| 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
δ18O/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 CO2) 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, CO2
and ice volume (marine δ18O) 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 CO2
response.
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 CO2 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. CO2 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
CO2/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 CO2 and their own feedback. |