Orbital forcing, or Milankovitch theory, describes the effect on climate of slow changes in the tilt of the Earth's axis and shape of the orbit. These orbital changes change the total amount of sunlight reaching the Earth by up to 25% at mid-latitudes (from 400 to 500 watts per cubic meter at latitudes of 60 degrees). In this context, the term "forcing" signifies a physical process that affects the Earth's climate.
This mechanism is believed to be responsible for the timing of the ice age cycles. A strict application of the Milankovitch theory does not allow the prediction of a "sudden" ice age (rapid being anything under a century or two), since the fastest orbital period is about 20,000 years. The timing of past glacial periods coincides very well with the predictions of the Milankovitch theory, and these effects can be calculated into the future.
Ice core data. Note length of glacial cycles averages ~100,000 years. Blue curve is temperature, black curve is CO2, and red curve is windblown glacial dust (loess). Today's date is on the left side of the graph.
It is common to see it asserted that the length of the current interglacial temperature peak is similar to the length of the preceding interglacial peak (Sangamon/Eem), and from this conclude that we might be nearing the end of this warm period. However, this conclusion may be mistaken: the lengths of previous interglacials were not particularly regular (see graphic at right).
Note in the graphic the strong 120,000 year periodicity of the cycles, and the striking asymmetry of the curves. This asymmetry is believed to result from complex interactions of feedback mechanisms. It has been observed that ice ages deepen by progressive steps, but the recovery to interglacial conditions occurs in one big step.
The arrangements of land masses on the Earth's surface are believed to reinforce the orbital forcing effects. Comparisons of plate tectonic continent reconstructions and paleoclimatic studies show that the Milankovitch cycles have the greatest effect during geologic eras when landmasses have been concentrated in polar regions, as is the case today. Greenland, Antarctica, and the northern portions of Europe, Asia, and North America are situated such that a minor change in solar energy will tip the balance between year-round snow/ice preservation and complete summer melting. The presence of snow and ice is a well-understood positive feedback mechanism for climate.
