glacial-interglacial regimes needs to be triggered by a
threshold overrun of the ice cap volume. This relationship is
what allows Calder’s model to finally work ‘well’, because
Calder is more successful at predicting glacial maxima than
transitions. In other words, for still unknown physical reasons, deglaciations are facilitated by the occurrence of a
glacial maximum, which triggers a relaxation oscillation,
propelling the system into the opposite state.
Although this astronomical forcing is at the root of the
climate variations in the Quaternary, the understanding of
the climate mechanisms in play is limited. In Milankovich’s
theory there is a very simple hypothesis which, in a way,
equates the concept of ‘climate’ with the expansion of the ice
caps in the northern hemisphere. Milankovitch’s theory,
strictly speaking, is not a climate theory, but rather, an ice
cap theory, a point emphasized by the conceptual models
above. The action of insolation on the ice caps of the
northern hemisphere accounts for certain Quaternary phenomena, but does not explain all the observations, in particular the existence of the 100,000-year cycles, punctuated
by exceptional deglaciations (or terminations). This theory
therefore needs to be further completed to make it a true
climate theory.
Recent Advances
The Vital Role of Atmospheric CO 2
Shortly after the discovery of the role of astronomical periodicities in the climate (Hays et al. 1976), analysis of air
bubbles in Antarctic ice cores demonstrated that the last
glacial period was also characterized by a significantly lower
atmospheric concentration of CO 2 , in line with the predictions of Arrhenius. Since the work on the Vostok ice cores
(Petit et al. 1999) and Dôme C ice cores (Monnin et al.
2001), it is now well established that the glacial-interglacial
cycles also correspond to cycles in atmospheric greenhouse
gas composition, and in particular of CO 2 , which varies
between about 280 ppm (cm
3 /m
3 of air) during the interglacial period and 180 ppm during the glacial period. These
measurements make it possible to demonstrate that the two
traditional theories, astronomical and geochemical, are not
mutually exclusive, but that both are necessary. This was
largely confirmed by numerous numerical experiments in the
simulation of the glacial climate: in order to explain the
paleoclimate observations, it is essential to take into account
the 30% decrease in partial CO 2 pressure. In addition, during
the terminations and in particular during the last deglaciation, it is well established that the concentration of CO 2
increased several thousand years before the rise of the sea
level associated with the melting of the ice caps, or, in other
words, the actual deglaciation, as illustrated in Fig. 28.8.
Although it is relatively easy to apply Milankovitch’s theory
to most of the past cycles, considerable difficulties arise for
the deglaciations as is highlighted in Fig. 28.5. It is therefore
for these specific moments, when the astronomical theory
alone is insufficient, that other mechanisms need to be
explored.
In other words, glacial-interglacial changes are not limited to changes in the expansion of the ice caps that could
subsequently influence the rest of the climate system. They
are, on the contrary, a combination of changes for the caps,
but also for the biogeochemical cycles and the climate as a
whole. Milankovitch’s theory only accounts for part of this
reality, the other part most likely involves the carbon cycle
coupled with climate variations.
Unfortunately, our understanding of the carbon cycle during the Last Glacial Maximum is very patchy. To make a first
approximation, it is reasonable to consider the ocean + atmosphere + terrestrial biosphere system as isolated, i.e. with
no significant exchanges of geological carbon (via volcanoes
or rivers). The problem is therefore to reduce the atmospheric
reservoir by about 200 GtC (billion tons of carbon) while
increasing the others by the same amount. However, the terrestrial biosphere was considerably reduced during the glacial
period (between 300 GtC and 700 GtC), making the problem
all the more difficult, since all this atmospheric and biospheric
carbon needs to be trapped in the ocean. Many hypotheses
have been put forward to try to explain this low level of pCO 2
during the Last Glacial Maximum, but no consensus has yet
emerged. A complex combination of multiple factors (physical and biogeochemical) is one possibility which would
explain a glacial-interglacial difference of 100 ppm, but the
high level of similarity between the climate recordings around
the Antarctic and the pCO 2 records argue for a relatively
simple mechanism which would link the Southern Ocean and
its climate with the atmospheric concentration of CO 2 .
Towards a Consolidation of Astronomical
and Geochemical Theories?
Nevertheless, very recent progress makes a forthcoming
solution possible. The conceptual models mentioned above
suggest viewing the ‘glacial’ and ‘interglacial’ states as
distinct states able to account for a relaxation oscillation
between two (or more) different modes of operation. In this
context, it is interesting to mention the hypothesis of a glacial ocean with very cold and above all salty bottom waters.
This hypothesis is largely supported by measurements of
interstitial fluids in marine sediment cores (Adkins et al.
