It is sometimes asserted that the length of the current interglacial temperature peak will be similar to that of the preceding interglacial peak (
Sangamonian/Eem Stage). Therefore, we might be nearing the end of this warm period. However, this conclusion is probably mistaken: the lengths of previous interglacials were not particularly regular (see graphic at right). Berger and Loutre (2002) argue that “with or without human perturbations, the current warm climate may last another 50,000 years. The reason is a minimum in the eccentricity of Earth's orbit around the Sun.” Also, Archer and Ganopolski (2005) report that probable future CO2 emissions may be enough to suppress the glacial cycle for the next 500 kyr. Note in the graphic, the strong
100,000 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 in progressive steps. However, the recovery to interglacial conditions occurs in a single large step. Orbital mechanics require that the length of the seasons be proportional to the swept areas of the seasonal quadrants, so when the eccentricity is extreme, the seasons on the far side of the orbit can last substantially longer. Today, when autumn and winter in the Northern Hemisphere occur at closest approach, the Earth is moving at its maximum velocity and therefore autumn and winter are slightly shorter than spring and summer. Today in the Northern Hemisphere, summer is 4.66 days longer than winter and spring is 2.9 days longer than autumn. 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 favourable 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 in the
climate of the Arctic, between
year-round snow/ice preservation and complete summer melting. The presence or absence of snow and ice is a well-understood
positive feedback mechanism for climate. ==See also==