For the past 30 years, the Milankovitch hypothesis, which posits that long term changes in the Earth’s climate are controlled by small, predictable changes in the Earth’s orbit about the sun, has been accepted by many in the scientific community but by no means all. The first experimental support of the orbital forcing hypothesis published in 1976 relied on deep sea sediment cores. However this analysis has been criticized because it was developed by tuning sediment timescales to insolation curves calculated from the astronomical effects calculated by Milankovitch. In 1997 Raymo was the first to provide observations supporting the orbital forcing hypothesis for glacial/deglacial cycles that did not rely on orbital tuning. However, her analysis also found that while periods of increased insolation generally corresponded to increasing temperatures at the end of ice ages, other factors needed to be taken into account in explaining deglaciation events. She suggested that ice sheet dynamics might help explain the dominant 100,000 year cycle.
Approximately 800 thousand years ago, something changed within the Earth’s climate system that led to the observed cycle of glacial and interglacial periods. The time interval between glacial terminations, often characterized as “100,000”, is not constant. It varies from 84,000 between terminations IV and V to 120,000 between terminations III and II. True terminations, where rapid transitions from full glacial to full interglacial conditions occur, have only been observed at Terminations I, II, IV, V and VII.
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 “100,000” 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.
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.
The first experimental support of the Milankovitch hypothesis was published by Hays, Imbrie and Shackleton in 1976 analyzed deep sea sediment cores. While this analysis found evidence in the sediment record for 41,000 and 26,000 cycles, it did find direct evidence for orbital forcing driving the dominant 100,000 year glacial/deglacial cycle. While many scientists have accepted the orbital forcing hypotheses, 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”.
The Hays et al. analysis was criticized because it used orbital tuning based on Milankovitch’s astronomical calculations for the timescale used to date the sediment record. In 1997 Raymo was the first provide evidence from the marine sediment record for the orbital forcing hypothesis that did not rely on a time scale determined using orbital tuning. Raymo used delta oxygen-18 records (a radiometric temperature proxy) from deep sea sediment cores from eleven sites 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 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.
Raymo concluded that her analysis supports the correlation of ice age 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 (which is not a true termination) correspond to increases in insolation.
But Raymo also observed that terminations do not always correspond to the largest increases in summer insolation. She concluded that to account for the observed 100,000 year cycle other factors need to be taken into account. She suggested that ice sheet dynamics might play an role in addition to orbital forcing. Based on her observations she suggested that once a large ice sheet has developed, which requires low temperatures for about 100 thousand years, the first significant warming event causes the ice mass to melt catastrophically triggering global warming and a glacial termination event.
Her conclusion is that the interaction between obliquity and eccentricity modulation of precession as it controls northern hemisphere summer radiation initiated deglaciation events, but that other factors need to be taken into account. She suggested that ice sheet dynamics may be responsible for amplifying the effect of orbital forcing leading to a glacial termination.