Signature Patterns of Large-Scale Climatic Controls on Orbital Timescales

P.J. Bartlein (Dept. Geography, Univ. Oregon, Eugene, OR 97403-1251; ph. 541-346-4967; fax 541-346-2067; Internet:

On orbital time scales (e.g. 3x10^3 to 3x10^6 yrs) the large-scale controls of regional climates include the (externally governed) variations in latitudinal and seasonal distriubtions of insolation, and the (internally generated) variations in the configuration of the ice sheets, atmospheric composition (including aerosols), sea-surface temperature, and land-surface cover and hydrology. Variations in each of these controls produce distinct regional patterns of surface-climate responses, in much the same way that the signature of warm and cold phases of ENSO-type variations can be recognized in the instrumental record. For example (as revealed by climate-experiments), variations in ice-sheet size or sea-ice limits reconfigure atmospheric circulation patterns, while variations in insolation alter the land-sea temperature contrast and the amplitude of the seasonal cycles of temperature and effective moisture, and so on. At any particular location, the paleoclimatic record will reflect the superimposition of these controls, mediated by interactions between the local water and energy balances. The search for mechanisms responsible for millennial-scale climatic variations and their transmission around the globe can be informed by the conceptual models that emerge from examining these orbital-scale responses in two ways. First, the orbital-scale responses provide a number candidate mechanisms for the generation and transmission of the kinds of regional-scale responses that are likely to be recorded by paleoclimatic indicators, and second, variations in the orbital-scale controls result in a continuously varying context that millennial-scale variations necessarily take place in.