The Milankovitch band.



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The Milankovitch band.

Imbrie recently wrote an editorial entitled ``A good year for Milankovitch'' [ Imbrie, 1992], and there is little doubt that the last decade has indeed been a successful one with respect to elucidating how insolation changes driven by variations in the Earth's orbit have forced climate dynamics on time scales of 10 to 10 years. Comprehensive work by the SPECMAP (Spectral Mapping Project) group refined the tools and data needed to unravel the role of astronomical forcing, defined the state-of-the-art understanding of Milankovitch theory, and has facilitated a much better understanding of how the Earth's climate system works [ Imbrie et al., 1992; 1993a]. Similar mechanisms internal to the climate system appear to respond to quasi-periodic insolation forcing in each of the three major Milankovitch frequency bands: 1 cycle per 23 kyrs, 41 kyrs, and 100 kyrs. Whereas the response of the climate system to orbitally-induced insolation changes seems mostly linear in the case of the 23 and 41 kyr cycles, the climatic response to the 100 kyr orbital eccentricity variations is nonlinear, with large northern hemisphere ice sheets providing the requisite source of climatic inertia [ Imbrie et al., 1992; 1993a; 1993b; Sowers et al., 1993; Winograd et al., 1992; Clemens et al., 1991; Anderson and Prell, 1993].

The last decade has also seen remarkable progress toward understanding the patterns and processes of climatic change over the last ca. 40,000 years (40 kyrs). Radiocarbon provides the tight chronological framework needed to unravel the exact response of the climate system to astronomical forcing over the last glacial to interglacial cycle, as well as to understand the decade- to millennia-scale (``sub-Milankovitch'') variability that is superimposed on this low-frequency component of variability. The COHMAP (Cooperative Holocene Mapping Project) group took up where the CLIMAP (Climate Mapping, Analysis and Prediction) effort left off, providing an international catalyst in efforts to map out the patterns of global climate change over the past 18,000 years using a variety of proxy data sources [ COHMAP, 1988; Wright et al., 1993]. Through the powerful combination of empirical paleoenvironmental reconstructions and climate model simulations for successive 3 kyr intervals (18,000 years ago [18 ka] to present), COHMAP and others have demonstrated the central role of Milankovitch forcing, along with the response of the climate system to this forcing, in explaining much of the observed low-frequency climatic variance of the last 18 kyrs [ COHMAP, 1988; Markgraf et al., 1992; Porter et al., 1992; Jarvis, 1993; R. Webb et al., 1993; Wright et al., 1993].

Despite remarkable progress toward understanding how forcing both external and internal to the climate system generated the observed patterns of change during the late Quaternary, puzzles still remain. Many areas of the Earth outside mid-latitude North America and Europe still lack well-dated networks of data. Controversy over the thermal history of the low-latitude surface oceans continues [ Webster and Streten, 1978; Bush and Colinvaux, 1990; Anderson and Webb, 1994; Guilderson et al., 1994; Sikes and Keigwin, 1994;
[4] Thunell et al., 1994]. Uncertainty over the last 20 kyrs of sea level and ice-sheet history has been greatly reduced, although the glacial volumes of the East Antarctic and North American ice-sheets are still uncertain [ Fairbanks, 1989; Ruddiman, 1994].



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Next: The millennial-centennial band. Up: The Tectonic Band: Previous: The Tectonic Band:



U.S. National Report to IUGG, 1991-1994
Rev. Geophys. Vol. 33 Suppl., © 1995 American Geophysical Union