2002), which directly estimate the salinity of the ocean floor
in the past. The glacial ocean was therefore likely to have
been profoundly different from the current ocean, with
strong stratification between the upper and lower halves of
28 Climate and Astronomical Cycles
397
threshold overrun of the ice cap volume. This relationship is
what allows Calder’s model to finally work ‘well’, because
Calder is more successful at predicting glacial maxima than
transitions. In other words, for still unknown physical reasons, deglaciations are facilitated by the occurrence of a
glacial maximum, which triggers a relaxation oscillation,
propelling the system into the opposite state.
Although this astronomical forcing is at the root of the
climate variations in the Quaternary, the understanding of
the climate mechanisms in play is limited. In Milankovich’s
theory there is a very simple hypothesis which, in a way,
equates the concept of ‘climate’ with the expansion of the ice
caps in the northern hemisphere. Milankovitch’s theory,
strictly speaking, is not a climate theory, but rather, an ice
cap theory, a point emphasized by the conceptual models
above. The action of insolation on the ice caps of the
northern hemisphere accounts for certain Quaternary phenomena, but does not explain all the observations, in particular the existence of the 100,000-year cycles, punctuated
by exceptional deglaciations (or terminations). This theory
therefore needs to be further completed to make it a true
climate theory.
Recent Advances
The Vital Role of Atmospheric CO 2
Shortly after the discovery of the role of astronomical periodicities in the climate (Hays et al. 1976), analysis of air
bubbles in Antarctic ice cores demonstrated that the last
glacial period was also characterized by a significantly lower
atmospheric concentration of CO 2 , in line with the predictions of Arrhenius. Since the work on the Vostok ice cores
(Petit et al. 1999) and Dôme C ice cores (Monnin et al.
2001), it is now well established that the glacial-interglacial
cycles also correspond to cycles in atmospheric greenhouse
gas composition, and in particular of CO 2 , which varies
between about 280 ppm (cm
3 /m
3 of air) during the interglacial period and 180 ppm during the glacial period. These
measurements make it possible to demonstrate that the two
traditional theories, astronomical and geochemical, are not
mutually exclusive, but that both are necessary. This was
largely confirmed by numerous numerical experiments in the
simulation of the glacial climate: in order to explain the
paleoclimate observations, it is essential to take into account
the 30% decrease in partial CO 2 pressure. In addition, during
the terminations and in particular during the last deglaciation, it is well established that the concentration of CO 2
increased several thousand years before the rise of the sea
level associated with the melting of the ice caps, or, in other
words, the actual deglaciation, as illustrated in Fig. 28.8.
Although it is relatively easy to apply Milankovitch’s theory
to most of the past cycles, considerable difficulties arise for
the deglaciations as is highlighted in Fig. 28.5. It is therefore
for these specific moments, when the astronomical theory
alone is insufficient, that other mechanisms need to be
explored.
In other words, glacial-interglacial changes are not limited to changes in the expansion of the ice caps that could
subsequently influence the rest of the climate system. They
are, on the contrary, a combination of changes for the caps,
but also for the biogeochemical cycles and the climate as a
whole. Milankovitch’s theory only accounts for part of this
reality, the other part most likely involves the carbon cycle
coupled with climate variations.
Unfortunately, our understanding of the carbon cycle during the Last Glacial Maximum is very patchy. To make a first
approximation, it is reasonable to consider the ocean + atmosphere + terrestrial biosphere system as isolated, i.e. with
no significant exchanges of geological carbon (via volcanoes
or rivers). The problem is therefore to reduce the atmospheric
reservoir by about 200 GtC (billion tons of carbon) while
increasing the others by the same amount. However, the terrestrial biosphere was considerably reduced during the glacial
period (between 300 GtC and 700 GtC), making the problem
all the more difficult, since all this atmospheric and biospheric
carbon needs to be trapped in the ocean. Many hypotheses
have been put forward to try to explain this low level of pCO 2
during the Last Glacial Maximum, but no consensus has yet
emerged. A complex combination of multiple factors (physical and biogeochemical) is one possibility which would
explain a glacial-interglacial difference of 100 ppm, but the
high level of similarity between the climate recordings around
the Antarctic and the pCO 2 records argue for a relatively
simple mechanism which would link the Southern Ocean and
its climate with the atmospheric concentration of CO 2 .
Towards a Consolidation of Astronomical
and Geochemical Theories?
Nevertheless, very recent progress makes a forthcoming
solution possible. The conceptual models mentioned above
suggest viewing the ‘glacial’ and ‘interglacial’ states as
distinct states able to account for a relaxation oscillation
between two (or more) different modes of operation. In this
context, it is interesting to mention the hypothesis of a glacial ocean with very cold and above all salty bottom waters.
This hypothesis is largely supported by measurements of
interstitial fluids in marine sediment cores (Adkins et al.
2002), which directly estimate the salinity of the ocean floor
in the past. The glacial ocean was therefore likely to have
been profoundly different from the current ocean, with
strong stratification between the upper and lower halves of
28 Climate and Astronomical Cycles
397