ORBITAL FORCING AND ENDOGENOUS NONLINEARITY IN THE PLEISTOCENE - THE GREENLAND ICE CORE

The succession of ice ages and interglacials during the Pleistocene is understood to have been caused primarily by shifts in the earth's orb it. At the same time, there is evidence of high variability in climate at suborbital frequencies. This paper conducts a statistical analysis of Pleistocene climate using the Greenland Ice Core Research Project (GRIP) data. Factoring temperature into the component explained by orb ital forcing and a residual demonstrates that variations at suborbital frequencies are nonlinear and aperiodic. There is evidence of a regul ar cycle at 7.9 kyr, evidently a subharmonic of the orbital frequencie s. Apart from this, however, the proximate memory of both the actual d ata and the residual decays slowly over a period of 15 kyr. Residual v ariations in temperature show two prominent features, alternating peri ods of high and low volatility, and states of distance from and proxim ity to the path implied by orbital forcing. A parametric model incorpo rating both of these properties is fit to the data, and is found to si gnificantly improve the forecastability of climate. Transitions betwee n states of proximity and distance from the orbital path can be partia lly predicted using the statistical model.