Abstract

The relative role of external forcing and of intrinsic variability is a key question of climate variability in general and of our planet’s paleoclimatic past in particular. Over the last 100 years since Milankovitch’s contributions, the role of orbital forcing has been well established for the last 2.6 Myr and their Quaternary glaciation cycles. A convincing case has also been made for the role of several internal mechanisms that are active on time scales both shorter and longer than the orbital ones. Such mechanisms clearly have a causal role in Dansgaard-Oeschger and Heinrich events, as well as in the mid-Pleistocene transition. We introduce herein a unified framework for the understanding of the interplay between internal mechanisms and orbital forcing on time scales from thousands to millions of years. This framework relies on the fairly recent theory of nonautonomous and random dynamical systems and it has been successfully applied so far in the climate sciences for problems like the El Niño-Southern Oscillation, the oceans’ wind-driven circulation, and other problems on interannual to interdecadal time scales. Finally, we provide further examples of climate applications and present preliminary results of interest for the Quaternary glaciation cycles in general and the mid-Pleistocene transition in particular.

Highlights

  • AND MOTIVATIONFor two centuries or more of modern geology, records of our planet’s physical and biological past were merely discrete sequences of strata with specific properties, like coloration and composition (Imbrie and Imbrie, 1986)

  • As a result of the twofold stimulation provided by data about past glaciations and concern about future ones, a number of 40 researchers in the early-to-mid 1970s worked on energy balance models (EBMs) of climate with multiple stable steady states (Held and Suarez, 1974; North, 1975; Ghil, 1976)

  • The section concluded by the formulation and study of a FitzHugh-Nagumo (FHN)-type model of recurrent Dansgaard-Oeschger (D-O) events, in which historical CO2 concentrations induced episodes of D-O events alternating with episodes of their absence, in excellent qualitative agreement with NGRIP δ18O data; see again Fig. 9

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Summary

INTRODUCTION

For two centuries or more of modern geology, records of our planet’s physical and biological past were merely discrete sequences of strata with specific properties, like coloration and composition (Imbrie and Imbrie, 1986). As a result of the twofold stimulation provided by data about past glaciations and concern about future ones, a number of 40 researchers in the early-to-mid 1970s worked on energy balance models (EBMs) of climate with multiple stable steady states (Held and Suarez, 1974; North, 1975; Ghil, 1976) Two such stable “equilibria” corresponded to the present climate and to a “deep-freeze,” as it was called at the time, i.e., to a totally ice-covered Earth. To coupling a “climate” equation, with temperature as its only dependent variable, with an ice-sheet equation (Källén et al, 1979; Ghil and Le Treut, 1981) or a carbon-dioxide equation (Saltzman et al, 1981; Saltzman and Maasch, 1988) These coupled climate models, albeit highly idealized, did produce oscillatory solutions that captured some of the features 55 of the Quaternary glaciation cycles as known at that time.

A Simple Mechanism for Climate Oscillations
Intrinsic Climate Oscillations and the Mid-Pleistocene Transition (MPT)
Orbital Forcing of a Climate Oscillator
Basic Facts of NDS and RDS Life
Application to Dansgaard-Oeschger (D-O) events
Conclusions
580 Acknowledgements
585 References
Full Text
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