Milankovitch cycles Nature December 8 2016

Link between Milankovitch’s theory of orbital forcing and ice ages remains an open question

A seminal book by Milankovitch in 1941 proposed that the sequence of ice ages that has characterized long term changes in the Earth’s climate over the past hundreds of thousands of years was due to changes in the amount of solar radiation reaching the Earth’s Northern Hemisphere as a result of small variations in the  Earth’s orientation and orbit with respect to the sun.  Since then there have been many attempts to find evidence in the paleoclimate record to support this hypothesis.  But while research suggests that orbital forcing may have a role in initiating deglaciation, other factors such as ice sheet dynamics, biofeedback, ocean degassing, and dust may all contribute to determining the Earth’s glacial/deglacial cycles.  A recent review concludes that at the present time we still lack a unified mechanism that links changes in Earth’s orbit to ice ages.

Milankovitch cycles Nature December 8 2016
Milankovitch cycles: eccentricity with a period of 96,000 years, obliquity with a period of 41,000 years, and precession of axis and orbit with a period of about 21,000 years. The effect of eccentricity on insolation is very weak and was not considered by Milankovitch.

Almost 80 years ago Milutin Milankovitch proposed that Earth’s glacial/deglacial cycles were caused by small changes in the planet’s obliquity and precession which changed the intensity of solar radiation reaching the Earth at 65 °N  in summer. (A change in Earth’s eccentricity has a very weak impact on insolation and was not studied by Milankovitch.) At this latitude the amount of solar radiation reaching the planet can vary by 25% as a result of Milankovitch cycles.

In 1976 a seminal paper by Hays, Imbrie, and Shackleton found clear evidence in the marine sediment record for the Milankovitch obliquity and precession cycles.  Since then research has shown that the ice ages in the early Pleistocene 2.58 million to 0.78 million years ago had a period identical to the 41,000 cycle of Earth’s obliquity.  Other research has found evidence in the paleoclimate record of Milankovitch cycles extending to 1.4 billion years ago.

There is also the question of how broadly or narrowly to interpret Milankovitch’s theory.  A narrow interpretation requires that the variation in insolation reaching 65 °N  in summer is the sole determining factor.  This seemed reasonable to Milankovitch because most of the Earth’s land is in the Northern Hemisphere and most of the ice sheet expansion and associated increased albedo during ice ages occurred in the Northern Hemisphere.

In 1941 Milankovitch would have had limited computing capacity, which restricted his theory to what he had the capacity to calculate.  A broad interpretation relates any changes in the amount of insolation reaching earth due to small variations in the Earth’s orbit as the determining factor.  This could include variations in the amount of insolation reaching the Southern Ocean, for example.

The major challenge Milankovitch theory is that there is no Milankovitch cycle that can be directly related to the dominant 100,000 (+/-20,000) year cycle of ice ages over the past 800,000 years. While many scientists have accepted the orbital forcing hypotheses as it relates to ice ages, others have found it unconvincing. For example, an assessment by Donald Rapp (Ice Ages and Interglacials 2009) concluded that “while there are innumerable books, reports, and articles claiming that the astronomical theory is proven, the basis for such claims remains flimsy”.

On the anniversary of the Hays, Imbrie and Shackleton paper in 2016, articles in Science and Nature reviewed the status of the link between Milankovitch’s orbital forcing theory and ice ages.  While there is little doubt that Milanovitch cycles are represented in the marine sediment, cave speleological, Antarctic and Greenland ice core, and other paleoclimate records, the link between Milankovitch cycles and ice-ages remains an open topic of research.  A paper by Raymo in 1997 provided evidence that Milankovitch cycles could have initiated deglaciations over the past 800,000 years, but argued that ice sheet dynamics also needed to be considered.  One of the first papers in 1999 based on the Vostok ice cores covering the last four deglaciations, suggested that while Milankovitch cycles may have initiated warming, amplification by growing greenhouse gas concentrations followed by ice sheet retreat and decreased albedo were together required for a full-scale interglacial.  Other researchers have suggested changes in atmospheric dust, biofeedback, variations in solar output, CO2 degassing from the Southern ocean, and other phenomena as contributing factors.

In conclusion the link between Milankovitch cycles and ice ages remains an open question.  In the words of David A. Hodell in his 2016 Science perspective article “The paper by Hays, Imbrie and Shackleton is often said to have solved the mystery of the ice ages.  But in fact it raised more questions than it answered…we still lack a unified mechanism that links changes in Earth’s orbit to ice ages.”

Sources:

Canon of Insolation and the Ice-age Problem, M. Milankovitch, Royal Serbian Academy, Belgrade, 1941.  Translated Jerusalem 1969 for the U.S. Dept of Commerce and the National Science Foundation.

Variations in the Earth’s Orbit: Pacemaker of the Ice Ages, J. D. Hays, John Imbrie, N. J. Shackleton, Science 10 Dec 1976: Vol. 194, Issue 4270, pp. 1121-1132 DOI: 10.1126/science.194.4270.1121

The timing of major climate terminations, M. E. Raymo, Paleooceanography, Volume 12, Issue 4, August 1997, Pages 577–585

Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica, J. R. Petit, J. Jouzel, D. Raynaud, N. I. Barkov, J.-M. Barnola, I. Basile, M. Bender, J. Chappellaz, M. Davis, G. Delaygue, M. Delmotte, V. M. Kotlyakov, M. Legrand, V. Y. Lipenkov, C. Lorius, L. PÉpin, C. Ritz, E. Saltzman & M. Stievenard, Nature 399, pages 429–436 (1999)

Forty years of linking orbits to ice ages, Mark Maslin, Nature 540, pages 208–210 (08 December 2016).

The smoking gun of the ice ages, David A. Hodell, Science, 09 Dec 2016: Vol. 354, Issue 6317, pp. 1235-1236 DOI: 10.1126/science.aal4111