TERMINATING THE 100 KYR ICE-AGE CYCLE

We report a simulation of the most recent 100,000-year glaciation-degl aciation cycle of the late Pleistocene ice age, a simulation that deli vers an ice sheet chronology that is in close accord with that inferre d from the geological record. Our analyses are performed with a reduce d model of the climate system that incorporates significant improvemen ts to the representation of both climate forcing and mass balance resp onse in a previously described theory based upon a coupled one-level e nergy balance model (EBM) and vertically integrated ice sheet model (I SM). Tile theory fully incorporates the influences of orbital insolati on forcing, glacial isostatic adjustment and variations in the atmosph eric concentrations of greenhouse gases. It correctly predicts the mai n geographical regions of the northern hemisphere that were glaciated at last glacial maximum 21,000 years ago as well as the abrupt termina tion of the glacial epoch that occurred subsequently. The latter featu re of the ice age cycle is obtained without the need to incorporate un constrained and therefore controversial physical processes into the mo del, a limitation of all previous attempts to understand this global s cale climate cycle. Our analyses suggest that the radiative impact on surface glaciation due to the changing atmospheric concentration of CO 2 is critical to the ability of the model to deliver a synthetic histo ry of glaciation and deglaciation that is in accord with inferences ba sed upon surface geological and geomorphological evidence. With the in corporation of this influence, model-predicted ice thickness distribut ions at last glacial maximum (LGM) are very similar to those of the re cently described ICE-4G reconstruction that was based upon the inversi on of postglacial relative sea level histories.