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.
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 desertifi-
cation 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 icealbedo
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.
Originally Posted by pbjosh
