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Ice ages controlled by dust

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Posted October 22, 2017

Image courtesy of the researchers.

The reason for the 100,000-year ice age cycle has remained a mystery for decades.  This new theory demonstrates that it is actually modulated by dust and albedo.

The mechanism is this:

Glacial cooling is initiated by orbital cycles, a fact that is well known.  But the primary temperature feedback mechanism is ice-sheet albedo, not CO2.  And the albedo feedback caused by northern ice sheets is so strong, it can even resist subsequent warming orbital cycles (the 100,000 year cycle problem).  So the Earth could and would continue cooling, until we almost reach a ‘snowball Earth’ situation, with almost global ice sheets.

So what initiates sudden interglacial warming?

Well, CO2 levels get so low during glacial periods, that all life begins to die off across the globe, especially at high altitude (CO2 is plant food).  This causes massive CO2 deserts (as opposed to aridity deserts) to form, and results in massive dust storms for thousands of years. And those dust storms have been detected in the glacial record.  These global dust storms coat the ice sheets with layers of dust, which lowers their albedo, and generates a huge amount of insolation absorption and warming. About 40,000 times the warming effect of CO2, when measured regionally and annually.

Since the discovery of ice-age cycles almost two centuries ago, a large amount of geological evidence has been assembled from a variety of sources, and many different hypotheses have been advanced to account for their approximate 100 kyr periodicity and asymmetric, saw-tooth temperature response. Improved calculations of Milankovitch insolation cycles and greater precision of Antarctic ice-core records demonstrate that each major deglaciation coincides with maximum summer insolation in the northern hemisphere. And yet many of the other insolation maxima only trigger minor warming events, and so interglacials only occur after four or five insolation cycles. No generally accepted explanation exists for this peculiar intermittent climate response, and any comprehensive explanation for ice-age modulation and periodicity has to be able to explain this anomaly.

Image courtesy of the researchers / Prof. Clive Best

The answer to this conundrum can be found in a novel reanalysis of the effects of decreasing atmospheric CO2 concentrations during an ice-age. Ice age CO2 reductions coincide with an increase in ice sheet extent and therefore an increase in global albedo, and this should result in further cooling of the climate.

But what actually happens is that when CO2 reaches a minimum and albedo reaches a maximum, the world rapidly warms into an interglacial. A similar effect can be seen at the peak of an interglacial, where high CO2 and low albedo results in cooling. This counterintuitive response of the climate system also remains unexplained, and so a hitherto unaccounted for agent must exist that is strong enough to counter and reverse the classical feedback mechanisms.

The answer to both of these conundrums lies in glacial dust, which was deposited upon the ice sheets towards the end of each glacial maximum. Previous research has considered two effects of this aeolian dust on the glacial climate: the increased albedo of atmospheric dust cooling the climate, and the mineral fertilization of marine life reducing atmospheric CO2 . But both of these effects would result in a cooling feedback, and therefore provide no explanation for the interglacial warming that appears to result from dust deposition.

In great contrast to these explanations it is proposed here that during the glacial maximum, CO2 depletion starves terrestrial plant life of a vital nutrient and causes a die-back of upland forests and savannahs, resulting in widespread desertification and soil erosion. The resulting dust storms deposit large amounts of dust upon the ice sheets and thereby reduce their albedo, allowing a much greater absorption of insolation. Up to 180 W/m2 of increased absorption can be provided to the northern ice sheets, when calculated seasonally and regionally instead of annually and globally.

This dramatic increase in insolation and absorption results in melting and dissipation of the northern ice sheets, and the establishment of a short interglacial period. Ice ages are therefore forced by orbital cycles and Milankovitch insolation, but regulated by ice-albedo and dust-albedo feedbacks. And the warming effects of dust-ice albedo are counterintuitively caused by a reduction in global temperatures and a corresponding reduction in CO2 concentrations. And while this proposal represents a reversal of conventional thinking it does explain each and every facet of the glacial cycle, and all of the many underlying mechanisms that control its periodicity and temperature excursions and limitations.

Source: Modulation of Ice Ages, via Dust and Albedo. https://www.sciencedirect.com/science/article/pii/S1674987116300305

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