Leigh A. Seldomridge
Satterley, A.K., 1996. The Milankovitch Theory: Earth-Science Reviews, v. 40. p. 181-207.
This article describes the Milankovitch theory and the ways in which the theory is justified. First of all, the theory is presented as the four different aspects of the Earth's rotation and how they affect the distribution of incoming solar radiation. Next, Satterley discusses how climatic variations could possibly be recorded in the sediments of lakes and evaporitic deposits. Another means of researching the climatic changes of the past is looking at shallow water carbonate platforms because of their sensitivity to sea level changes. Finally, Satterley discusses how sea level expansion could also occur in response to thermal expansion of surface ocean waters. The question is whether or not fossil fuel burning of this century could have been the main cause of thermal changes.
Kerr, Richard, 1994. Milankovitch Plays Climate in Double-Time in the Tropics: Geotimes, v.39. p. 10.
This article discusses how the Milankovitch cycle theories have always been used to explain climatic change over the last tens of thousands to millions of years. Now some scientists believe that this theory may explain shifts in climate over periods of 1,000 to 15,000 years also. The research done in the tropical regions of the Earth shows that geography such as land masses that straddle the tropics and northern or southern regions could possible cause the cycle to be cut in half for the tropical regions (10,500 years rather than 21,000 years). The research includes proof that the carbonate content of plankton varied in periods of 10,000 to 12,000 years which would show that upwelling varied over those periods also. In other words, the Milankovitch cycle is not constant over the entire Earth, as was previously believed.
Clemens, S., Tiedemann, R., 1997. Eccentricity forcing of Pliocene-Early Pleistocene climate revealed in a marine oxygen-isotope record: Nature, v. 385, p. 801-803.
Crowley, T., Kim, K., 1994. Milankovitch Forcing of the Last Interglacial Sea Level: Science, v. 265, p. 1566-1567.
Three elements of Milankovitch shown by graphicsThis web site graphically displays the 3 aspects of the Milankovitch cycle-eccentricity of earth's orbit, obliquity of earth's axis and precession of earth's axis.
Total
solar energy flux from Colorado School of MinesThis web site includes
a discussion of the total solar energy flux to earth's surface and links
to specific information on Milankovitch and solar radiation.
In the last fifty years, global warming has become an issue of concern for
scientists, industrialists, and virtually everyone who has some stake in
the future of the Earth. Most of the attention of the public has been
focused upon how greenhouse gases affect the atmosphere and how consumers
can help the environment. In addition to these factors, however, global
warming includes some external causes which scientists continue to
research with hopes that the global warming process is more naturally
occurring than was previously determined. The two main sources of
external control of the Earth's climate are the Milankovitch cycle and the
output of radiation by the sun.
The Milankovitch theory involves three aspects of the Earth's rotation
around the sun--the eccentricity of the Earth's orbit, the obliquity of
the Earth's axis, and the precession of the Earth's axis. The shape of
the Earth's orbit varies from a circular path to a more elliptical path,
while the orbital axis about which the Earth rotates precesses with a
frequency of 21,000 years. The inclination of the axis can also change by
up to three degrees. The distribution of incoming solar radiation,
therefore, is greatly affected by these variances, and certainly these
factors may drive cyclic climatic changes on Earth. The Milankovitch
theory has been justified by the extraction of sediment layers that vary
in carbonate material, indicating a cycle of warming and cooling trends.
In addition to orbital variations, the total solar radiation output of the
sun may be an external control of the Earth's climate. Beginning in 1980,
researchers have been studying sunspots to measure the total solar output.
Between 1980 and 1985, solar output decreased by 0.1 percent, but in 1987
it increased again. Such measurements did not relate to the warming trend
of the Earth's climate during those years, however, some correlations have
been made between sunspot frequency and climate. Because research on
sunspots is fairly recent, assessment of long-term climate changes due to
these factors is difficult.
The external controls of the Earth's climate can be summed up in the list
below:
Two general sources on the external controls of the Earth's climate:
Friis-Christensen, E., Lassen, K., 1991. Length of the Solar Cycle: An
Indicator of Solar Activity Closely Associated with Climate: Science, v.
254, p. 698ff.
Hays, J., Imbrie, J., and Shackleton, N., 1976. Variations in the Earth's
Orbit: Pacemaker of the Ice Ages: Science, v. 194, p. 1121.
Summary of External Controls of the Earth's Climate
External Factor How Climate is Affected
Milankovitch Cycle The amount of solar radiation
*Eccentricity of orbit entering atmosphere varies
*Obliquity of axis with change of Earth's position
*Precession of axis
Total Solar Output The amount of solar radiation
leaving the sun varies
(sunspots?)
Researching external controls of climate change
The best way to go about researching global change is to use a combination
of journal/magazine articles and information from the Internet from
reliable sources. The problem with journal articles is that many are
written for only other experts on the subject to read. Sometimes,
scientific magazines provide a more reader friendly discussion of the
topic. The Internet is a valuable resource if it is used discriminately.
When searching for "Milankovitch cycle" on the Internet, 666 sources were
listed, however, only a handful provided trustworthy information from
scientific agencies or universities. In order to find the most recent,
accurate information on a topic such as global change, one must be
persistent and cautious.