Milankovitch cycles

Milankovitch cycles describe the collective effects of changes in the Earth's movements on its climate over thousands of years.

In the 1920s, he provided a more definitive and quantitative analysis than James Croll's earlier hypothesis that variations in eccentricity, axial tilt, and precession combined to result in cyclical variations in the intra-annual and latitudinal distribution of solar radiation at the Earth's surface, and that this orbital forcing strongly influenced the Earth's climatic patterns.

[1][2][3] 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.

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.

Similarly, the very large thermal inertia of the global ocean delays changes to Earth's average surface temperature when gradually driven by other forcing factors.

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.

[9] Longer-term variations are caused by interactions involving the perihelia and nodes of the planets Mercury, Venus, Earth, Mars, and Jupiter.

[15] 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.

[15] Because most of the planet's snow and ice lies at high latitude, decreasing tilt may encourage the termination of an interglacial period (and lead to an overall cooler climate) and the onset of a glacial period for two reasons: 1) there is less overall summer insolation, and 2) there is less insolation at higher latitudes (which melts less of the previous winter's snow and ice).

[15] Axial precession is the trend in the direction of the Earth's axis of rotation relative to the fixed stars, with a period of about 25,700 years.

This precession is caused by the tidal forces exerted by the Sun and the Moon on the rotating Earth; both contribute roughly equally to this effect.

Axial tilt and orbital eccentricity will both contribute their maximum increase in solar radiation during the northern hemisphere's summer.

[citation needed] The orbital ellipse itself precesses in space, in an irregular fashion, completing a full cycle in about 112,000 years relative to the fixed stars.

When the Earth's apsides (extremes of distance from the sun) are aligned with the equinoxes, the length of spring and summer combined will equal that of autumn and winter.

When measured independently of Earth's orbit, but relative to the invariable plane, however, precession has a period of about 100,000 years.

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.

[20] Analysis of deep-ocean cores and of lake depths,[21][22] and a seminal paper by Hays, Imbrie, and Shackleton[23] provide additional validation through physical evidence.

[24] 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.

Various explanations for this discrepancy have been proposed, including frequency modulation[29] or various feedbacks (from carbon dioxide, or 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.

After one million years ago, the Mid-Pleistocene Transition (MPT) occurred with a switch to the 100,000-year cycle matching eccentricity.

[37] The MPT can now be reproduced in numerical simulations that include a decreasing trend in carbon dioxide and glacially induced removal of regolith.

[40] Deep-sea core samples show that the interglacial interval known as marine isotope stage 5 began 130,000 years ago.

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.

[55][56] Saturn's moon Titan has a cycle of approximately 60,000 years that could change the location of the methane lakes.

[57] Neptune's moon Triton has a variation similar to Titan's, which could cause its solid nitrogen deposits to migrate over long time scales.

Past and future Milankovitch cycles via VSOP model
  • Graphic shows variations in five orbital elements:
    Axial tilt or obliquity (ε).
    Eccentricity ( e ).
    Longitude of perihelion (sin(ϖ)).
    Precession index ( e sin(ϖ))
  • Precession index and obliquity control insolation at each latitude:
    Daily-average insolation at top of atmosphere on summer solstice ( ) at 65° N
  • Ocean sediment and Antarctic ice strata record ancient sea levels and temperatures:
    Benthic forams (57 widespread locations)
    Vostok ice core (Antarctica)
  • Vertical gray line shows present (2000 CE)
22.1–24.5° range of Earth's obliquity.
Axial precessional movement.
Planets orbiting the Sun follow elliptical (oval) orbits that rotate gradually over time (apsidal precession). The eccentricity of this ellipse, as well as the rate of precession, are exaggerated for visualization.
Effects of precession on the seasons (using the Northern Hemisphere terms)
Tabernas Desert , Spain: Cycles can be observed in the colouration and resistance of different sediment strata
Variations of cycle times, curves determined from ocean sediments.
420,000 years of ice core data from Vostok, Antarctica research station, with more recent times on the left.
Past and future estimations of daily average insolation at top of the atmosphere on the day of the summer solstice, at 65° N latitude. The green curve is with eccentricity e hypothetically set to 0. The red curve uses the actual (predicted) value of e ; the blue dot indicates current conditions (2000 CE).