Full paper available here: http://www.sciencedirect.com/science/article/pii/S1674987116300305
The primary forcing agent regulating ice-age glaciation is precession.
The primary feedback system regulating ice-age glaciation is albedo.
Albedo modulation is controlled by desertification and dust contamination of ice sheets.
Desertification and dust productions are caused by low CO2 concentrations.
We present here a simple and novel proposal for the modulation and rhythm of ice-ages and interglacials during the late Pleistocene. While the standard Milankovitch-precession theory fails to explain the long intervals between interglacials, these can be accounted for by a novel forcing and feedback system involving CO2, dust and albedo. During the glacial period, the high albedo of the northern ice sheets drives down global temperatures and CO2 concentrations, despite subsequent precessional forcing maxima. Over the following millennia more CO2 is sequestered in the oceans and atmospheric concentrations eventually reach a critical minima of about 200 ppm, which combined with arid conditions, causes a die-back of temperate and boreal forests and grasslands, especially at high altitude. The ensuing soil erosion generates dust storms, resulting in increased dust deposition and lower albedo on the northern ice sheets. As northern hemisphere insolation increases during the next Milankovitch cycle, the dust-laden ice-sheets absorb considerably more insolation and undergo rapid melting, which forces the climate into an interglacial period. The proposed mechanism is simple, robust, and comprehensive in its scope, and its key elements are well supported by empirical evidence.
6. Summary and conclusions
The primary orbital cycle responsible for interglacial initiation is the precessional Great Summer, which can provide large increases in annual midsummer insolation in the northern hemisphere for several millennia. However, not all Great Summers produce a warming event, while full interglacials only occur every four or five cycles. The additional factor that can achieve this selective regulation is the high albedo of the northern ice sheets, which can reject and reduce the increased insolation of a Great Summer. In order for a Great Summer to generate a significant warming response the northern ice sheets need to be laden with dust, so that the increased insolation can get some leverage on the highly reflective ice. And the mechanism required to achieve this involves surface CO2concentrations reducing below 200 ppm, which results in a die-back of high altitude flora, widespread desertification, and dust deposition upon the ice sheets. Fig. 14 depicts all the key elements that play a part in these complex climatic interactions. Note that dust (purple) is only generated once CO2 (yellow) has reached critically low levels, and that interglacial warming (red) only occurs after dust deposition and when an eccentricity-enhanced Great Summer (dark blue peak) is reached.
The apparent correlation between dust (purple) and eccentricity minima (black) on this graph is merely a function of the eccentricity-enhanced inception of interglacial periods (red peak). An interglacial is only initiated when eccentricity is rising and northern Great Summer Milankovitch insolation is enhanced. Following this temporary warm period, the rate of polar ice regrowth and its associated increase in albedo, controls the cooling-rate of the oceans and climate. These steadily reducing temperatures control the equally steady oceanic absorption and sequestration of atmospheric CO2, which in turn eventually controls the exponential increase in dust production, which then lowers ice-sheet albedo and primes the world for another interglacial warming. Thus one of the primary climatic regulators of interglacial periodicity is the steady rate of increase in polar ice extent. And since it takes about 70 kyr before the ice-sheets are large enough for temperatures and CO2 to reach a minima, this coincidentally places the increased dust production era close to the next eccentricity minima.
Thus the rate of ice-sheet regrowth plays a key role in determining the ∼100 kyr length of the glacial cycle. If temperatures and CO2 have not reached their critical minimum values before the onset of an eccentricity-enhanced Great Summer, there would be no dust-ice albedo feedbacks. And so the world would wait patiently until the next enhanced Great Summer, when hopefully all the participants in this stand-off between orbital forcing and climate feedbacks are ready to play their part. The glacial world’s dust-ice Achilles heel needs to be primed and ready to fire before an interglacial can be fully successful, otherwise the result is merely a ‘flash in the pan’ – one of the many minor warming events of no consequence in the paleoclimatic record. In which case, interglacial warming is eccentricity and polar ice regrowth regulated, Great Summer forced, and dust-ice albedo amplified. And the greenhouse-gas attributes of CO2 play little or no part in this complex feedback system.