Editor’s comments

Glacial terminations

There are between seven and eleven recognized terminations, where relatively rapid increases in Earth’s surface temperatures have been recorded. The seven most recent ones are listed in the table.

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.

Milankovitch cycles
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.

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 Finally, 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.

However, the “100 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. The time interval between glacial terminations is not constant, but varies from 84 kyr between terminations IV and V to 120 kyr between terminations III and II.

Challenges to the Milankovitch hypothesis
Although astronomical frequencies corresponding to Milankovitch cycles are found in almost all paleoclimatic records, it is clear that the climatic system does not respond linearly to insolation variations.

• The best well-known disagreement between the astronomical theory of climate and observation for the past million years is the ‘100 kyr problem’. The intervals between glacial terminations over the past million years range form 84 kyr to 120 kyr, or roughly every 100 kyr. But the variation in insolation computed from the astronomical model does not does not predict a strong signal at this frequency.
• There is no simple relation between the amplitudes of the insolation extrema and the corresponding ice volume extrema.
• The ‘400 kyr problem’. The amplitude of summer high latitude insolation variations is maximum every 400 kyr, due to the dominance of this periodicity in the eccentricity modulation of the precessional forcing, but this frequency is not found in paleoclimatic records.
• The phase relationship between terminations and the corresponding insolation extremum may not be constant through time. For example, Termination II was in advance of the insolation maximum, the opposite seems to be the case for termination III.

Raymo noticed that terminations occurred only after considerable build-up of ice sheet, and that beyond this point, the next northern latitude summer insolation maximum, even a relatively weak one, will cause deglaciation.

To address the deficiencies of the Milankovitch hypothesis, Parrenin and Paillard have suggested that ice volume and insolation together play a role in triggering deglaciations. Tthier conceptual model hypothesizes that teerminations occur when a combination of insolation and ice volume is large. A deglaciation can occur when insolation forcing is moderate if ice volume is very large, or reciprocally when ice volume is moderate if insolation forcing is very large. Parrenin and Paillard’s model is driven by changes in the June Solstice insolation at 65 deg N and by obliquity.

According to Parrenin and Paillard, their simple model not only reproduces sea level transitions at the correct time, but also sea level extrema with the right amplitude.

In addition, despite high latitude northern insolation being the only external forcing, they obtain significant phase variations between climatic transitions and insolation,in agreement with chronologies for terminations II and III. They argue that this shows that an astronomical theory of glacial cycles can explain these phase variations.
Amplitude and phase of glacial cycles from a conceptual model, Frédéric Parrenin and Didier Paillard, Earth and Planetary Science Letters 214 (2003) 243-250