======================================================================
= Milankovitch_cycles =
======================================================================
Introduction
======================================================================
| Graphic shows variations in five orbital elements:
|Precession index and obliquity control insolation at each latitude:
{{legend|#323232|border=1px solid #00289E|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:
Milankovitch cycles describe the collective effects of changes in the
Earth's movements on its climate over thousands of years. The term was
coined and named after the Serbian geophysicist and astronomer Milutin
Milanković. 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.
Earth movements
======================================================================
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 of precession with
eccentricity 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 surface
temperature more quickly than oceans, mainly because convective mixing
between shallow and deeper waters keeps the ocean surface relatively
cooler. 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.
Orbital eccentricity
======================
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 (theoretically the eccentricity
can hit zero) and mildly elliptical (highest eccentricity was 0.0679
in the last 250 million years). Its geometric or logarithmic mean is
0.0019. The major component of these variations occurs with a period
of 405,000 years (eccentricity variation of ±0.012). Other components
have 95,000-year and 124,000-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 0.0167 and
decreasing.
Eccentricity varies primarily due to the gravitational pull of Jupiter
and Saturn. The semi-major axis of the orbital ellipse, however,
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. Longer-term variations are caused by interactions
involving the perihelia and nodes of the planets Mercury, Venus,
Earth, Mars, and Jupiter.
Effect on temperature
=======================
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% ().
Perihelion presently occurs around 3 January, while aphelion is around
4 July. 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 so small (at least at present)
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.
Effect on lengths of seasons
==============================
Season durations
!Year !Northern hemisphere !Southern hemisphere !Date (UTC) !Season
duration
|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
align="right" | 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 3 January, so the Earth's greater velocity shortens
winter and autumn in the northern hemisphere, and summer and spring in
the southern hemisphere. Summer in the northern hemisphere is 4.66
days longer than winter, and spring is 2.9 days longer than autumn. In
the southern hemisphere this is the reverse, winter is 4.66 days
longer than summer, and autumn is 2.9 days longer than spring. Greater
eccentricity increases the variation in the Earth's orbital velocity.
Currently, however, the Earth's orbit is becoming less eccentric (more
nearly circular). This will make the seasons in the immediate future
more similar in length.
Axial tilt (obliquity)
========================
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, which correlates with the beginning of the Holocene, the
current geological epoch. 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 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).
Axial precession
==================
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. Also known as the precession of the equinoxes, this motion
means that eventually Polaris will no longer be the north pole star.
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.
Currently, perihelion occurs during the southern hemisphere's summer.
This means that solar radiation due to both the axial tilt inclining
the southern hemisphere toward the Sun, and the Earth's proximity to
the Sun, will reach maximum during the southern summer and reach
minimum during the southern winter. These 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.
Apsidal precession
====================
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. 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 25,700-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 about
21,000 years, at present. According to a relatively old source (1965),
the average value over the last 300,000 years was 23,000 years,
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 (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 they are aligned with the solstices,
the difference in the length of these seasons will be greatest.
Orbital inclination
=====================
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.
When measured independently of Earth's orbit, but relative to the
invariable plane, however, 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.
Theory constraints
======================================================================
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. Similar astronomical hypotheses had been
advanced in the 19th century by Joseph Adhemar, James Croll, and
others.
Analysis of deep-ocean cores and of lake depths, and a seminal paper
by Hays, Imbrie, and Shackleton provide additional validation through
physical evidence. Climate records contained in a 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.
100,000-year issue
====================
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 modulation 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. In particular, the mechanism of
the stochastic resonance was originally proposed in order to describe
this interaction.
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 others have argued that the length of the
climate record is insufficient to establish a statistically
significant relationship between climate and eccentricity variations.
Transition changes
====================
From 1-3 million years ago, climate cycles matched the 41,000-year
cycle in obliquity. After one million years ago, the Mid-Pleistocene
Transition (MPT) occurred with a switch to the 100,000-year cycle
matching eccentricity. The 'transition problem' refers to the need to
explain what changed one 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.
Interpretation of unsplit peak variances
==========================================
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. Some researchers,
however, say the records do not show these peaks, but only indicate a
single cycle of 100,000 years. The split between the two eccentricity
components, however, is observed at least once in a drill core from
the 500-million year-old Scandinavian Alum Shale.
Unsynced stage five observation
=================================
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.)
Present and future conditions
======================================================================
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." Another work suggests that solar insolation at 65°
N will reach a peak of 460 W·m−2 in around 6,500 years, before
decreasing back to current levels (450 W·m−2) in around 16,000 years.
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.
Mars
======
Since 1972, speculation sought a relationship between the formation of
Mars' alternating bright and dark layers in the polar layered
deposits, and the planet's orbital climate forcing. In 2002, Laska,
Levard, and Mustard showed ice-layer radiance, as a function of depth,
correlate with the insolation variations in summer at the Martian
north pole, similar to palaeoclimate variations on Earth. They also
showed Mars' precession had a period of about 51 kyr, obliquity had a
period of about 120 kyr, and eccentricity had a period ranging between
95 and 99 kyr. In 2003, Head, Mustard, Kreslavsky, Milliken, and
Marchant proposed Mars was in an interglacial period for the past 400
kyr, and in a glacial period between 400 and 2100 kyr, due to Mars'
obliquity exceeding 30°. At this extreme obliquity, insolation is
dominated by the regular periodicity of Mars' obliquity variation.
Fourier analysis of Mars' orbital elements, show an obliquity period
of 128 kyr, and a precession index period of 73 kyr.
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.
Outer Solar system
====================
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.
Exoplanets
============
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.
Bibliography
======================================================================
* 'This is the first work that investigated the derivative of the ice
volume in relation to insolation (page 698).'
*[
https://news.climate.columbia.edu/2018/05/07/milankovitch-cycles-deep-time
In Ancient Rocks Scientists See a Climate Cycle Working Across Deep
Time (Columbia Climate School, Kevin Krajick, May 7, 2018)]
* .
*
* 'This shows that Milankovitch theory fits the data extremely well,
over the past million years, provided that we consider derivatives.'
* The oldest reference for Milankovitch cycles is:
*[
https://physicstoday.scitation.org/doi/10.1063/PT.3.4474 Tying
celestial mechanics to Earth's ice age (Physics Today 73 (5), Maslin
M. A. 01 May 2020)]
* 'This review article discusses cycles and great-scale changes in
the global climate during the Cenozoic Era.'
External links
======================================================================
* Campisano, C. J. (2012)
[
https://www.nature.com/scitable/knowledge/library/milankovitch-cycles-paleoclimatic-change-and-hominin-evolution-68244581
Milankovitch Cycles, Paleoclimatic Change, and Hominin Evolution].
Nature Education Knowledge 4(3):5
*
[
https://web.archive.org/web/20121011002043/http://channel.nationalgeographic.com/channel/videos/ice-age-cycles/
Ice Age - Milankovitch Cycles - National Geographic Channel]
*
[
https://web.archive.org/web/20080729060933/http://www.agu.org/revgeophys/overpe00/node6.html
The Milankovitch band], Internet Archive of American Geophysical Union
lecture
License
=========
All content on Gopherpedia comes from Wikipedia, and is licensed under CC-BY-SA
License URL:
http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/Milankovitch_cycles
