For the past 30 years, the Milankovitch hypothesis, which posits that the Earth’s climate is controlled by variations in incoming solar radiation which are determined by small, predictable changes in the Earth’s orbit about the sun, has been widely accepted by the scientific community. However, the “100 kyr” (in reality 84 to 120 kyr) glacial/interglacial cycle which represents the dominant feature of the Earth’s climate in the last 800 kyr has been difficult to reconcile with the Milankovitch hypothesis.

Orbital forcing

Milankovitch studied small variations in the Earth’s orbit about the sun and its axis of rotation. The eccentricity of the Earth’s orbit varies with a period of 413 kyr with smaller cycles varying between 95 and 125 kyr. The angle of the Earth’s axial tilt (obliquity of the elliptic) takes approximately 41 kyr to shift between a tilt of 22.1° and 24.5° and back again. The Earth’s axis of rotation precesses with a period of roughly 26 kyr. The Earth’s orbital ellipse precesses in space, primarily as a result of interactions with Jupiter and Saturn (this was not studied by Milankovitch). In combination with changes to the eccentricity it alters the length of the seasons. The inclination of Earth’s orbit drifts up and down relative to the invariable plane (corresponding to Jupiter’s orbit) with a 100 kyr cycle.

Observations

The first experimental support of the Milankovitch hypothesis was published by Hays, Imbrie and Shackleton in 1976 which relied on deep sea sediment cores and resulted in the SPECMAP timescale. Using this timescale to support the Milankovitch hypothesis has been criticized because it was developed by tuning sediment timescales to insolation curves calculated from the astronomical affects studied by Milankovitch.

In 1997 Raymo was the first to support the Milankovitch hypothesis without relying on observations determined using orbital tuning. Raymo used delta oxygen-18 records (a radiometric temperature proxy) from deep sea sediment cores from 11 sites. None of these records were used in the development of the SPECMAP timescale. The records are from three oceans, high and low latitudes, and eastern and western equatorial regions.

The process used to derive a common chronology was to scale each core on a simple radiometric timescale pinned to recognizable events, typically termination midpoints, in the glacial/interglacial cycles. Radiometric measurements based on carbon-14, protactimium-231, and thorium-230 were used to determine calendar dates for these recognizable events. The resulting timescale for each core was then used to estimate the dates of the remaining terminations. These dates were then averaged (referred to as GSS97) and compared with SPECMAP dates. For all dates the GSS97 dates were within 1.5% of the SPECMAP dates. For all but Termination III, the agreement between Raymo’s dates and SPECMAP dates are within 1%.

Results

Raymo also concluded that her analysis supports the correlation of terminations and increases in summer insolation at high northern latitudes. For example, comparing delta oxygen-18 data for one of the Pacific core sites shows that all termination midpoints except for Termination III correspond to increases in insolation.

Terminations II through VII in the ocean sediment record compared with summer insolation at 65 degrees N. Vertical bold lines define the termination mid points.

But Raymo also observed that the terminations do not always correspond to the largest increases in summer insolation. She explains this by basing his argument on the importance of large ice sheet growth to the climate cycle. She argues that once a large ice sheet has developed, which requires low temperatures for about 100 kyr, the first warming event of any note, causes the ice mass to melt catastrophically, triggering global warming, and a glacial termination event.

Her most important conclusion is that the interaction between obliquity and eccentricity modulation of precession as it controls northern hemisphere summer radiation together with large ice sheet formation are responsible for the pattern of ice growth and decay in the late Quaternary (since 800 kya).

Editor’s comments

This research, which was published in 1997, is the first attempt to correlate unbiased observed temperature profiles around glacial terminations with high northern latitude insolation for the past 800,000 years in support of the orbital forcing hypothesis. The formation of large ice sheets is a necessary part of her argument that orbital forcing triggers control the glacial/interglacial cycle with a period of roughly 100,000 years.

Source

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