ORBITAL FORCING AND ENDOGENOUS INTERACTIONS - NONLINEARITY, PERSISTENCE AND CONVERGENCE IN LATE PLEISTOCENE CLIMATE

The succession of ice ages and interglacials during the late Pleistoce ne has been linked with changes in the eccentricity, obliquity and pre cession of the earth's orbit. The combined effect of orbital cycles wi th different periodicities argues that climate should exhibit both per sistence - significant dependence between observations at distant inte rvals - and non-linearity. At the same time, a significant component o f climatic variation derives from endogenous interactions which exhibi t non-linear properties at both higher and lower frequencies than the orbital variations. In order to analyze the effect of orbital shifts a nd endogenous interactions, climate is factored into an orbitally forc ed and a residual component. The component explained by orbital forcin g shows some evidence of non-linearity, strong dependence between obse rvations over the first 10 ka, and additional dependence between obser vations at frequencies corresponding to the orbital cycles. The residu al component shows greater dependence between observations over the fi rst 20 ka, some evidence of dependence at the orbital cycles, and sign ificantly greater degrees of non-linearity. Neither shows evidence of actual chaoticity. The climatic residual generally moves in the same d irection as the path implied by orbital forcing, meaning that endogeno us interactions reinforce and accentuate changes in climate implied by orbital shifts. Despite the high variability of the residual, orbital forcing acts as a long-run attractor both for temperature and for atm ospheric CO2. Actual temperatures converge to the path implied by orbi tal forcing, on average within a period of 7.14 ka. Because orbital fo rcing is a long-term attractor for temperature but short-term movement s show strong serial dependence, the best forecasts for Pleistocene cl imate are obtained from a model including distributed lags of both orb ital shifts and temperature.