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Cosmic rays--high-speed bits of atoms thought to be created by the blasts of exploding stars--bombard Earth's atmosphere constantly. Not many people besides physicists pay much attention to them, but that may soon change. A Danish physicist has found evidence that cosmic rays greatly influence Earth's climate, and may even trigger ice ages.

Henrik Svensmark of the Danish Space Research Institute was led to this idea while studying eras of climate change, including the little ice age, one of the most jarring shifts on record. From about 1300 to 1850, global temperatures plunged. Svensmark found that the most plausible explanation--changes in the sun's temperature--didn't hold up: they were too small.

Svensmark speculated that as cosmic rays streak through the atmosphere, they collide with atoms, creating reactions that help trigger cloud formation. Increased cloud cover would lower global temperatures by reflecting sunlight away from Earth's surface.

Fortunately for Svensmark, Earth preserves a record of this barrage. When cosmic rays hit carbon atoms in the atmosphere, they create radioactive carbon 14. The amount of carbon 14 in Earth's sediment layers thus reflects cosmic-ray bombardments throughout the centuries. During the little ice age, Svensmark says carbon 14 levels "went up by almost a factor of two."

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