Orbital forcing

Today, northern hemisphere summer is 4.66 days longer than winter and spring is 2.9 days longer than autumn.[3] As axial precession changes the place in the Earth's orbit where the solstices and equinoxes occur, Northern hemisphere winters will get longer and summers will get shorter, eventually creating conditions believed to be favorable for triggering the next glacial period.

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 clima