing effect wanes, where the obliquity could lose its scant current range and the poles could point almost directly towards the Sun.

Additional bibliography