Orbital forcing

Overview

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.

References

References

1. (1 August 2018). "Terrestrial sedimentary responses to astronomically forced climate changes during the Early Paleogene in the Bohai Bay Basin, eastern China". [[Palaeogeography, Palaeoclimatology, Palaeoecology]].
2. (1 November 2011). "Astronomical forcing and chronology of the early Toarcian (Early Jurassic) oceanic anoxic event in Yorkshire, UK". [[Paleoceanography and Paleoclimatology]].
3. (15 January 2003). "The mid-Cenomanian Event: prelude to OAE 2". [[Palaeogeography, Palaeoclimatology, Palaeoecology]].
4. (1 March 2008). "Oceanic anoxic cycles? Orbital prelude to the Bonarelli Level (OAE 2)". [[Earth and Planetary Science Letters]].
5. (11 August 2017). "Unraveling the onset of Cretaceous Oceanic Anoxic Event 2 in an extended sediment archive from the Tarfaya-Laayoune Basin, Morocco". [[Paleoceanography and Paleoclimatology]].
6. Berger, A.. (23 August 2002). "An Exceptionally Long Interglacial Ahead?". Science.
7. (5 May 2005). "A Movable Trigger: Fossil Fuel CO₂ And The Onset Of The Next Glaciation". Geochemistry, Geophysics, Geosystems.
8. Benson, Gregory. (11 December 2007). "Global Warming, Ice Ages, and Sea Level Changes: Something new or an astronomical phenomenon occurring in present day?".