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K. D. Pang (La Canada, CA), K. K. Yau (JPL)
The Sun, once thought to be constant, has actually undergone nine long brightness cycles in the past 1800 years. Although these luminosity changes were < 1%, they have clearly influenced in the Earth's climate. For example, during the Maunder (Solar) Minimum between 1645 and 1715, sunspots were rarely seen, indicative of a weak Sun. Astronomers at Mt. Wilson, e.g., Hale, Nicholson and Wilson, have linked the luminosity changes to solar magnetic cycles, and similar behaviors in Sun-like stars. Others have elucidated the Sun-climate connection. For example, Eddy suggested that the enfeebled Sun was responsible for the cold climate of the Little Ice Age. The estimated average global temperature then was half a degree Centigrade lower than that prevailed in the 1970's (Lean, Geophys. Res. Lett. 22, 3195, 1995). Yau researched East Asian historical sunspot records, and found that there were seven Maunder-like solar minima before 1600 (Quart. J. Roy. Astron. Soc. 29, 175, 1988). The earliest was in the third century, and the Sporer Minimum occurred in 1400-1600. These findings agree with analyses of historical records of aurora sightings. Energetic electrons and protons emitted by an active Sun create auroras on Earth. When sunspots were rare auroras were, likewise, seldom seen. Minima in sunspot/aurora sightings are, in turn, coincident with peaks in long-term carbon-14 and beryllium-10 records. The radioisotopes are made high in atmosphere by cosmic rays. Their production is low when strong solar magnetic storms deflect such rays from the Solar System, and vice versa (Pang and Yau, Eos 83, No. 43, 481, 2002). Pang analyzed Chinese historical weather records, and discovered that periods of severely cold climate coincide with the Maunder and Sporer Minima. Lakes and rivers froze up, seas icebound, unseasonable frosts killed crops, and famines were widespread during these cold spells, consistent with general circulation model hindcast of contemporary winter conditions for China (Shindell, Science 294, 2149, 2001). Although variable solar luminosity is important, other strong forces also drive the Earth's climate system. These include greenhouse warming, volcanic eruptions, and changing ocean current systems, e.g., the El Nino. The solar variations, however, act over periods ranging from decades to centuries.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.