|What is The Milankovitch Theory? The Milankovitch or
astronomical theory of climate change is an explanation for changes in the
seasons which result from changes in the earth's orbit around the sun. The
theory is named for Serbian astronomer Milutin Milankovitch, who
calculated the slow changes in the earth's orbit by careful measurements
of the position of the stars, and through equations using the
gravitational pull of other planets and stars. He determined that the
earth "wobbles" in its orbit. The earth's "tilt" is what causes seasons,
and changes in the tilt of the earth change the strength of the seasons.
The seasons can also be accentuated or modified by the eccentricity
(degree of roundness) of the orbital path around the sun, and the
precession effect, the position of the solstices in the annual orbit.
What does The Milankovitch Theory say about future climate
Orbital changes occur over thousands of years, and the
climate system may also take thousands of years to respond to orbital
forcing. Theory suggests that the primary driver of ice ages is the total
summer radiation received in northern latitude zones where major ice
sheets have formed in the past, near 65 degrees north. Past ice ages
correlate well to 65N summer insolation (Imbrie 1982). Astronomical
calculations show that 65N summer insolation should increase gradually
over the next 25,000 years, and that no 65N summer insolation declines
sufficient to cause an ice age are expected in the next 50,000 - 100,000
years ( Hollan 2000, Berger 2002).
Milankovitch, M. 1920. Theorie Mathematique des
Phenomenes Thermiques produits par la Radiation Solaire. Gauthier-Villars
Milankovitch, M. 1930. Mathematische Klimalehre und Astronomische
Theorie der Klimaschwankungen, Handbuch der Klimalogie Band 1 Teil A
Milankovitch, M. 1941 Kanon der Erdbestrahlungen und seine Anwendung
auf das Eiszeitenproblem Belgrade.
(New English Translation, 1998,
Canon of Insolation and the Ice Age Problem. With introduction and
biographical essay by Nikola Pantic. 636 pp. $79.00 Hardbound. Alven
Global. ISBN 86-17-06619-9.)
Recent Calculations of Earth Orbital
Parameters and Insolation by A. Berger are archived at the WDC Paleo.
For more detailed explanations of orbital variations with graphic
representations, please see WDC Paleo's educational slide set "The Ice Ages".
See also the "Past Cycles: Ice Age Speculations" section of "The
Discovery of Global Warming" from the American Institute of Physics
for a history of the development of the astronomical theory of
The tilt of the earth relative to its plane of travel about the sun is
what causes seasons. The hemisphere "pointing toward" the sun is in summer,
while the opposite hemisphere is in winter. The earth makes one full orbit
around the sun each year. The northern hemisphere is in summer in the left
image, while 6 months later, the southern hemisphere has summer, as in the
center image. If the earth's axis were "straight up and down" relative to
the orbital plane, as in the right-hand image, there would be no seasons,
since every point on the earth would receive the same amount of sun each day
of the year.
Changes in the "tilt" of the earth can change the severity of the seasons
- more "tilt" means more severe seasons - warmer summers and colder winters;
less "tilt" means less severe seasons - cooler summers and milder winters.
The earth wobbles in space so that its tilt changes between about 22 and 25
degrees on a cycle of about 41,000 years. It is the cool summers which are
thought to allow snow and ice to last from year to year in high latitudes,
eventually building up into massive ice sheets. There are positive feedbacks
in the climate system as well, because an earth covered with more snow
reflects more of the sun's energy into space, causing additional cooling. In
addition, it appears that the amount of Carbon Dioxide in the atmosphere
falls as ice sheets grow, also adding to the cooling of the climate.
The earth's orbit around the sun is not quite circular, which means that
the earth is slightly closer to the sun at some times of the year than
others. The closest approach of the earth to the sun is called perihelion,
and it now occurs in January, making northern hemisphere winters slightly
milder. This change in timing of perihelion is known as the precession of
the equinoxes, and occurs on a period of 22,000 years. 11,000 years ago,
perihelion occurred in July, making the seasons more severe than today. The
"roundness", or eccentricity, of the earth's orbit varies on cycles of
100,000 and 400,000 years, and this affects how important the timing of
perihelion is to the strength of the seasons. The combination of the 41,000
year tilt cycle and the 22,000 year precession cycles, plus the smaller
eccentricity signal, affect the relative severity of summer and winter, and
are thought to control the growth and retreat of ice sheets. Cool summers in
the northern hemisphere, where most of the earth's land mass is located,
appear to allow snow and ice to persist to the next winter, allowing the
development of large ice sheets over hundreds to thousands of years.
Conversely, warmer summers shrink ice sheets by melting more ice than the
amount accumulating during the winter.