Milankovitch Cycles

by Eric R. Pianka

The seasons are produced by the annual elliptical orbit of Earth around its Sun. However, these orbital movements do not repeat themselves exactly, but because of ever changing gravitational forces imposed by the other planets, they follow three complex celestial periodicities measured on timescales of thousands of years, known as the Milankovitch Cycles named after their discover Milutin Milankovitch. One of these cycles involves the inclination of Earth's axis (around which the planet spins) which is presently inclined at about 23.44o degrees with the northern hemisphere leaning towards the Sun at the Summer Solstice (see figure).
The Tropic of Cancer and the Tropic of Capricorn are defined as the latitude at which the Sun is directly overhead at noon at the Summer and Winter Solstices, respectively. But the angle of this inclination changes from about 22o to about 24.5o over a period of 41,000 years -- in other words, the angle of Earth's axis changes over a timescale of thousands of years. This long-term cyclic change in the axis is called "Obliquity."

Another Milankovitch Cycle involves the shape of Earth's orbit which varies from elliptical to more circular and back again ("Eccentricity") over longer complex periodicities of 95, 125 and 400 thousand years. These are shown in the the graph below. Finally, the third most complicated Milankovitch Cycle involves wobbling of the orbital axis and movement of the orbital plane itself -- at times, the Sun is closest to Earth during the Summer Solstice, but it is now closest during the Winter Solstice (perihelion). These periodic movements (known as "Precession") have complex periodicites of about 19-24,000 years. When added together, these three components generate a complex Solar Forcing (next to bottom in the graph below).
Taken together, the Milankovitch cycles generate ice ages, alternately cooling and warming Earth's surface over time intervals of roughly 100,000 years (see next graph). Our cave man ancestors had to have been pretty tough to have endured those many centuries long frigid cold spells.
















This plot is based on air samples of known age dating back almost half a million years taken from Antarctic and Greenland ice cores. Temperature changes are plotted in red, CO2 in fuzzy gray, and methane levels in black. These changes are caused by complex periodic fluctuations in Earth's orbit and the inclination of its axis (Milankovitch cycles).

Four prolonged ice ages are clearly evident. Notice the four spikes in temperature spaced roughly every 100,000+ years. Earth is presently in a warm interglacial phase with CO2 levels well above any that have been experienced during the last 400,000 years.

Notice also that the last thermal spike has been prolonged a lot compared to the three earlier ones. Earth should have gone into a colder glacial state but has stayed warm for roughly the last 10,000 years. This extended warm period (called "the Long Summer") corresponds to the invention of agriculture and the resulting surge in human population and it is almost certainly due to human activities, especially deforestation and burning of fossil fuels. Climate change is not a new thing, but actually began 100 centuries ago.

The very recent, exceedingly steep, spike in methane production is quite alarming. Methane levels are now more than twice as high as they have been over the last million years. The greenhouse gas effect of one molecule of methane is equivalent to 20-25 molecules of carbon dioxide. As the oceans and permafrost warm, long frozen methane is vaporizing and bubbling out into the atmosphere faster and faster! This could well portend the onset of a powerful positive feedback loop for very rapid global warming.


See also Wikipedia webpage on Milankovitch Cycles




Last updated 13 January 2009 by Eric R. Pianka