What Do We Know about Potential Modes of Operation of the Quaternary Earth System?

The problem of Earth system stability during the Quaternary, approximately the last two million years, is addressed from a global perspective. Despite efforts over the last 160 years to obtain geological evidence of ice ages, we conclude that the question of which potential modes of operation impacted the Quaternary Earth system is as yet unsolved. However, there are some clues as to which elements should be included in a theory of the Quaternary Earth system. The search for direct paleo-analogues is unlikely to answer the question of potential modes in a physically meaningful manner. Assessment of a number of conceptual models—ranging from models in which forcing is necessary to yield observed climate variability to models of free climate oscillations — does not favor any particular model because of the difficulties of tuning each model to the time series of global ice volume. Hence, geographically explicit fully coupled climate system, or natural Earth system, models are required to analyze the system's response to geographically varying forcing and internal feedbacks. Evidence emerges that much of Quaternary climate variability was due to internal feedbacks involving ice sheets and biogeochemical cycles as critical elements and orbital forcing as the pacemaker.