August 4, 2006
Understanding Glacial Cycles
Three articles published in Science provide background on glacial cycles. Those of you attempting to understand the science of global warming may want to check them out:
The episodic nature of Earth’s glacial and interglacial periods is believed to be caused by long-term changes in the amount of energy Earth receives from the Sun (insolation) and in seasonal variations driven by three major cycles: a 100,000-year eccentricity cycle (regular changes in the Earth’s orbit around the sun), a 41,000-yr obliquity cycle (oscillations in the tilt of Earth’s axis), and a 23,000-year precession cycle (changes in the direction of Earth’s axis of rotation). Curiously, Earth’s glacial oscillations between 3 and 1 millions years ago have followed a 41,000-year cycle when the 20,000-year precessional effects should have been stronger. Two studies in the 28 Jul 2006 Science (published online 22 Jun) offered two new explanations for this paradox. Raymo et al. ( http://www.sciencemag.org/cgi/content/short/313/5786/492 ) argue that the early glacial cycles appear to have a 40,000-year cycle because the opposing 23,000-year insolation cycles in the Northern and Southern hemispheres may have canceled one another. Huybers ( http://www.sciencemag.org/cgi/content/short/313/5786/508 ), on the other hand, maintain that ice age models have been incorrectly using peak summer insolation to estimate ice mass variability, when they should have instead used the integrated amount of solar energy received over the duration of the summer. An accompanying Perspective by D. Paillard ( http://www.sciencemag.org/cgi/content/short/313/5786/455 ) lent historical context to these new arguments.
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