New research finds Late Pleistocene glaciations terminated by Earth's axis tilt rather than ...

[Jim__]

... orbital eccentricity

From phys.org

Glacial cyclicity of the Earth has often been considered on 100,000 year timescales, particularly for the Late Pleistocene (~11,700 to 129,000 years ago) swapping between periods of extensive polar and mountain glacier ice sheets, to warmer interglacial periods when ice sheets and glaciers retreated, with subsequent sea level rise. This is thought to be related to three key drivers affecting the amount of solar radiation reaching Earth from the sun.

Termed Milankovitch cycles, eccentricity considers the shape of Earth's orbit changing from circular to more elliptical over 100,000 year timescales, while obliquity refers to the varying 'tilt' of the planet's axis between 22.1 and 24.5 degrees over 41,000 years (contributing to seasons) and precession, which in simple terms is the direction Earth's axis is pointed and can make the contrast between seasons more extreme in one hemisphere compared to the other.

While the eccentricity cycle has been a major factor thought to drive glacial/interglacial cycles, newer research has suggested that they instead may result from a series of obliquity or precession cycles (especially as the former dominated up to 800,000 years ago). To test this theory, Bethany Hobart, a Doctoral Researcher at the University of California, and colleagues modeled the impacts of glacial termination on 23,000 and 41,000 year cycles.

Three hypotheses were proposed in a Nature Geoscience publication: 1) precession forced by eccentricity, whereby weak precession cycles are associated with near-circular orbits and so summer insolation reaching Earth's surface is low, encouraging ice sheets to accumulate; 2) glaciation terminated every two or three obliquity cycles, therefore roughly every 100,000 years; or 3) both precession and obliquity drove the switch between glaciation and interglaciation.

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[John1956PA]

1. My understanding is that, currently, the Earth's orbit is elliptical.

It is also my understanding that, at the current rate of precession, it takes about 25,700 years for the direction of the Earth's axis to make one pass through its cycle. Currently, the position of the earth in the northern hemisphere's winter season is closer to the sun than it is in the summer season. In 13,000 years, the opposite will prevail, and the winter months will see the sun farther from the sun than in the summer months.