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Science 28 July 2006:
Vol. 313. no. 5786, p. 405
DOI: 10.1126/science.313.5786.405b
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This Week in Science

Long-term variation in terrestrial insolation (the amount of energy that Earth receives from the Sun) results from the precession of Earth's axis of rotation, the variability of the obliquity of the spin axis, and the eccentricity of Earth's orbit around the Sun, which lead to signals with periods of ~20,000, 41,000, and 100,000 years, respectively. However, these cycles do not always appear in various paleoclimate records in the way predicted by current models. For example, the 41,000-year obliquity signal in geological records dominates between 3 million and 1 million years ago, rather than the expected ~20,000-year precessional signal. Two studies offer different solutions to this problem (see the Perspective by Paillard). Raymo et al. (p. 492, published online 22 June) maintain that the ~20,000-year cycles are largely absent from sea-level records of that period because ice volume changes in the Northern and Southern polar ice sheets are always opposite in sign and essentially cancel each other. Huybers (p. 508; published online 22 June) argues that models have been incorrectly using peak summer insolation to calculate melting, rather than the correct quantity, the integrated amount of solar energy received during the course of the season.




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