Climate of the Past
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2006-07-17
Ice-driven CO<sub>2</sub> feedback on ice volume
W. F. Ruddiman
rudds2@ntelos.net
Department of Environmental Sciences, University of Virginia, Charlottesville, VA, USA
The origin of the major ice-sheet variations during the last 2.7 million
years is a long-standing mystery. Neither the dominant 41 000-year cycles in
δ<sup>18</sup>O/ice-volume during the late Pliocene and early Pleistocene
nor the late-Pleistocene oscillations near 100 000 years is a linear
("Milankovitch") response to summer insolation forcing. Both responses must
result from non-linear behavior within the climate system. Greenhouse gases
(primarily CO<sub>2</sub>) are a plausible source of the required non-linearity,
but confusion has persisted over whether the gases force ice volume or are a
positive feedback. During the last several hundred thousand years, CO<sub>2</sub>
and ice volume (marine δ<sup>18</sup>O) have varied in phase at the
41 000-year obliquity cycle and nearly in phase within the ~100 000-year band.
This timing rules out greenhouse-gas forcing of a very
slow ice response and instead favors ice control of a fast CO<sub>2</sub>
response.
<P>
In the schematic model proposed here, ice sheets responded linearly to
insolation forcing at the precession and obliquity cycles prior to 0.9
million years ago, but CO<sub>2</sub> feedback amplified the ice response at the
41 000-year period by a factor of approximately two. After 0.9 million years
ago, with slow polar cooling, ablation weakened. CO<sub>2</sub> feedback continued
to amplify ice-sheet growth every 41 000 years, but weaker ablation
permitted some ice to survive insolation maxima of low intensity. Step-wise
growth of these longer-lived ice sheets continued until peaks in northern
summer insolation produced abrupt deglaciations every ~85 000 to ~115 000 years.
Most of the deglacial ice melting resulted from the same
CO<sub>2</sub>/temperature feedback that had built the ice sheets. Several
processes have the northern geographic origin, as well as the requisite
orbital tempo and phasing, to be candidate mechanisms for ice-sheet control
of CO<sub>2</sub> and their own feedback.
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